Linear Technology No Rsense LTC3736 User manual

1
LTC3736
3736fa
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode
is a registered trademark of Linear Technology Corporation. No R
SENSE
is a trademark of
Linear Technology Corporation. All other trademarks are the property of their respective
owners. Protected by U.S. Patents including 5481178, 5929620, 6144194, 6580258,
6304066, 6611131, 6498466.
Dual 2-Phase, No RSENSETM,
Synchronous Controller
with Output Tracking
High Efficiency, 2-Phase, Dual Synchronous DC/DC Step-Down Converter
■
No Current Sense Resistors Required
■
Out-of-Phase Controllers Reduce Required
Input Capacitance
■
Tracking Function
■
Wide V
IN
Range: 2.75V to 9.8V
■
Constant Frequency Current Mode Operation
■
0.6V ±1.5% Voltage Reference
■
Low Dropout Operation: 100% Duty Cycle
■
True PLL for Frequency Locking or Adjustment
■
Selectable Burst Mode
®
/Forced Continuous Operation
■
Auxiliary Winding Regulation
■
Internal Soft-Start Circuitry
■
Power Good Output Voltage Monitor
■
Output Overvoltage Protection
■
Micropower Shutdown: I
Q
= 9µA
■
Tiny Low Profile (4mm ×4mm) QFN and Narrow
SSOP Packages
The LTC
®
3736 is a 2-phase dual synchronous step-down
switching regulator controller with tracking that drives
external complementary power MOSFETs using few exter-
nal components. The constant frequency current mode
architecture with MOSFET V
DS
sensing eliminates the
need for sense resistors and improves efficiency. Power
loss and noise due to the ESR of the input capacitance are
minimized by operating the two controllers out of phase.
BurstModeoperationprovideshigh efficiency atlight loads.
100%dutycycle capability provideslowdropout operation,
extending operating time in battery-powered systems.
Theswitching frequency canbe programmed up to750kHz,
allowing the use of small surface mount inductors and ca-
pacitors. For noise sensitive applications, the LTC3736
switching frequency can be externally synchronized from
250kHz to 850kHz. Burst Mode operation is inhibited dur-
ingsynchronizationor when theSYNC/FCBpin is pulledlow
inorder toreducenoise andRFinterference. Automaticsoft-
start is internally controlled.
The LTC3736 is available in the tiny thermally enhanced
(4mm ×4mm) QFN package or 24-lead SSOP narrow
package.
■
One or Two Lithium-Ion Powered Devices
■
Notebook and Palmtop Computers, PDAs
■
Portable Instruments
■
Distributed DC Power Systems
SENSE1+
VIN
LTC3736
SGND
SENSE2+
TG1 TG2
SW1 SW2
BG1 BG2
PGND PGND
VFB1 VFB2
220pF
VOUT1
2.5V
VOUT2
1.8V
47µF47µF
15k
220pF
15k
59k 59k
187k 118k
2.2µH2.2µH
ITH1
3736 TA01a
ITH2
10µF
×2
VIN
2.75V TO 9.8V
LOAD CURRENT (mA)
65
EFFICIENCY (%)
95
100
60
55
90
75
85
80
70
1 100 1000 10000
3736 TA01b
50
10
V
IN
= 3.3V
FIGURE 16 CIRCUIT
V
OUT
= 2.5V
V
IN
= 5V
V
IN
= 4.2V
Efficiency vs Load Current
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U

2
LTC3736
3736fa
Input Supply Voltage (V
IN
) ........................ –0.3V to 10V
PLLLPF, RUN/SS, SYNC/FCB,
TRACK, SENSE1
+
, SENSE2
+
,
IPRG1, IPRG2 Voltages ................. –0.3V to (V
IN
+ 0.3V)
V
FB1
, V
FB2
, I
TH1
, I
TH2
Voltages .................. –0.3V to 2.4V
SW1, SW2 Voltages ............ –2V to V
IN
+ 1V or 10V Max
PGOOD ..................................................... –0.3V to 10V
ABSOLUTE AXI U RATI GS
WWWU
(Note 1)
TG1, TG2, BG1, BG2 Peak Output Current (<10µs) ..... 1A
Operating Temperature Range (Note 2) ... –40°C to 85°C
Storage Temperature Range .................. –65°C to 125°C
Junction Temperature (Note 3) ............................ 125°C
Lead Temperature (Soldering, 10 sec)
(LTC3736EGN) ..................................................... 300°C
PACKAGE/ORDER I FOR ATIO
UU
W
24 23 22 21 20 19
7 8 9
TOP VIEW
25
UF PACKAGE
24-LEAD (4mm ×4mm) PLASTIC QFN
10 11 12
6
5
4
3
2
1
13
14
15
16
17
18
ITH1
IPRG2
PLLLPF
SGND
VIN
TRACK
SYNC/FCB
TG1
PGND
TG2
RUN/SS
BG2
VFB1 IPRG1 SW1 SENSE1+ PGND BG1
VFB2
ITH2
PGOOD
SW2
SENSE2+
PGND
T
JMAX
= 125°C, θ
JA
= 37°C/W
EXPOSED PAD (PIN 25) IS PGND
MUST BE SOLDERED TO PCB
1
2
3
4
5
6
7
8
9
10
11
12
TOP VIEW
GN PACKAGE
24-LEAD PLASTIC SSOP
24
23
22
21
20
19
18
17
16
15
14
13
SW1
IPRG1
VFB1
ITH1
IPRG2
PLLLPF
SGND
VIN
TRACK
VFB2
ITH2
PGOOD
SENSE1+
PGND
BG1
SYNC/FCB
TG1
PGND
TG2
RUN/SS
BG2
PGND
SENSE2+
SW2
ORDER PART
NUMBER
UF PART MARKING
3736
LTC3736EUF
T
JMAX
= 125°C, θ
JA
= 130°C/ W
ORDER PART
NUMBER
LTC3736EGN
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ●denotes specifications that apply over the full operating temperature
range, otherwise specifications are at TA= 25°C. VIN = 4.2V unless otherwise specified.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Main Control Loops
Input DC Supply Current (Note 4)
Sleep Mode 300 425 µA
Shutdown RUN/SS = 0V 9 20 µA
UVLO V
IN
= UVLO Threshold –200mV 3 10 µA
Undervoltage Lockout Threshold V
IN
Falling ●1.95 2.25 2.55 V
V
IN
Rising ●2.15 2.45 2.75 V
Shutdown Threshold at RUN/SS 0.45 0.65 0.85 V
Start-Up Current Source RUN/SS = 0V 0.4 0.7 1 µA
Regulated Feedback Voltage 0°C to 85°C (Note 5) 0.591 0.6 0.609 V
–40°C to 85°C●0.588 0.6 0.612 V
Output Voltage Line Regulation 2.75V < V
IN
< 9.8V (Note 5) 0.05 0.2 mV/V

