Linear Technology LT3591 User manual

LT3591
1
3591f
■Cellular Phones
■PDAs, Handheld Computers
■Digital Cameras
■MP3 Players
■GPS Receivers
TYPICAL APPLICATION
FEATURES
APPLICATIONS
DESCRIPTION
White LED Driver with
Integrated Schottky in
3mm ×2mm DFN
The LT
®
3591 is a fixed frequency step-up DC/DC converter
specifically designed to drive up to ten white LEDs in series
from a Li-Ion cell. Series connection of the LEDs provides
identical LED currents resulting in uniform brightness
and eliminating the need for ballast resistors. The device
features a unique high side LED current sense that enables
the part to function as a “one wire current source;” one
side of the LED string can be returned to ground anywhere,
allowing a simpler one wire LED connection. Traditional
LED drivers use a grounded resistor to sense LED current,
requiring a 2-wire connection to the LED string.
The high switching frequency allows the use of tiny induc-
tors and capacitors. A single pin performs both shutdown
and accurate LED dimming control. Few external compo-
nents are needed: open-LED protection and the Schottky
diode are all contained inside a low profile 3mm ×2mm
DFN package.
Li-Ion Driver for Ten White LEDs
■Drives Up to Ten White LEDs from a 3V Supply
■High Side Sense Allows “One Wire Current Source”
■Internal Schottky Diode
■One Pin Dimming and Shutdown
■80:1 True Color PWM
TM
Dimming Range
■42V Open LED Protection
■1MHz Switching Frequency
■±5% Reference Accuracy
■V
IN Range: 2.5V to 12V
■Requires Only 2.2µF Output Capacitor
■Low Profile 8-Lead DFN Package
(3mm ×2mm ×0.75mm)
Conversion Efficiency
CTRL
VIN
22µH
VIN
3V TO 5V
RSENSE
10Ω
3591 TA01a
LT3591
SHUTDOWN AND
DIMMING CONTROL
SW
CAP
LED
GND 2.2µF
1µF
LED CURRENT (mA)
0
EFFICIENCY (%)
60
20
3591 TA01b
510 15
80
70
55
65
75
VIN = 3.6V
10 LEDs
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are
the property of their respective owners.

LT3591
2
3591f
PACKAGE/ORDER INFORMATION
ELECTRICAL CHARACTERISTICS
ABSOLUTE MAXIMUM RATINGS
Input Voltage (VIN)................................................... 12V
CTRL Voltage ........................................................... 12V
SW Voltage .............................................................. 45V
CAP Voltage ............................................................. 45V
LED Voltage ............................................................. 45V
Operating Junction Temperature Range
(Note 2) ...............................................–40°C to 85°C
Maximum Junction Temperature ........................ 125°C
Storage Temperature Range...................–65°C to 150°C
(Note 1)
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Operating Voltage 2.5 V
LED Current Sense Voltage (VCAP – VLED)V
CAP = 24V, ISW = 300mA ●190 200 210 mV
CAP Pin Bias Current VCAP = 36V, VLED = 35.8V 40 80 µA
LED Pin Bias Current VCAP = 36V, VLED = 35.8V 20 40 µA
Supply Current VCAP = 24V, VLED = 23V
CTRL = 0V
4
9
5
11
mA
µA
Switching Frequency 0.75 1 1.2 MHz
Maximum Duty Cycle 92 94 %
Switch Current Limit ●500 800 mA
Switch VCESAT ISW = 300mA 200 mV
Switch Leakage Current VSW = 24V 0.1 5 µA
VCTRL for Full LED Current VCAP = 44V ●1.5 V
VCTRL to Shut Down IC 50 mV
VCTRL to Turn On IC ●100 mV
CTRL Pin Bias Current 100 nA
CAP Pin Overvoltage Protection ●40 42 44 V
Schottky Forward Drop ISCHOTTKY = 200mA 0.8 V
Schottky Leakage Current VR= 30V 4 µA
TOP VIEW
9
DDB PACKAGE
8-LEAD (3mm ×2mm) PLASTIC DFN
5
6
7
8
4
3
2
1VIN
GND
NC
SW
CTRL
LED
NC
CAP
TJMAX = 125°C, θJA = 76°C/W
EXPOSED PAD (PIN 9) SHOULD BE CONNECTED TO PCB GROUND
ORDER PART NUMBER DDB PART MARKING
LT3591EDDB LCPG
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The ●denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA= 25°C. VIN = 3V, VCTRL = 3V, unless otherwise specified.
