Linear Technology LT1952-1 User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F
ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER
1
LT1952-1
DESCRIPTION
Demonstration circuit 1317A-F is isolated input to
high current output 1/8th Brick footprint converter
featuring the LT®1952 switching controller with Ac-
tive Reset circuit. The Active Reset circuit can im-
prove the efficiency in wide input voltage applica-
tions. Also, the Active Reset allows the implementa-
tion of self-driven synchronous secondary rectifiers
in some applications.
The DC1317A-F converts isolated 9V to 36V input to
3.3V output and provides over 22A of output current
depending on cooling. When determining the cooling
requirements the actual input voltage range and con-
tinuous maximum output current must be taken into
account. The converter operates at 200kHz with the
peak efficiency greater than 92%. The DC1317 can be
easily modified to generate output voltages in the
range from 0.6V to 48V. The output currents are lim-
ited by total output power of up to 150W.
The available versions of DC1317A are:
DC1317A-A, 34-75Vin to 3.3V, 35A
DC1317A-B, 18-72Vin to 5V, 25A
DC1317A-C, 18-72Vin to 12V, 8A-12A
DC1317A-D, 18-72Vin to 24V, 5A
DC1317A-E, 36-72Vin to 5V, 12A
DC1317A-F, 9-36Vin to 3.3V, 20A
DC1317A-G, 9-36Vin to 12V, 5A
DC1317A-H, 9-36Vin to 48V, 1.5A
The DC1317 circuit features soft-start which prevents
output voltage overshoot on startup or when recover-
ing from overload condition.
The DC1317 has precise over-current protection cir-
cuit that allows for continuous operation under short
circuit conditions. The low power dissipation under
short circuit conditions insures high reliability even
during short circuits.
The LT1952 can be synchronized to an external clock
of up to 400kHz. Please refer to LT1952 data sheet for
design details and applications information.
Design files for this circuit board are available. Call
the LTC factory.
LT is a trademark of Linear Technology Corporation
Table 1. Performance Summary
PARAMETER CONDITION VALUE
Minimum Input Voltage IOUT = 0A to 22A 9V
Maximum Input Voltage IOUT = 0A to 22A 36V
VOUT VIN = 9V to 36V, IOUT = 0A to 22A
(27Amax)
3.3V ±3%
Typical Output Ripple VOUT VIN = 9V to 36V, IOUT = 0A to 22A 50mVP–P
Nominal Switching Frequency 200kHz

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F
ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER
2
QUICK START PROCEDURE
Demonstration circuit 1317 is easy to set up to evalu-
ate the performance of LT1952-1 circuit. Refer to Fig-
ure 1 for proper measurement equipment setup and
follow the procedure below:
NOTE: When measuring the input or output voltage
ripple, care must be taken to avoid a long ground lead
on the oscilloscope probe. Measure the input or out-
put voltage ripple by touching the probe tip directly
across the Vin or Vout and GND terminals. See Figure
2. for proper scope probe technique.
1. With power off, connect the input power supply to
Vin and GND. Make sure that the input power sup-
ply has sufficient current rating at minimum input
voltage for the required output load.
2. Turn on the power at the input.
NOTE: Make sure that the input voltage does not
exceed 36V.
3. Check for the proper output voltage.
Vout = 3.3V.
If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
4. Once the proper output voltage is established, ad-
just the load within the operating range and ob-
serve the output voltage regulation, ripple voltage,
efficiency and other parameters.
5. The DC1317 is equipped with an output capacitor
CSYS (470uF) that approximates typical system
rail capacitance. If system board already has ca-
pacitance of similar value CSYS can be removed.
Figure 1. Proper Measurement Equipment Setup

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F
ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER
3
Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
ACTIVE RESET CIRCUIT
The Active Reset circuit on DC1317A-F demo board
consists of a small P-Channel MOSFET Q13 and
reset capacitor C25. The MOSFET Q13 is used to
connect the reset capacitor across the transformer
T1 primary winding during the reset period when
Q1 MOSFET is off. The voltage across capacitor
C25 automatically adjusts with the duty cycle to
provide complete transformer reset under all oper-
ating conditions.
Also the active reset circuit shapes the reset voltage
into a square waveform that results in lower drain
voltages for Q1 and Q2 MOSFETs. The lower
MOSFET drain voltages allow lower voltage and
lower Rdson MOSFETs to be used. The MOSFETs
must be avalanche rated for the peak reset voltage.
If non-avalanche rated MOSFETs are used a proper
drain voltage derating should be used.
The main benefit of active reset circuit in the case of
DC1317A-F demo board is high efficiency (shown
in Figure 3), wide input range, high power density
and small size. To achieve such high efficiency all of
the power components were carefully selected.
Please consult LT factory for assistance if any
changes to the circuit are required.
DC1317A-F (LT1952) 3.3V Output Efficiency
72%
74%
76%
78%
80%
82%
84%
86%
88%
90%
92%
94%
0 5 10 15 20 25
Iout [A]
Efficiency
12V input
24V input
Figure 3. High efficiency of DC1317A-F allows the
board to be used in thermally critical applications
OUTPUT LOAD STEP RESPONSE
The load step response of DC1317A-F is very fast
even though relatively small amount of output ca-
pacitance is present (200uF ceramic and 470uF
electrolytic). This is thanks to fast error amplifier of
LT4430, optimal amount of current slope compen-
sation of LT1952, fast opto coupler and fast error
amplifier of LT1952. If higher load steps need to be
handled more output capacitance can be added in
order to keep the voltage transients at the desired
level. The load step transients are shown in Figure
4.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F
ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER
4
Figure 4. Fast transient response of DC1317A-F is
superior to many competing power modules with-
out the additional output capacitors.