3
LTC3736
3736fa
ELECTRICAL CHARACTERISTICS
The ●denotes specifications that apply over the full operating temperature
range, otherwise specifications are at TA= 25°C. VIN = 4.2V unless otherwise specified.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LTC3736E is guaranteed to meet specified performance from
0°C to 70°C. Specifications over the –40°C to 85°C operating range are
assured by design, characterization and correlation with statistical process
controls.
Note 3: T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
T
J
= T
A
+ (P
D
• θ
JA
°C/W)
Note 4: Dynamic supply current is higher due to gate charge being
delivered at the switching frequency.
Note 5: The LTC3736 is tested in a feedback loop that servos I
TH
to a
specified voltage and measures the resultant V
FB
voltage.
Note 6: Peak current sense voltage is reduced dependent on duty cycle to
a percentage of value as shown in Figure 1.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Output Voltage Load Regulation I
TH
= 0.9V (Note 5) 0.12 0.5 %
I
TH
= 1.7V –0.12 –0.5 %
V
FB1,2
Input Current (Note 5) 10 50 nA
TRACK Input Current TRACK = 0.6V 10 50 nA
Overvoltage Protect Threshold Measured at V
FB
0.66 0.68 0.7 V
Overvoltage Protect Hysteresis 20 mV
Auxiliary Feedback Threshold SYNC/FCB Ramping Positive 0.525 0.6 0.675 V
Top Gate (TG) Drive 1, 2 Rise Time C
L
= 3000pF 40 ns
Top Gate (TG) Drive 1, 2 Fall Time C
L
= 3000pF 40 ns
Bottom Gate (BG) Drive 1, 2 Rise Time C
L
= 3000pF 50 ns
Bottom Gate (BG) Drive 1, 2 Fall Time C
L
= 3000pF 40 ns
Maximum Current Sense Voltage (∆V
SENSE(MAX)
) IPRG = Floating ●110 125 140 mV
(SENSE
+
– SW) IPRG = 0V ●70 85 100 mV
IPRG = V
IN
●185 204 223 mV
Soft-Start Time Time for V
FB1
to Ramp from 0.05V to 0.55V 0.667 0.833 1 ms
Oscillator and Phase-Locked Loop
Oscillator Frequency Unsynchronized (SYNC/FCB Not Clocked)
PLLLPF = Floating ●480 550 600 kHz
PLLLPF = 0V ●260 300 340 kHz
PLLLPF = V
IN
●650 750 825 kHz
Phase-Locked Loop Lock Range SYNC/FCB Clocked
Minimum Synchronizable Frequency ●200 250 kHz
Maximum Synchronizable Frequency ●850 1150 kHz
Phase Detector Output Current
Sinking f
OSC
> f
SYNC/FCB
–4 µA
Sourcing f
OSC
< f
SYNC/FCB
4 µA
PGOOD Output
PGOOD Voltage Low I
PGOOD
Sinking 1mA 125 mV
PGOOD Trip Level V
FB
with Respect to Set Output Voltage
V
FB
< 0.6V, Ramping Positive –13 –10.0 –7 %
V
FB
< 0.6V, Ramping Negative –16 –13.3 –10 %
V
FB
> 0.6V, Ramping Negative 7 10.0 13 %
V
FB
> 0.6V, Ramping Positive 10 13.3 16 %

4
LTC3736
3736fa
TYPICAL PERFOR A CE CHARACTERISTICS
UW
Efficiency and Power Loss
vs Load Current
V
OUT
AC-COUPLED
100mV/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/FCB = V
IN
FIGURE 17 CIRCUIT
100µs/DIV
3736 G02
I
L
2A/DIV
V
OUT
AC-COUPLED
100mV/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/FCB = 0V
FIGURE 17 CIRCUIT
100µs/DIV 3736 G03
I
L
2A/DIV
Load Step (Burst Mode Operation)
Load Step
(Forced Continuous Mode)
Load Step (Pulse Skipping Mode)
V
OUT
AC-COUPLED
100mV/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/FCB = 550kHz EXTERNAL CLOCK
FIGURE 17 CIRCUIT
100µs/DIV 3736 G04
I
L
2A/DIV
Burst Mode
OPERATION
SYNC/FCB = V
IN
FORCED
CONTINUOUS
MODE
SYNC/FCB = 0V
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 200mA
FIGURE 17 CIRCUIT
4µs/DIV
3736 G05
PULSE
SKIPPING MODE
SYNC/FCB = 550kHz
I
L
1A/DIV
VIN = 5V
RLOAD1 = RLOAD2 = 1Ω
FIGURE 15 CIRCUIT
200µs/DIV 3736 G06
500mV/
DIV
VOUT1
2.5V
VOUT2
1.8V
Inductor Current at Light Load
Tracking Start-Up with Internal
Soft-Start (CSS = 0µF)
VIN = 5V
RLOAD1 = RLOAD2 = 1Ω
FIGURE 15 CIRCUIT
40ms/DIV 3736 G07
500mV/
DIV
VOUT1
2.5V
VOUT2
1.8V
INPUT VOLTAGE (V)
2
–5
NORMALIZED FREQUENCY SHIFT (%)
–4
–2
–1
0
5
2
467
3736 G08
–3
3
4
1
35 8910
Oscillator Frequency
vs Input Voltage
Tracking Start-Up with External
Soft-Start (CSS = 0.15µF)
T
A
= 25°C unless otherwise noted.
LOAD CURRENT (mA)
65
EFFICIENCY (%)
95
100
60
55
90
75
85
80
70
1 100 1000 10000
3736 G01
50
10
FIGURE 15 CIRCUIT
POWER LOSS (W)
10
1
0.1
0.01
0.001
V
IN
= 5V
SYNC/FCB = V
IN
V
OUT
= 3.3V
V
OUT
= 2.5V
V
OUT
= 1.8V
V
OUT
= 1.2V

5
LTC3736
3736fa
Maximum Current Sense Voltage
vs ITH Pin Voltage
ITH VOLTAGE (V)
0.5
–20
CURRENT LIMIT (%)
0
20
40
60
100
1 1.5
3736 G09
2
80
Burst Mode OPERATION
(ITH RISING)
Burst Mode OPERATION
(ITH FALLING)
FORCED CONTINUOUS
MODE
PULSE SKIPPING
MODE
LOAD CURRENT (mA)
65
EFFICIENCY (%)
95
100
60
55
90
75
85
80
70
1 100 1000 10000
3736 G10
50
10
PULSE SKIPPING
MODE
(SYNC/FCB = 550kHz)
FIGURE 15 CIRCUIT
V
IN
= 5V
V
OUT
= 2.5V
Burst Mode
OPERATION
(SYNC/FCB = V
IN
)
FORCED
CONTINUOUS
(SYNC/FCB = 0V)
TEMPERATURE (°C)
–60
FEEDBACK VOLTAGE (V)
0.601
0.603
0.605
100
3736 G11
0.599
0.597
0.591 –20 20 60
–40 040 80
0.595
0.593
0.609
0.607
Efficiency vs Load Current
Regulated Feedback Voltage
vs Temperature
Shutdown (RUN) Threshold
vs Temperature
RUN/SS Pull-Up Current
vs Temperature
Maximum Current Sense Threshold
vs Temperature
TEMPERATURE (°C)
–60
0
RUN/SS VOLTAGE (V)
0.1
0.3
0.4
0.5
1.0
0.7
–20 20 40
3736 G12
0.2
0.8
0.9
0.6
–40 0 60 80 100
TEMPERATURE (°C)
–60
0.4
RUN/SS PULL-UP CURRENT (µA)
0.5
0.6
0.7
0.8
–20 20 60 100
3736 G13
0.9
1.0
–40 0 40 80
TEMPERATURE (°C)
–60
115
MAXIMUM CURRENT SENSE THRESHOLD (mV)
120
125
130
135
–40 –20 0 20
3736 G14
40 60 80 100
I
PRG
= FLOAT
Oscillator Frequency
vs Temperature
TEMPERATURE (°C)
–60
–10
NROMALIZED FREQUENCY (%)
–8
–4
–2
0
10
4
–20 20 40
3736 G15
–6
6
8
2
–40 0 60 80 100
TEMPERATURE (°C)
–60
INPUT (V
IN
) VOLTAGE (V)
2.30
2.40
100
3736 G16
2.20
2.10 –20 20 60
–40 040 80
2.50
2.25
2.35
2.15
2.45 V
IN
RISING
V
IN
FALLING
Undervoltage Lockout Threshold
vs Temperature
TYPICAL PERFOR A CE CHARACTERISTICS
UW
T
A
= 25°C unless otherwise noted.