Note 2: The LT3591E is guaranteed to meet performance specifications
from 0°C to 85°C operating junction temperature range. Specifications
over the –40°C to 85°C operating junction temperature range are assured
by design, characterization and correlation with statistical process controls.

LT3591
3
3591f
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Saturation Voltage
(VCESAT)
Schottky Forward Voltage Drop Shutdown Current (VCTRL = 0V)
Sense Voltage (VCAP – VLED)
vs VCTRL
Open-Circuit Output
Clamp Voltage
Input Current in Output
Open Circuit
Switching Waveform Transient Response
SWITCH CURRENT (mA)
0
500
400
300
200
100
0300 600
3591 G01
100 200 400 800700
500
SWITCH SATURATION VOLTAGE (mV)
–50°C
125°C
25°C
SCHOTTKY FORWARD DROP (mV)
SCHOTTKY FORWARD CURRENT (mA)
200
300
3591 G02
100
0
600
500
400
0 200 400 600 800 1000 1200
–50°C
125°C
25°C
VIN (V)
0
SHUTDOWN CURRENT (µA)
9
12
15
12
3591 G03
6
3
0369
–50°C
125°C
25°C
VCTRL (mV)
SENSE VOLTAGE (mV)
80
120
3591 G04
40
0
240
200
160
0 500 1000 1500 2000 2500 3000
–50°C
25°C
125°C
VIN (V)
0
OUTPUT CLAMP VOLTAGE (V)
42
43
12
3591 G05
41
40 369
45
44
–50°C
125°C
25°C
VIN (V)
0
INPUT CURRENT (mA)
36 912
3591 G06
–50°C
125°C
4
5
6
3
2
0
1
8
7
25°C
VSW
20V/DIV
VCAP
50mV/DIV
IL
200mA/DIV
VIN = 3.6V
FRONT PAGE
APPLICATION CIRCUIT
500ms/DIV 3591 G07
VCTRL
5V/DIV
VCAP
5V/DIV
IL
500mA/DIV
VIN = 3.6V
FRONT PAGE
APPLICATION CIRCUIT
1ms/DIV 3591 G08
TA= 25°C, unless otherwise specified.

LT3591
4
3591f
VIN (V)
0
0
QUIESCENT CURRENT (mA)
1
2
3
4
5
6
36 912
3591 G09
–50°C
125°C
25°C
TEMPERATURE (°C)
0
CURRENT LIMIT (mA)
400
1000
3591 G10
200
600
800
–50 –25 0 25 50 75 100 125 –50 25 75
–25 0 50 100 125
TEMPERATURE (°C)
0
SCHOTTKY LEAKAGE CURRENT (µA)
3
6
9
12
15
3591 G11
VR= 10V
VR= 16V
VR= 20V
TEMPERATURE (°C)
–50 –25
40
OUTPUT CLAMP VOLTAGE (V)
42
45
050 75
3591 G12
41
44
43
25 100 125
TEMPERATURE (°C)
–50
INPUT CURRENT (mA)
25 75
3591 G13
–25 0 50 100 125
VIN = 3V
4
5
6
3
2
0
1
8
7
TEMPERATURE (°C)
–50
SWITCHING FREQUENCY (kHz)
25 75
3591 G14
–25 0 50 100 125
1000
1050
1100
950
900
750
800
850
1200
1150
TEMPERATURE (°C)
–50
MAXIMUM DUTY CYCLE (%)
25 75
3591 G15
–25 0 50 100 125
98
96
94
90
92
100
VCAP (V)
5
SENSE VOLTAGE (mV)
20 30
3591 G16
10 15 25 35
204
200
196
188
192
208
–50°C
125°C
25°C
TEMPERATURE (°C)
–50 –25
188
SENSE VOLTAGE (mV)
196
208
050 75
3591 G17
192
204
200
25 100 125
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current
(VCTRL = 3V) Current Limit vs Temperature
Schottky Leakage Current
vs Temperature
Open-Circuit Output Clamp
Voltage vs Temperature
Input Current in Output Open
Circuit vs Temperature
Switching Frequency
vs Temperature
Maximum Duty Cycle
vs Temperature
Sense Voltage (VCAP – VLED)
vs VCAP
Sense Voltage (VCAP – VLED)
vs Temperature
TA= 25°C, unless otherwise specified.