6
LTC3736
3736fa
UU
U
PI FU CTIO S
I
TH1
/I
TH2
(Pins 1, 8/ Pins 4, 11): Current Threshold and
Error Amplifier Compensation Point. Nominal operating
range on these pins is from 0.7V to 2V. The voltage on
these pins determines the threshold of the main current
comparator.
PLLLPF (Pin 3/Pin 6): Frequency Set/PLL Lowpass Filter.
When synchronizing to an external clock, this pin serves
as the lowpass filter point for the phase-locked loop. Nor-
mallyaseries RC isconnectedbetween this pin andground.
Whennotsynchronizing to anexternalclock, this pinserves
as the frequency select input. Tying this pin to GND selects
300kHz operation; tying this pin to V
IN
selects 750kHz op-
eration. Floating this pin selects 550kHz operation.
SGND (Pin 4/Pin 7): Small-Signal Ground. This pin serves
as the ground connection for most internal circuits.
V
IN
(Pin 5/Pin 8): Chip Signal Power Supply. This pin
powerstheentire chip except forthegate drivers. Externally
filtering this pin with a lowpass RC network (e.g.,
R = 10Ω, C = 1µF) is suggested to minimize noise pickup,
especially in high load current applications.
TRACK (Pin 6/Pin 9): Tracking Input for Second Control-
ler. Allows the start-up of V
OUT2
to “track” that of V
OUT1
according to a ratio established by a resistor divider on
V
OUT1
connected to the TRACK pin. For one-to-one track-
ing of V
OUT1
and V
OUT2
during start-up, a resistor divider
(UF/GN Package)
with values equal to those connected to V
FB2
from V
OUT2
should be used to connect to TRACK from V
OUT1
.
PGOOD(Pin 9/Pin 12): Power Good Output Voltage Moni-
tor Open-Drain Logic Output. This pin is pulled to ground
when the voltage on either feedback pin (V
FB1
, V
FB2
) is not
within ±13.3% of its nominal set point.
PGND (Pins 12, 16, 20, 25/ Pins 15, 19, 23): Power
Ground. These pins serve as the ground connection for the
gate drivers and the negative input to the reverse current
comparators. The Exposed Pad (UF package) must be
soldered to PCB ground.
RUN/SS (Pin 14/Pin 17): Run Control Input and Optional
ExternalSoft-Start Input. Forcingthis pinbelow0.65V shuts
down the chip (both channels). Driving this pin to V
IN
or
releasing this pin enables the chip, using the chip’s inter-
nal soft-start. An external soft-start can be programmed by
connecting a capacitor between this pin and ground.
TG1/TG2(Pins17, 15/Pins 20,18): Top (PMOS)GateDrive
Output.Thesepins drive the gates of theexternalP-channel
MOSFETs. These pins have an output swing from PGND to
SENSE
+
.
SYNC/FCB (Pin 18/Pin 21): This pin performs three
functions: 1) auxiliary winding feedback input, 2) external
clock synchronization input for phase-locked loop, and 3)
Burst Mode operation or forced continuous mode select.
Shutdown Quiescent Current
vs Input Voltage
INPUT VOLTAGE (V)
2
0
SHUTDOWN CURRENT (µA)
2
6
8
10
20
14
467
3736 G17
4
16
18
12
35 8910
RUN/SS = 0V
RUN/SS Start-Up Current
vs Input Voltage
INPUT VOLTAGE (V)
2
RUN/SS PIN PULL-UP CURRENT (µA)
0.5
0.6
0.7
10
3736 G18
0.4
0.3
0
0.1
468
3579
0.2
0.9
0.8
RUN/SS = 0V
TYPICAL PERFOR A CE CHARACTERISTICS
UW
T
A
= 25°C unless otherwise noted.

7
LTC3736
3736fa
FU CTIO AL DIAGRA
UU W
–
+
–
+
–
+
–
+
SHDN
0.6V
V
REF
EXTSS
0.7µA
BURSTDIS
CLK1
CLK2
FCB
0.54V
V
FB1
V
FB2
FCB
0.6V
SLOPE1
SLOPE2
RUN/SS
V
IN
C
VIN
V
IN
(TO CONTROLLER 1, 2)
R
VIN
SYNC/FCB
PLLLPF
UNDERVOLTAGE
LOCKOUT
BURST DEFEAT/
SYNC DETECT
VOLTAGE
CONTROLLED
OSCILLATOR
SLOPE
COMP
VOLTAGE
REFERENCE
t
SEC
= 1ms
INTSS
PHASE
DETECTOR
IPROG1
IPROG2
IPRG1
IPRG2
VOLTAGE
CONTROLLED
OSCILLATOR
MAXIMUM
SENSE VOLTAGE
SELECT
PGOOD
SHDN
OV1
UV1
UV2 OV2
3736 FD
(Common Circuitry)
For auxiliary winding applications, connect to a resistor
divider from the auxiliary output. To synchronize with an
external clock using the PLL, apply a CMOS compatible
clock with a frequency between 250kHz and 850kHz. To
selectBurstMode operation at light loads,tiethis pin to V
IN
.
Grounding this pin selects forced continuous operation,
which allows the inductor current to reverse. When
synchronizedto an externalclock,pulse-skipping operation
is enabled at light loads.
BG1/BG2 (Pins 19, 13/Pins 22, 16): Bottom (NMOS) Gate
Drive Output. These pins drive the gates of the external N-
channel MOSFETs. These pins have an output swing from
PGND to SENSE
+
.
SENSE1
+
/SENSE2
+
(Pins 21, 11/Pins 24, 14): Positive
Input to Differential Current Comparator. Also powers the
gate drivers. Normally connected to the source of the ex-
ternal P-channel MOSFET.
SW1/SW2 (Pins 22, 10/Pins 1, 13): Switch Node Connec-
tion to Inductor. Also the negative input to differential peak
current comparator and an input to the reverse current
comparator. Normally connected to the drain of the exter-
nalP-channelMOSFETs, the drainofthe external N-channel
MOSFET and the inductor.
IPRG1/IPRG2 (Pins 23, 2/Pins 2, 5): Three-State Pins to
SelectMaximum Peak SenseVoltage Threshold. Thesepins
select the maximum allowed voltage drop between the
SENSE
+
and SW pins (i.e., the maximum allowed drop
across the external P-channel MOSFET) for each channel.
Tie to V
IN
, GND or float to select 204mV, 85mV or 125mV
respectively.
V
FB1
/V
FB2
(Pins 24, 7/Pins 3,10): FeedbackPins. Receives
theremotelysensed feedback voltageforits controller from
an external resistor divider across the output.
UU
U
PI FU CTIO S
(UF/GN Package)

8
LTC3736
3736fa
FU CTIO AL DIAGRA
UU W
Q
OV1
CLK1
SC1
FCB
SLEEP1
SLOPE1
SW1
SENSE1
+
IREV1
S
R
RS1
ANTISHOOT
THROUGH
PGND
TG1
SENSE1
+
V
IN
V
OUT1
C
IN
C
OUT1
MP1
MN1
BG1
R1B
L1
PGND
V
FB1
I
TH1
R
ITH1
C
ITH1
0.6V
0.12V
SC1
V
FB1
SW1
SENSE1
+
R1A
–
+
–
+
EXTSS
INTSS
EAMP
SHDN
–
+
BURSTDIS
SLEEP1
0.3V
IPROG1
–
+
ICMP
0.15V
BURSTDIS
–
+
V
FB1
OV1
0.68V
+
–
PGND
IREV1
IPROG1 FCB
SW1
3736 CONT1
–
+
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
SCP
RICMPOVP
(Controller 1)

9
LTC3736
3736fa
FU CTIO AL DIAGRA
UU W
(Controller 2)
Q
OV2
CLK2
SC2
FCB
SLEEP2
BURSTDIS
SLOPE2
SW2
SENSE2
+
SHDN
IREV2
S
R
RS2
ANTISHOOT
THROUGH
PGND
SENSE2
+
TG2
SENSE2
+
V
IN
V
OUT2
C
OUT2
MP2
MN2
BG2
R2B
R
TRACKB
R
TRACKA
L2
PGND
V
FB2
I
TH2
TRACK
R
ITH2
C
ITH2
0.6V
0.12V
SC2
TRACK
V
FB2
SW2
R2A
V
OUT1
–
+
–
+
EAMP
–
+
BURSTDIS
SLEEP2
–
+
ICMP
0.15V
–
+
V
FB2
OV2
0.68V
+
–
PGND
IREV2
IPROG2 FCB
SW2
3736 CONT2
–
+
0.3V
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
OVP
SCP

10
LTC3736
3736fa
Main Control Loop
The LTC3736 uses a constant frequency, current mode
architecture with the two controllers operating 180 de-
grees out of phase. During normal operation, the top
external P-channel power MOSFET is turned on when the
clock for that channel sets the RS latch, and turned off
when the current comparator (I
CMP
) resets the latch. The
peak inductor current at which I
CMP
resets the RS latch is
determined by the voltage on the I
TH
pin, which is driven
by the output of the error amplifier (EAMP). The V
FB
pin
receives the output voltage feedback signal from an exter-
nal resistor divider. This feedback signal is compared to
the internal 0.6V reference voltage by the EAMP. When the
load current increases, it causes a slight decrease in V
FB
relative to the 0.6V reference, which in turn causes the I
TH
voltage to increase until the average inductor current
matches the new load current. While the top P-channel
MOSFET is off, the bottom N-channel MOSFET is turned
on until either the inductor current starts to reverse, as
indicated by the current reversal comparator, I
RCMP
, or the
beginning of the next cycle.
Shutdown, Soft-Start and Tracking Start-Up
(RUN/SS and TRACK Pins)
The LTC3736 is shut down by pulling the RUN/SS pin low.
In shutdown, all controller functions are disabled and the
chip draws only 9µA. The TG outputs are held high (off)
and the BG outputs low (off) in shutdown. Releasing
RUN/SS allows an internal 0.7µA current source to charge
up the RUN/SS pin. When the RUN/SS pin reaches 0.65V,
the LTC3736’s two controllers are enabled.
The start-up of V
OUT1
is controlled by the LTC3736’s
internal soft-start. During soft-start, the error amplifier
EAMP compares the feedback signal V
FB1
to the internal
soft-start ramp (instead of the 0.6V reference), which rises
linearly from 0V to 0.6V in about 1ms. This allows the
output voltage to rise smoothly from 0V to its final value,
while maintaining control of the inductor current.
The 1ms soft-start time can be increased by connecting
the optional external soft-start capacitor C
SS
between the
RUN/SS and SGND pins. As the RUN/SS pin continues to
OPERATIO
U
rise linearly from approximately 0.65V to 1.3V (being
charged by the internal 0.7µA current source), the EAMP
regulates the V
FB1
proportionally linearly from 0V to 0.6V.
The start-up of V
OUT2
is controlled by the voltage on the
TRACK pin. When the voltage on the TRACK pin is less
than the 0.6V internal reference, the LTC3736 regulates
the V
FB2
voltage to the TRACK pin instead of the 0.6V
reference. Typically, a resistor divider on V
OUT1
is con-
nected to the TRACK pin to allow the start-up of V
OUT2
to
“track” that of V
OUT1
. For one-to-one tracking during start-
up, the resistor divider would have the same values as the
divider on V
OUT2
that is connected to V
FB2
.
Light Load Operation (Burst Mode or Continuous
Conduction) (SYNC/FCB Pin)
The LTC3736 can be enabled to enter high efficiency Burst
Mode operation or forced continuous conduction mode at
low load currents. To select Burst Mode operation, tie the
SYNC/FCB pin to a DC voltage above 0.6V (e.g., V
IN
). To
select forced continuous operation, tie the SYNC/FCB to a
DC voltage below 0.6V (e.g., SGND). This 0.6V threshold
betweenBurst Modeoperation andforcedcontinuous mode
can be used in secondary winding regulation as described
in the Auxiliary Winding Control Using SYNC/FCB Pin dis-
cussion in the Applications Information section.
When a controller is in Burst Mode operation, the peak
current in the inductor is set to approximate one-fourth of
the maximum sense voltage even though the voltage on
the I
TH
pin indicates a lower value. If the average inductor
current is lower than the load current, the EAMP will
decrease the voltage on the I
TH
pin. When the I
TH
voltage
drops below 0.85V, the internal SLEEP signal goes high
and both external MOSFETs are turned off.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3736 draws.
The load current is supplied by the output capacitor. As
the output voltage decreases, the EAMP increases the I
TH
voltage. When the I
TH
voltage reaches 0.925V, the SLEEP
signal goes low and the controller resumes normal
operation by turning on the external P-channel MOSFET
on the next cycle of the internal oscillator.
(Refer to Functional Diagram)