LT3591
5
3591f
BLOCK DIAGRAM
PIN FUNCTIONS
VIN (Pin 1): Input Supply Pin. Must be locally bypassed.
GND (Pin 2): Ground Pin. Should be tied directly to local
ground plane.
SW (Pin 4): Switch Pin. Minimize trace area at this pin to
minimize EMI. Connect the inductor at this pin.
CAP (Pin 5): Output of the Driver. This pin is connected
to the cathode of internal Schottky. Connect the output
capacitor to this pin and the sense resistor from this pin
to the LED pin.
LED (Pin 7): Connection Point for the Anode of the First
LED and the Sense Resistor. The LED current can be
programmed by :
ImV
R
LED SENSE
=200
CTRL (Pin 8): Dimming and Shutdown Pin. Connect this
pin below 50mV to disable the driver. As the pin voltage
is ramped from 0V to 1.5V, the LED current ramps from
0 to ILED ( = 200mV/RSENSE). The CTRL pin must not be
left floating.
Exposed Pad (Pin 9): Ground. The Exposed Pad must
be soldered to PCB ground to achieve the rated thermal
performance.
4
–
+
–
+
R
SW
CAP
1
VIN
Q
S
DRIVER
Σ
OVERVOLTAGE
PROTECTION
RAMP
GENERATOR
OSCILLATOR
VREF
1.25V
START-UP
CONTROL
SHDN
Q1
R
–
+
+
5
LED
3591 F01
CTRL GND
7
82
–
+
A = 6.25
A1
RC
CC
A2
A3
PWM
COMP
Figure 1. Block Diagram

LT3591
6
3591f
Figure 2. Switching Waveforms
OPERATION
The LT3591 uses a constant frequency, current mode
control scheme to provide excellent line and load regula-
tion. Operation can be best understood by referring to the
Block Diagram in Figure 1.
At power-up, the capacitor at the CAP pin is charged up
to VIN (input supply voltage) through the inductor and
the internal Schottky diode. If CTRL is pulled higher than
100mV, the bandgap reference, the start-up bias and the
oscillator are turned on. At the start of each oscillator cycle,
the power switch Q1 is turned on. A voltage proportional
to the switch current is added to a stabilizing ramp and the
resulting sum is fed into the positive terminal of the PWM
comparator, A2. When this voltage exceeds the level at the
negative input of A2, the PWM logic turns off the power
switch. The level at the negative input of A2 is set by the
error amplifier A1, and is simply an amplified version of
the difference between the VCAP and VLED voltage and the
bandgap reference. In this manner the error amplifier, A1,
sets the correct peak current level in inductor L1 to keep
the output in regulation. The CTRL pin is used to adjust
the LED current. The LT3591 enters into shutdown when
CTRL is pulled lower than 50mV.
Minimum Output Current
The LT3591 can drive a 2-LED string at 2mA LED current
without pulse skipping using the same external compo-
nents shown in the application circuit on the front page of
this data sheet. As current is further reduced, the device
will begin skipping pulses. This will result in some low
frequency ripple, althoughthe average LED currentremains
regulated down to zero. The photo in Figure 2 details circuit
operation driving two white LEDs at 2mA load. Peak induc-
tor current is less than 40mA and the regulator operates
in discontinuous mode, meaning the inductor current
reacheszeroduring the discharge phase. After the inductor
current reaches zero, the SW pin exhibits ringing due to
the LC tank circuit formed by the inductor in combination
with the switch and the diode capacitance. This ringing is
not harmful; far less spectral energy is contained in the
ringing than in the switch transitions.