11
LTC3736
3736fa
When a controller is enabled for Burst Mode operation, the
inductor current is not allowed to reverse. Hence, the
controller operates discontinuously. The reverse current
comparator (RICMP) senses the drain-to-source voltage
of the bottom external N-channel MOSFET. This MOSFET
is turned off just before the inductor current reaches zero,
preventing it from reversing and going negative.
In forced continuous operation, the inductor current is
allowed to reverse at light loads or under large transient
conditions. The peak inductor current is determined by the
voltage on the I
TH
pin. The P-channel MOSFET is turned on
every cycle (constant frequency) regardless of the I
TH
pin
voltage. In this mode, the efficiency at light loads is lower
than in Burst Mode operation. However, continuous mode
has the advantages of lower output ripple and less inter-
ference with audio circuitry.
When the SYNC/FCB pin is clocked by an external clock
source to use the phase-locked loop (see Frequency
Selection and Phase-Locked Loop), the LTC3736 operates
in PWM pulse skipping mode at light loads. In this mode,
the current comparator I
CMP
may remain tripped for
several cycles and force the external P-channel MOSFET to
stay off for the same number of cycles. The inductor
current is not allowed to reverse, though (discontinuous
operation). This mode, like forced continuous operation,
exhibits low output ripple as well as low audio noise and
reduced RF interference as compared to Burst Mode
operation. However, it provides low current efficiency
higher than forced continuous mode, but not nearly as
high as Burst Mode operation. During start-up or a short-
circuit condition (V
FB1
or V
FB2
≤0.54V), the LTC3736
operates in pulse skipping mode (no current reversal
allowed), regardless of the state of the SYNC/FCB pin.
Short-Circuit Protection
When an output is shorted to ground (V
FB
< 0.12V), the
switching frequency of that controller is reduced to 1/5 of
the normal operating frequency. The other controller is
unaffected and maintains normal operation.
The short-circuit threshold on V
FB2
is based on the smaller
of 0.12V and a fraction of the voltage on the TRACK pin.
This also allows V
OUT2
to start up and track V
OUT1
more
easily. Note that if V
OUT1
is truly short-circuited
OPERATIO
U
(Refer to Functional Diagram)
(V
OUT1
= V
FB1
= 0V), then the LTC3736 will try to regulate
V
OUT2
to 0V if a resistor divider on V
OUT1
is connected to
the TRACK pin.
Output Overvoltage Protection
As a further protection, the overvoltage comparator (OV)
guards against transient overshoots, as well as other more
serious conditions that may overvoltage the output. When
the feedback voltage on the V
FB
pin has risen 13.33%
above the reference voltage of 0.6V, the external P-chan-
nel MOSFET is turned off and the N-channel MOSFET is
turned on until the overvoltage is cleared.
Frequency Selection and Phase-Locked Loop
(PLLLPF and SYNC/FCB Pins)
The selection of switching frequency is a tradeoff between
efficiency and component size. Low frequency operation
increasesefficiencyby reducing MOSFETswitchinglosses,
but requires larger inductance and/or capacitance to main-
tain low output ripple voltage.
The switching frequency of the LTC3736’s controllers can
be selected using the PLLLPF pin.
If the SYNC/FCB is not being driven by an external clock
source, the PLLLPF can be floated, tied to V
IN
or tied to
SGND to select 550kHz, 750kHz or 300kHz respectively.
A phase-locked loop (PLL) is available on the LTC3736 to
synchronize the internal oscillator to an external clock
source that connected to the SYNC/FCB pin. In this case,
a series RC should be connected between the PLLLPF pin
and SGND to serve as the PLL’s loop filter. The LTC3736
phase detector adjusts the voltage on the PLLLPF pin to
align the turn-on of controller 1’s external P-channel
MOSFET to the rising edge of the synchronizing signal.
Thus, the turn-on of controller 2’s external P-channel
MOSFET is 180 degrees out of phase with the rising edge
of the external clock source.
The typical capture range of the LTC3736’s phase-locked
loop is from approximately 200kHz to 1MHz, and is
guaranteed over temperature to be between 250kHz and
850kHz. In other words, the LTC3736’s PLL is guaranteed
to lock to an external clock source whose frequency is
between 250kHz and 850kHz.

12
LTC3736
3736fa
OPERATIO
U
(Refer to Functional Diagram)
Dropout Operation
When the input supply voltage (V
IN
) decreases towards
the output voltage, the rate of change of the inductor
current while the external P-channel MOSFET is on (ON
cycle) decreases. This reduction means that the P-channel
MOSFET will remain on for more than one oscillator cycle
if the inductor current has not ramped up to the threshold
set by the EAMP on the I
TH
pin. Further reduction in the
input supply voltage will eventually cause the P-channel
MOSFET to be turned on 100%; i.e., DC. The output
voltage will then be determined by the input voltage minus
the voltage drop across the P-channel MOSFET and the
inductor.
Undervoltage Lockout
To prevent operation of the external MOSFETs below safe
inputvoltage levels,anundervoltage lockoutis incorporated
in the LTC3736. When the input supply voltage (V
IN
) drops
below 2.3V, the external P- and N-channel MOSFETs and
all internal circuitry are turned off except for the undervolt-
age block, which draws only a few microamperes.
Peak Current Sense Voltage Selection and Slope
Compensation (IPRG1 and IPRG2 Pins)
When a controller is operating below 20% duty cycle, the
peak current sense voltage (between the SENSE
+
and SW
pins) allowed across the external P-channel MOSFET is
determined by:
∆=
()
VAV V
SENSE MAX ITH ()
–.07
10
where A is a constant determined by the state of the IPRG
pins. Floating the IPRG pin selects A = 1; tying IPRG to V
IN
selects A = 5/3; tying IPRG to SGND selects A = 2/3. The
maximum value of V
ITH
is typically about 1.98V, so the
maximum sense voltage allowed across the external
P-channel MOSFET is 125mV, 85mV or 204mV for the
three respective states of the IPRG pin. The peak sense
voltages for the two controllers can be independently
selected by the IPRG1 and IPRG2 pins.
However, once the controller’s duty cycle exceeds 20%,
slope compensation begins and effectively reduces the
peak sense voltage by a scale factor given by the curve in
Figure 1.
The peak inductor current is determined by the peak sense
voltage and the on-resistance of the external P-channel
MOSFET:
IV
R
PK SENSE MAX
DS ON
=∆()
()
DUTY CYCLE (%)
10
SF = I/I
MAX
(%)
60
80
110
100
90
3736 F01
40
20
50
70
90
30
10
030 50 70
200 40 60 80 100
Figure 1. Maximum Peak Current vs Duty Cycle
Power Good (PGOOD) Pin
A window comparator monitors both feedback voltages
and the open-drain PGOOD output pin is pulled low when
either or both feedback voltages are not within ±10% of
the 0.6V reference voltage. PGOOD is low when the
LTC3736 is shut down or in undervoltage lockout.
2-Phase Operation
Why the need for 2-phase operation? Until recently, con-
stant frequency dual switching regulators operated both
controllers in phase (i.e., single phase operation). This
means that both topside MOSFETs (P-channel) are turned
on at the same time, causing current pulses of up to twice
the amplitude of those from a single regulator to be drawn
from the input capacitor. These large amplitude pulses
increase the total RMS current flowing in the input capaci-
tor, requiring the use of larger and more expensive input
capacitors, and increase both EMI and power losses in the
input capacitor and input power supply.

13
LTC3736
3736fa
With2-phaseoperation, the two controllersofthe LTC3736
are operated 180 degrees out of phase. This effectively
interleaves the current pulses coming from the topside
MOSFET switches, greatly reducing the time where they
overlap and add together. The result is a significant
reduction in the total RMS current, which in turn allows the
use of smaller, less expensive input capacitors, reduces
shielding requirements for EMI and improves real world
operating efficiency.
Figure 2 shows qualitatively example waveforms for a
single phase dual controller versus a 2-phase LTC3736
system. In this case, 2.5V and 1.8V outputs, each drawing
a load current of 2A, are derived from a 7V (e.g., a 2-cell
Li-Ion battery) input supply. In this example, 2-phase
operation would reduce the RMS input capacitor current
from 1.79A
RMS
to 0.91A
RMS
. While this is an impressive
reduction by itself, remember that power losses are pro-
portional to I
RMS2
, meaning that actual power wasted is
reduced by a factor of 3.86.
The reduced input ripple current also means that less
power is lost in the input power path, which could include
batteries, switches, trace/connector resistances, and pro-
tection circuitry. Improvements in both conducted and
radiated EMI also directly accrue as a result of the reduced
RMS input current and voltage. Significant cost and board
footprint savings are also realized by being able to use
smaller, less expensive, lower RMS current-rated input
capacitors.
Of course, the improvement afforded by 2-phase opera-
tion is a function of the relative duty cycles of the two
controllers, which in turn are dependent upon the input
supply voltage. Figure 3 depicts how the RMS input
current varies for single phase and 2-phase dual control-
lers with 2.5V and 1.8V outputs over a wide input voltage
range.
It can be readily seen that the advantages of 2-phase
operation are not limited to a narrow operating range, but
in fact extend over a wide region. A good rule of thumb for
most applications is that 2-phase operation will reduce the
input capacitor requirement to that for just one channel
operating at maximum current and 50% duty cycle.
OPERATIO
U
(Refer to Functional Diagram)
Figure 2. Example Waveforms for a Single Phase
Dual Controller vs the 2-Phase LTC3736
Single Phase
Dual Controller
2-Phase
Dual Controller
SW1 (V)
SW2 (V)
I
L1
I
L2
I
IN
3736 F02
INPUT VOLTAGE (V)
2
0
INPUT CAPACITOR RMS CURRENT
0.2
0.6
0.8
1.0
2.0
1.4
467
3736 F03
0.4
1.6
1.8
1.2
35 8910
SINGLE PHASE
DUAL CONTROLER
2-PHASE
DUAL CONTROLER
VOUT1 = 2.5V/2A
VOUT2 = 1.8V/2A
Figure 3. RMS Input Current Comparison

14
LTC3736
3736fa
The typical LTC3736 application circuit is shown in Fig-
ure 13. External component selection for each of the
LTC3736’s controllers is driven by the load requirement
and begins with the selection of the inductor (L) and the
power MOSFETs (MP and MN).
Power MOSFET Selection
Each of the LTC3736’s two controllers requires two exter-
nal power MOSFETs: a P-channel MOSFET for the topside
(main) switch and an N-channel MOSFET for the bottom
(synchronous)switch.Important parameters for thepower
MOSFETs are the breakdown voltage V
BR(DSS)
, threshold
voltage V
GS(TH)
, on-resistance R
DS(ON)
, reverse transfer
capacitance C
RSS
, turn-off delay t
D(OFF)
and the total gate
charge Q
G
.
The gate drive voltage is the input supply voltage. Since the
LTC3736 is designed for operation down to low input
voltages, a sublogic level MOSFET (R
DS(ON)
guaranteed at
V
GS
= 2.5V) is required for applications that work close to
this voltage. When these MOSFETs are used, make sure
that the input supply to the LTC3736 is less than the abso-
lute maximum MOSFET V
GS
rating, which is typically 8V.
The P-channel MOSFET’s on-resistance is chosen based
on the required load current. The maximum average
output load current I
OUT(MAX)
is equal to the peak inductor
current minus half the peak-to-peak ripple current I
RIPPLE
.
The LTC3736’s current comparator monitors the drain-to-
source voltage V
DS
of the P-channel MOSFET, which is
sensed between the SENSE
+
and SW pins. The peak
inductor current is limited by the current threshold, set by
the voltage on the I
TH
pin of the current comparator. The
voltage on the I
TH
pin is internally clamped, which limits
the maximum current sense threshold ∆V
SENSE(MAX)
to
approximately 128mV when IPRG is floating (86mV when
IPRG is tied low; 213mV when IPRG is tied high).
The output current that the LTC3736 can provide is given
by:
IV
R
I
OUT MAX SENSE MAX
DS ON
RIPPLE
() ()
()
–=∆
2
A reasonable starting point is setting ripple current I
RIPPLE
to be 40% of I
OUT(MAX)
. Rearranging the above equation
yields:
RV
I
DS ON MAX SENSE MAX
OUT MAX
()( ) ()
()
•=∆
5
6
for Duty Cycle < 20%.
However, for operation above 20% duty cycle, slope
compensation has to be taken into consideration to select
the appropriate value of R
DS(ON)
to provide the required
amount of load current:
RSF
V
I
DS ON MAX SENSE MAX
OUT MAX
()( ) ()
()
••=∆
5
6
where SF is a scale factor whose value is obtained from the
curve in Figure 1.
These must be further derated to take into account the
significant variation in on-resistance with temperature.
The following equation is a good guide for determining the
required R
DS(ON)MAX
at 25°C (manufacturer’s specifica-
tion), allowing some margin for variations in the LTC3736
and external component values:
RSF
V
I
DS ON MAX SENSE MAX
OUT MAX T
()( ) ()
()
•.• • • =∆
5
609 ρ
The ρ
T
is a normalizing term accounting for the tempera-
ture variation in on-resistance, which is typically about
0.4%/°C, as shown in Figure 4. Junction to case tempera-
ture T
JC
is about 10°C in most applications. For a maxi-
mum ambient temperature of 70°C, using ρ
80°C
~ 1.3 in
the above equation is a reasonable choice.
The power dissipated in the top and bottom MOSFETs
strongly depends on their respective duty cycles and load
current. When the LTC3736 is operating in continuous
mode, the duty cycles for the MOSFETs are:
Top P-Channel Duty Cycle = V
Bottom N-Channel Duty Cycle = V
OUT
IN
V
V
V
IN
OUT
IN
–
APPLICATIO S I FOR ATIO
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15
LTC3736
3736fa
The MOSFET power dissipations at maximum output
current are:
PV
VIRV
ICf
PVV
VIR
TOP OUT
IN OUT MAX T DS ON IN
OUT MAX RSS OSC
BOT IN OUT
IN OUT MAX T DS ON
=+
=
••• •
•••
–•••
() ()
()
() ()
22
2
2r
r
Both MOSFETs have I
2
R losses and the P
TOP
equation
includes an additional term for transition losses, which are
largest at high input voltages. The bottom MOSFET losses
are greatest at high input voltage or during a short circuit
when the bottom duty cycle is nearly 100%.
TheLTC3736utilizesa nonoverlapping, antishoot-through
gate drive control scheme to ensure that the P- and
N-channel MOSFETs are not turned on at the same time.
To function properly, the control scheme requires that the
MOSFETs used are intended for DC/DC switching applica-
tions. Many power MOSFETs, particularly P-channel
MOSFETs, are intended to be used as static switches and
therefore are slow to turn on or off.
Reasonable starting criteria for selecting the P-channel
MOSFET are that it must typically have a gate charge (Q
G
)
less than 25nC to 30nC (at 4.5V
GS
) and a turn-off delay
(t
D(OFF)
) of less than approximately 140ns. However, due
to differences in test and specification methods of various
MOSFET manufacturers, and in the variations in Q
G
and
t
D(OFF)
withgatedrive(V
IN
)voltage,theP-channel MOSFET
ultimately should be evaluated in the actual LTC3736
application circuit to ensure proper operation.
Shoot-through between the P-channel and N-channel
MOSFETs can most easily be spotted by monitoring the
input supply current. As the input supply voltage in-
creases, if the input supply current increases dramatically,
then the likely cause is shoot-through. Note that some
MOSFETs that do not work well at high input voltages (e.g.,
V
IN
> 5V) may work fine at lower voltages (e.g., 3.3V).
Table 1 shows a selection of P-channel MOSFETs from
different manufacturers that are known to work well in
LTC3736 applications.
Selecting the N-channel MOSFET is typically easier, since
for a given R
DS(ON)
, the gate charge and turn-on and turn-
off delays are much smaller than for a P-channel MOSFET.
Table 1. Selected P-Channel MOSFETs Suitable for LTC3736
Applications
PART
NUMBER MANUFACTURER TYPE PACKAGE
Si7540DP Siliconix Complementary PowerPak
P/N SO-8
Si9801DY Siliconix Complementary SO-8
P/N
FDW2520C Fairchild Complementary TSSOP-8
P/N
FDW2521C Fairchild Complementary TSSOP-8
P/N
Si3447BDV Siliconix Single P TSOP-6
Si9803DY Siliconix Single P SO-8
FDC602P Fairchild Single P TSOP-6
FDC606P Fairchild Single P TSOP-6
FDC638P Fairchild Single P TSOP-6
FDW2502P Fairchild Dual P TSSOP-8
FDS6875 Fairchild Dual P SO-8
HAT1054R Hitachi Dual P SO-8
NTMD6P02R2-D On Semi Dual P SO-8
Operating Frequency and Synchronization
The choice of operating frequency, f
OSC
, is a trade-off
between efficiency and component size. Low frequency
APPLICATIO S I FOR ATIO
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JUNCTION TEMPERATURE (°C)
–50
ρT
NORMALIZED ON RESISTANCE
1.0
1.5
150
3736 F04
0.5
0050 100
2.0
Figure 4. RDS(ON) vs Temperature