VSW
5V/DIV
IL
20mA/DIV
VIN = 4.2V
ILED = 2mA
2 LEDs
500ns/DIV 3591 F02

LT3591
7
3591f
INDUCTOR SELECTION
A 22µH inductor is recommended for most LT3591 ap-
plications. Although small size and high efficiency are
major concerns, the inductor should have low core losses
at 1MHz and low DCR (copper wire resistance). Some
small inductors in this category are listed in Table 1. The
efficiency comparison of different inductors is shown in
Figure 3.
Table 1. Recommended Inductors
PART
L
(µH)
CURRENT
RATING
(mA)
MAX
DIMENSION
L × W × H
(mm) VENDOR
VLF4012AT-
220MR51
22 510 4× 3.8 × 1.2 TDK
www.tdk.com
VLCF4018T-
220MR49-2
22 490 4.1 × 4.1 × 1.8
VLCF4020T-
220MR56
22 560 4.1 × 4.1 × 2
LQH43CN220K03 22 420 4.8 × 3.4 × 2.8 Murata
www.murata.com
NR4018T220M 22 590 4.2 × 4.2 × 1.8 Taiyo Yuden
www.t-yuden.com
NR4012T220M 22 510 4.2 × 4.2 × 1.2
CDRH3D18-
220NC
22 600 4 × 4× 2Sumida
www.sumida.com
B82470-A1223-M 22 480 4.8 × 4.8 × 1.2 Epcos
www.epcos.com
APPLICATIONS INFORMATION
CAPACITOR SELECTION
The small size of ceramic capacitors make them ideal for
LT3591 applications. Use only X5R and X7R types because
they retain their capacitance over wider temperature
ranges than other types such as Y5V or Z5U. A 1µF input
capacitor and a 50V, 2.2µF output capacitor are sufficient
for most applications.
A limited number of manufacturers produce small 50V
capacitors. Table 2 shows a list of several recommended
50V capacitors. Consult the manufacturer for detailed
information on their entire selection of ceramic parts.
Table 2. Recommended Output Capacitors
PART
C
(µF)
VOLTAGE CASE SIZE
VENDORTEMP.
HEIGHT
(mm)
GRM21BR71H105KA12L 1 50V 0805 Murata
www.murata.com
X7R 1.25 ± 0.15
GRM31MR71H105KA88 1 50V 1206
X7R 1.15 ± 0.1
GRM31CR71H225KA88 2.2 50V 1206
X7R 1.6 ± 0.2
GRM31CR71H475KA12L 4.7 50V 1206
X7R 1.6 ± 0.2
UMK316BJ475KL-T 4.7 50V 1206 Taiyo Yuden
www.t-yuden.com
X7R 1.6 ± 0.2
Figure 3. Efficiency Comparison of Different Inductors
LED CURRENT (mA)
0
50
EFFICIENCY (%)
55
85
80
65
510
3591 F03
70
75
60
15 20
TAIYO YUDEN NR4018T220M
TDK VLCF4018T-220MR49-2
TAIYO YUDEN NR4012T220M
TDKVLCF4012AT-220MR51
MURATA LQH43CN220K03
TDK VLCF4020T-220MR56
SUMIDA CDRH3D18-220NC
EPCOS B82470-A1223-M
VIN = 3.6V
10 LEDs

LT3591
8
3591f
APPLICATIONS INFORMATION
SCHOTTKY DIODE
The LT3591 has a built-in Schottky diode. The internal
schottky saves board space in space constrained appli-
cations. In less space sensitive applications, an external
schottky diode connected between the SW node and the
CAP node increases efficiency one to two percent. It is
important to use a properly rated schottky diode that can
handle the peak switch current of the LT3591. In addition,
the schottkydiode must havea breakdownvoltage of atleast
40V along with a low forward voltage in order to achieve
higher efficiency. One recommended external schottky
diode for the LT3591 is the Phillips PMEG4005AEA.