16
LTC3736
3736fa
operationimprovesefficiency by reducingMOSFET switch-
ing losses, both gate charge loss and transition loss.
However, lower frequency operation requires more induc-
tance for a given amount of ripple current.
The internal oscillator for each of the LTC3736’s control-
lers runs at a nominal 550kHz frequency when the PLLLPF
pin is left floating and the SYNC/FCB pin is a DC low or
high. Pulling the PLLLPF to V
IN
selects 750kHz operation;
pulling the PLLLPF to GND selects 300kHz operation.
Alternatively, the LTC3736 will phase-lock to a clock signal
applied to the SYNC/FCB pin with a frequency between
250kHz and 850kHz (see Phase-Locked Loop and Fre-
quency Synchronization).
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency f
OSC
directly determine the
inductor’s peak-to-peak ripple current:
IV
V
VV
fL
RIPPLE OUT
IN
IN OUT
OSC
=⎛
⎝
⎜⎞
⎠
⎟
–
•
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest efficiency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specified maximum,
the inductor should be chosen according to:
LVV
fI
V
V
IN OUT
OSC RIPPLE
OUT
IN
≥–
••
Burst Mode Operation Considerations
The choice of R
DS(ON)
and inductor value also determines
the load current at which the LTC3736 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
IV
R
BURST PEAK SENSE MAX
DS ON
() ()
()
•=∆
1
4
Thecorrespondingaverage current dependsonthe amount
of ripple current. Lower inductor values (higher I
RIPPLE
)
will reduce the load current at which Burst Mode operation
begins.
The ripple current is normally set so that the inductor
current is continuous during the burst periods. Therefore:
I
RIPPLE
≤I
BURST(PEAK)
This implies a minimum inductance of:
LVV
fI
V
V
MIN IN OUT
OSC BURST PEAK
OUT
IN
≤–
••
()
A smaller value than L
MIN
could be used in the circuit,
although the inductor current will not be continuous
during burst periods, which will result in slightly lower
efficiency. In general, though, it is a good idea to keep
I
RIPPLE
comparable to I
BURST(PEAK)
.
Inductor Core Selection
Once the inductance value is determined, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of ferrite, molyper-
malloy or other cores. Actual core loss is independent of
core size for a fixed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance re-
quires more turns of wire and therefore copper losses will
increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive
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17
LTC3736
3736fa
than ferrite. A reasonable compromise from the same
manufacturer is Kool Mµ. Toroids are very space efficient,
especially when you can use several layers of wire.
Because they lack a bobbin, mounting is more difficult.
However, designs for surface mount are available which
do not increase the height significantly.
Schottky Diode Selection (Optional)
The Schottky diodes D1 and D2 in Figure 16 conduct
current during the dead time between the conduction of
the power MOSFETs . This prevents the body diode of the
bottom N-channel MOSFET from turning on and storing
charge during the dead time, which could cost as much as
1% in efficiency. A 1A Schottky diode is generally a good
size for most LTC3736 applications, since it conducts a
relatively small average current. Larger diodes result in
additional transition losses due to their larger junction
capacitance. This diode may be omitted if the efficiency
loss can be tolerated.
C
IN
and C
OUT
Selection
The selection of C
IN
is simplified by the 2-phase architec-
ture and its impact on the worst-case RMS current drawn
through the input network (battery/fuse/capacitor). It can
be shown that the worst-case capacitor RMS current
occurs when only one controller is operating. The control-
ler with the highest (V
OUT
)(I
OUT
) product needs to be used
in the formula below to determine the maximum RMS
capacitor current requirement. Increasing the output cur-
rent drawn from the other controller will actually decrease
the input RMS ripple current from its maximum value. The
out-of-phase technique typically reduces the input
capacitor’s RMS ripple current by a factor of 30% to 70%
when compared to a single phase power supply solution.
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle (V
OUT
)/(V
IN
). To
prevent large voltage transients, a low ESR capacitor sized
for the maximum RMS current of one channel must be
used. The maximum RMS capacitor current is given by:
CI
VVVV
IN MAX
IN
OUT IN OUT
Required IRMS ≈
()( )
[]
–/12
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturers’
ripple current ratings are often based on only 2000 hours
of life. This makes it advisable to further derate the
capacitor, or to choose a capacitor rated at a higher
temperature than required. Several capacitors may be
paralleled to meet size or height requirements in the
design.Due to thehigh operating frequencyof the LTC3736,
ceramic capacitors can also be used for C
IN
. Always
consult the manufacturer if there is any question.
The benefit of the LTC3736 2-phase operation can be cal-
culated by using the equation above for the higher power
controller and then calculating the loss that would have
resulted if both controller channels switched on at the
same time. The total RMS power lost is lower when both
controllers are operating due to the reduced overlap of
current pulses required through the input capacitor’s ESR.
This is why the input capacitor’s requirement calculated
above for the worst-case controller is adequate for the
dual controller design. Also, the input protection fuse re-
sistance, battery resistance, and PC board trace resistance
losses are also reduced due to the reduced peak currents
in a 2-phase system. The overall benefit of a multiphase
design will only be fully realized when the source imped-
ance of the power supply/battery is included in the effi-
ciency testing. The sources of the P-channel MOSFETs
should be placed within 1cm of each other and share a
common C
IN
(s). Separating the sources and C
IN
may pro-
duce undesirable voltage and current resonances at V
IN
.
A small (0.1µF to 1µF) bypass capacitor between the chip
V
IN
pin and ground, placed close to the LTC3736, is also
suggested. A 10Ωresistor placed between C
IN
(C1) and
the V
IN
pin provides further isolation between the two
channels.
The selection of C
OUT
is driven by the effective series
resistance (ESR). Typically, once the ESR requirement is
satisfied, the capacitance is adequate for filtering. The
output ripple (∆V
OUT
) is approximated by:
∆≈ +
⎛
⎝
⎜
⎞
⎠
⎟
V I ESR fC
OUT RIPPLE
OUT
1
8
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Kool Mµis a registered trademark of Magnetics, Inc.

18
LTC3736
3736fa
function. This diode (and capacitor) can be deleted if the
external soft-start is not needed.
During soft-start, the start-up of V
OUT1
is controlled by
slowly ramping the positive reference to the error amplifier
from 0V to 0.6V, allowing V
OUT1
to rise smoothly from 0V
to its final value. The default internal soft-start time is 1ms.
This can be increased by placing a capacitor between the
RUN/SS pin and SGND. In this case, the soft-start time will
be approximately:
tC mV
A
SS SS1
600
07
=µ •.
Tracking
The start-up of V
OUT2
is controlled by the voltage on the
TRACK pin. Normally this pin is used to allow the start-up
of V
OUT2
to track that of V
OUT1
as shown qualitatively in
Figures 7a and 7b. When the voltage on the TRACK pin is
less than the internal 0.6V reference, the LTC3736 regu-
lates the V
FB2
voltage to the TRACK pin voltage instead of
0.6V. The start-up of V
OUT2
may ratiometrically track that
of V
OUT1
, according to a ratio set by a resistor divider
(Figure 7c):
V
V
RA
R
RR
RB RA
OUT
OUT TRACKA
TRACKA TRACKB1
2
2
22
=+
+
•
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3.3V OR 5V RUN/SS RUN/SS
C
SS
C
SS
D1
3736 F06
Figure 6. RUN/SS Pin Interfacing
where f is the operating frequency, C
OUT
is the output
capacitance and I
RIPPLE
is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since I
RIPPLE
increases with input voltage.
Setting Output Voltage
The LTC3736 output voltages are each set by an external
feedback resistor divider carefully placed across the out-
put, as shown in Figure 5. The regulated output voltage is
determined by:
VV
R
R
OUT B
A
=+
⎛
⎝
⎜
⎞
⎠
⎟
06 1.•
To improve the frequency response, a feed-forward ca-
pacitor, C
FF
, may be used. Great care should be taken to
route the V
FB
line away from noise sources, such as the
inductor or the SW line.
1/2 LTC3736
VFB
VOUT
RBCFF
RA
3736 F05
Figure 5. Setting Output Voltage
Run/Soft Start Function
The RUN/SS pin is a dual purpose pin that provides the
optional external soft-start function and a means to shut
down the LTC3736.
Pulling the RUN/SS pin below 0.65V puts the LTC3736
into a low quiescent current shutdown mode (I
Q
= 9µA). If
RUN/SS has been pulled all the way to ground, there will
be a delay before the LTC3736 comes out of shutdown and
is given by:
tV
C
AsFC
DELAY SS SS
=µ=µ065 07 093.•
../•
This pin can be driven directly from logic as shown in
Figure 6. Diode D1 in Figure 6 reduces the start delay but
allows C
SS
to ramp up slowly providing the soft-start
LTC3736
VFB2
VOUT2
VOUT1
VFB1
TRACK
R2B
R2A
3736 F07a
R1B
R1A
RTRACKA
RTRACKB
Figure 7a. Using the TRACK Pin

19
LTC3736
3736fa
For coincident tracking (V
OUT1
= V
OUT2
during start-up),
R2A = R
TRACKA
R2B = R
TRACKB
The ramp time for V
OUT2
to rise from 0V to its final value
is:
tt
R
RA
RA RB
RR
SS SS TRACKA
TRACKA TRACKB
211
11
=+
+
••
For coincident tracking,
tt
V
V
SS SS OUT F
OUT F
21 2
1
=•
where V
OUT1F
and V
OUT2F
are the final, regulated values of
V
OUT1
and V
OUT2
. V
OUT1
should always be greater than
V
OUT2
when using the TRACK pin. If no tracking function
is desired, then the TRACK pin may be tied to V
IN
. How-
ever, in this situation there would be no (internal nor
external) soft-start on V
OUT2
.
Phase-Locked Loop and Frequency Synchronization
The LTC3736 has a phase-locked loop (PLL) comprised
of an internal voltage-controlled oscillator (VCO) and a
phase detector. This allows the turn-on of the external P-
channel MOSFET of controller 1 to be locked to the rising
edge of an external clock signal applied to the SYNC/FCB
pin. The turn-on of controller 2’s external P-channel
MOSFET is thus 180 degrees out of phase with the
external clock. The phase detector is an edge sensitive
digital type that provides zero degrees phase shift
between the external and internal oscillators. This type of
phase detector does not exhibit false lock to harmonics of
the external clock.
The output of the phase detector is a pair of complemen-
tary current sources that charge or discharge the external
filter network connected to the PLLLPF pin. The relation-
ship between the voltage on the PLLLPF pin and operating
frequency, when there is a clock signal applied to SYNC/
FCB, is shown in Figure 8 and specified in the Electrical
Characteristics table. Note that the LTC3736 can only be
synchronized to an external clock whose frequency is
within range of the LTC3736’s internal VCO, which is
nominally 200kHz to 1MHz. This is guaranteed, over
temperature and variations, to be between 300kHz and
750kHz. A simplified block diagram is shown in Figure 9.
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TIME
(7b) Coincident Tracking
VOUT1
VOUT2
OUTPUT VOLTAGE
TIME
3736 F07b,c
(7c) Ratiometric Tracking
VOUT1
VOUT2
OUTPUT VOLTAGE
Figures 7b and 7c. Two Different Modes of Output Voltage Tracking
PLLLPF PIN VOLTAGE (V)
0
0
FREQUENCY (kHz)
0.5 1 1.5 2
3736 F08
2.4
200
400
600
800
1000
1200
1400
Figure 8. Relationship Between Oscillator Frequency and Voltage
at the PLLLPF Pin When Synchronizing to an External Clock

20
LTC3736
3736fa
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If the external clock frequency is greater than the internal
oscillator’s frequency, f
OSC
, then current is sourced con-
tinuously from the phase detector output, pulling up the
PLLLPF pin. When the external clock frequency is less than
f
OSC
,current is sunkcontinuously, pulling downthe PLLLPF
pin. If the external and internal frequencies are the same
but exhibit a phase difference, the current sources turn on
for an amount of time corresponding to the phase differ-
ence. The voltage on the PLLLPF pin is adjusted until the
phase and frequency of the internal and external oscilla-
tors are identical. At the stable operating point, the phase
detector output is high impedance and the filter capacitor
C
LP
holds the voltage.
The loop filter components, C
LP
and R
LP
, smooth out the
current pulses from the phase detector and provide a
stable input to the voltage-controlled oscillator. The filter
components C
LP
and R
LP
determine how fast the loop
acquires lock. Typically R
LP
= 10k and C
LP
is 2200pF to
0.01µF.
Typically, the external clock (on SYNC/FCB pin) input high
level is 1.6V, while the input low level is 1.2V.
Table 2 summarizes the different states in which the
PLLLPF pin can be used.
Table 2
PLLLPF PIN SYNC/FCB PIN FREQUENCY
0V DC Voltage 300kHz
Floating DC Voltage 550kHz
V
IN
DC Voltage 750kHz
RC Loop Filter Clock Signal Phase-Locked to External Clock
Auxiliary Winding Control Using SYNC/FCB Pin
The SYNC/FCB can be used as an auxiliary feedback to
provide a means of regulating a flyback winding output.
When this pin drops below its ground-referenced 0.6V
threshold, continuous mode operation is forced.
During continuous mode, current flows continuously in
the transformer primary. The auxiliary winding draws
current only when the bottom, synchronous N-channel
MOSFET is on. When primary load currents are low and/or
the V
IN
/V
OUT
ratio is close to unity, the synchronous
MOSFET may not be on for a sufficient amount of time to
transfer power from the output capacitor to the auxiliary
load. Forced continuous operation will support an auxil-
iary winding as long as there is a sufficient synchronous
MOSFET duty factor. The FCB input pin removes the
requirement that power must be drawn from the trans-
former primary in order to extract power from the auxiliary
winding. With the loop in continuous mode, the auxiliary
output may nominally be loaded without regard to the
primary output load.
The auxiliary output voltage V
AUX
is normally set as shown
in Figure 10 by the turns ratio N of the transformer:
V
AUX
≅(N + 1) V
OUT
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then V
AUX
will droop. An external resistor divider from
V
AUX
to the FCB sets a minimum voltage V
AUX(MIN)
:
VV
R
R
AUX MIN()
.=+
⎛
⎝
⎜
⎞
⎠
⎟
06 1 6
5
DIGITAL
PHASE/
FREQUENCY
DETECTOR OSCILLATOR
2.4V
R
LP
C
LP
3736 F09
PLLLPF
EXTERNAL
OSCILLATOR
SYNC/
FCB
Figure 9. Phase-Locked Loop Block Diagram