OVERVOLTAGE PROTECTION
The LT3591 has an internal open-circuit protection circuit.
In the cases of output open circuit, when the LEDs are
disconnected from the circuit or the LEDs fail open circuit,
VCAP is clamped at 42V (typ). The LT3591 will then switch
at a very low frequency to minimize input current. The VCAP
and input current during output open circuit are shown in
the Typical Performance Characteristics. Figure 4 shows
the transient response when the LEDs are disconnected.
For low DCR inductors, which is usually the case for this
application, the peak inrush current can be simplified as
follows:
IV
L
r
L
LC
PK IN
=⎛
⎝
⎜⎞
⎠
⎟
=
=
–.
•• exp – •
•
•–
06
2
2
1
ω
α
ω
π
α
ωrr
L
2
2
4•
where L is the inductance, r is the DCR of the inductor
and C is the output capacitance.
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Currents
VIN (V) r (Ω)L(µH) COUT (µF) IP(A)
4.2 0.3 22 2.2 1.06
4.2 0.71 22 2.2 0.96
4.2 0.58 15 1 0.83
4.2 1.6 15 1 0.68
PROGRAMMING LED CURRENT
The feedback resistor (RSENSE) and the sense voltage
(VCAP – VLED) control the LED current.
The CTRL pin controls the sense reference voltage as
shown in the Typical Performance Characteristics. For
CTRL higher than 1.5V, the sense reference is 200mV,
which results in full LED current. In order to have accurate
LED current, precision resistors are preferred (1% is rec-
ommended). The formula and table for RSENSE selection
are shown below.
RmV
I
SENSE LED
=200
VCAP
20V/DIV
IL
500mA/DIV
VIN = 3.6V
CIRCUIT OF
FRONT PAGE
APPLICATION
LEDs DISCONNECTED
AT THIS INSTANT
500µs/DIV 3591 F04
Figure 4. Output Open-Circuit Waveform
INRUSH CURRENT
The LT3591 has a built-in Schottky diode. When supply
voltage is applied to the VIN pin, an inrush current flows
through the inductor and the Schottky diode and charges
up the CAP voltage. The Schottky diode inside the LT3591
can sustain a maximum current of 1A.

LT3591
9
3591f
APPLICATIONS INFORMATION
Table 4. RSENSE Value Selection for 200mV Sense
ILED (mA) RSENSE (Ω)
540
10 20
15 13.3
20 10
DIMMING CONTROL
There are three different types of dimming control circuits.
The LED current can be set by modulating the CTRL pin
with a DC voltage, a filtered PWM signal or directly with
a PWM signal.
Using a DC Voltage
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
The CTRL pin voltage can be modulated to set the dim-
ming of the LED string. As the voltage on the CTRL pin
increases from 0V to 1.5V, the LED current increases from
0 to ILED. As the CTRL pin voltage increases beyond 1.5V,
it has no effect on the LED current.
The LED current can be set by:
ImV
Rwhen V V
IV
LED SENSE CTRL
LED CTRL
≈>
≈
200 15
6
,.
.225 125
•,. Rwhen V V
SENSE CTRL <
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
Using a Filtered PWM Signal
A filtered PWM signal can be used to control the
brightness of the LED string. The PWM signal is filtered
(Figure 5) by a RC network and fed to the CTRL pin.
The corner frequency of R1, C1 should be much lower
than the frequency of the PWM signal. R1 needs to be
much smaller than the internal impedance of the CTRL
pin which is 10MΩ(typ).
Direct PWM Dimming
Changing the forward current flowing in the LEDs not only
changes the intensity of the LEDs, it also changes the color.
The chromaticity of the LEDs changes with the change in
forward current. Many applications cannot tolerate any
shift in the color of the LEDs. Controlling the intensity of
the LEDs with a direct PWM signal allows dimming of the
LEDs without changing the color. In addition, direct PWM
dimming offers a wider dimming range to the user.
Dimming the LEDs via a PWM signal essentially involves
turning the LEDs on and off at the PWM frequency. The
typical human eye has a limit of ~60 frames per second.
By increasing the PWM frequency to ~80Hz or higher,
the eye will interpret that the pulsed light source is con-
tinuously on. Additionally, by modulating the duty cycle
(amount of “on-time”), the intensity of the LEDs can be
controlled. The color of the LEDs remains unchanged in
this scheme since the LED current value is either zero or
a constant value.
Figure 6 shows a Li-Ion powered driver for ten white LEDs.
Direct PWM dimming method requires an external NMOS
tied between the cathode of the lowest LED in the string
LT3591
CTRL
C1
0.1µF
PWM
10kHz TYP
3591 F05
R1
100k
Figure 5. Dimming Control Using a Filtered PWM Signal
CTRL
PWM
FREQ
VIN
L1
22µH
VIN
3V TO
5V
RSENSE
10Ω
3591 F06
LT3591
SW
CAP
LED
100k
GND
0V
5V
C2
2.2µF
Q1
Si2308
C1
1µF
Figure 6. Li-Ion to Ten White LEDs with Direct PWM Dimming

LT3591
10
3591f
and ground as shown in Figure 6. A Si2308 MOSFET can
be used since its source is connected to ground. The PWM
signal is applied to the CTRL pin of the LT3591 and the gate
of the MOSFET. The PWM signal should traverse between
0V to 5V, to ensure proper turn on and off of the driver
and the NMOS transistor Q1. When the PWM signal goes
high, the LEDs are connected to ground and a current of
ILED = 200mV/RSENSE flows through the LEDs. When the
PWM signal goes low, the LEDs are disconnected and
turn off. The MOSFET ensures that the LEDs quickly turn
off without discharging the output capacitor which in turn
allows the LEDs to turn on faster. Figure 7 shows the PWM
dimming waveforms for the circuit in Figure 6.
APPLICATIONS INFORMATION
The calculations show that for a 100Hz signal the dimming
range is 83 to 1. In addition, the minimum PWM duty cycle
of 1.2% ensures that the LED current has enough time
to settle to its final value. Figure 8 shows the dimming
range achievable for different frequencies with a settling
time of 120µs.
IL
500mA/DIV
ILED
20mA/DIV
PWM
5V/DIV
VIN = 3.6V
10 LEDs
2ms/DIV 3591 F07
Figure 7. Direct PWM Dimming Waveforms
The time it takes for the LED current to reach its pro-
grammed value sets the achievable dimming range for a
given PWM frequency. For example, the settling time of
the LED current in Figure 7 is approximately 120µs for a
3.6V input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
Example:
ƒ
ƒ
==
== =
100 120
11
100 001
Hz t µs
ts
SETTLE
PERIOD
,
.
DDim Range t
t
s
µs
Mi
PERIOD
SETTLE
.:=== 001
120 83 1
nn Duty Cycle t
t
µs
s
SETTLE
PERIOD
•. ==100 120
001 ••.%
%.%
100 1 2
100 1 2 100
=
=→Duty Cycle Range at HHz
PWM DIMMING FREQUENCY (Hz)
10
PWM DIMMING RANGE
100
1000
10000
100 1000 10000
3591 F08
1
10
PULSING MAY BE VISIBLE
Figure 8. Dimming Range vs Frequency
In addition to extendingthe dimming range, PWMdimming
improves the efficiency of the converter for LED currents
below 20mA. Figure 9 shows the efficiency for traditional
analog dimming of the front page application and PWM
dimming of the application in Figure 6.
Figure 9. PWM vs Analog Dimming Efficiency
LED CURRENT (mA)
0
EFFICIENCY (%)
60
65
70
20
3591 F09
55 510 15
80
75
PWM DIMMING
ANALOG DIMMING
VIN = 3.6V
10 LEDs