SOFT START FUNCTION
The DC1317 features LT4430 opto coupler driver
that has soft start function which produces mono-
tonic startup ramp shown if Figure 5. The rise time
of output voltage is controlled by capacitor C19 that
is connected to OC (Overshoot Control) pin of
LT4430. Also, the soft-start function will prevent
input current surges even with full load at the out-
put.
Figure 5. The LT4430 opto coupler driver pro-
duces monotonic output voltage rise at startup
without output voltage overshoot.
DEBUGGING AND TESTING
The DC1317 can easily be tested and debugged by
powering the bias circuit separately from the main
power circuit. To place DC1317 into debug mode
remove the resistor R1 and connect 12V, 100mA
power source to +Vb node (right side of R1). By
doing this, the primary PWM controller LT1952 can
be activated without the main primary power being
applied to +Vin.
To activate the secondary side control circuit
LT4430 diode OR a 5V, 100mA power source into
pin 1 of LT4430 controller.
Once the primary and secondary controllers are
running the main power (+Vin) can be applied
slowly while observing the switching waveforms
and output voltage.
The input current supplying the power transformer
T1 should not exceeded 200mA without the output
load. If one of the MOSFETs is damaged, the input
current will exceed 200mA.
PCB LAYOUT
The PCB layout should be carefully planned to avoid
potential noise problems. The PCB layout for DC1317A
can be used as a guide. Since demo board DC1317A
has 8 versions the PCB layout has optional compo-
nents that can be removed. Also, the PCB layout has a
common schematic that is used just for the layout.
The actual circuit schematic shows component values.
The PCB layout schematic does not show the compo-
nent values.
In some cases, a different component like a diode is
used in a place holder for a capacitor such as in the
case of C6. Please modify the reference designators in
your schematic to reflect the actual component used.
The following simple PCB layout rules should be help-
ful.
If possible use solid ground planes on layers 2 and
n-1. The ground planes will prevent the switching
noise from coupling into sensitive lines.
Place sensitive lines on the inner layers that will be
shielded by grounds on layers 2 and n-1.
Keep the loop formed by Q1, RCS1, Cin and T1 tight.
Keep the loop formed by Q2, Q3 and T1 tight.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F
ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER
5
Keep noise sensitive nodes like SD/VSEC, ROSC, FB,
COMP, ISENSE, BLANK and DELAY as small as posible
by placing the associated components close to the
LT1952 and LT4430 chips.
Use local vias for all components that connect to
ground planes.
Do not place any traces on the layers 2 and n-1 to
avoid ground planes from being compromised.
If the PCB layout has to be done on 2 or 4-layer PCB
try to stay close to the guidelines outlined above. Also,
maximize the ground connections between compo-
nents by placing the components tight together.
Please contact LT factory for additional assistance.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F
ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER
6
Rcs1
0.005R
+Vin
R9
47R
R54
2.2R
Cu1
4.7u
R23 910R
R24
22k
Vu1
R15 178k
R20
100k
C10
0.1u
Q1
Si7138
D10
BAT760
R3
249R
C15
2.2nF, 2kV
C11
4.7n
R28 115k
R22
13.3k
G45R2-0302.007
GND
8
SD
7
MaxDC
5
FB=1.23V
2
Comp
1
Rosc
3
Vr=2.5V
6
Blank 9
Delay 12
OC 11
Sync
4
Isense 10
PGND 13
Out 14
Vin 15
Sout 16
U1
LT1952-1
R33
158k
C13
.47u
SS
SS
R26
220R
R30
1.2k
R14
33K
C23
1uF
Vfb
Vfb
R13
22K
Co1
100uF, X7R
+Vo1
R17
560R
Co2
T2A
PE-68386
C14
1u
C12
470p
R19
56K
CS-
2
CS+
1
CG
3
Vcc
4FG 5
GND 6
Timer 7
Sync 8
U5
LTC3900
R21
10k
R27
470R
R38
18.2K
R35
82.5k
R34
8.66k
C19
1uF, X5R
C16
.22u, 10V
C17
15n
+Vo1
R31
10R
SS
3
Vcc
1
FB 4
Opto 6
GND
2Comp 5
U4
LT4430
R18
10k
Cin
3x6.8uF
+Vin
D5
PDZ10B
Vu1
D2
BAS516
R11
10k
-Vin
+Vin
9V-36V Input
R1
2.2R
R32 196k
L1
PA1671.112
Q2
HAT2165
Q3
HAT2165
R16
33k
C5
0.22u
D6
B0540
Q9
BCX55
R10
1k
D9
PDZ7.5B
R8
2.2R
FG
FG
CG
CG
C3
1u
Q12
BC856T
D3
Q10
PBSS8110
+Vb
+Vb
L2
1.5mH
U6
PS2801-1
T1
Inp
3
Vcc
1
TS 4
Boost 6
GND
2TG 5
U2
LTC4440-5
C21
470p
D1
BAS516
Vu1
Out
D11
BAS516
GND
+Vout
C7
220p
Out
+
Csys 470u, 6.3V, 4m
C24
39p
C9
100p
C25 33n
Q13
Si7309
D14
BAS516
R45
10k
+Vr2
+Vr2
D17
BAS516
2
5
7, 8
10, 11
C33
0.1u
* R29
PMEG3002
C34
0.33uF
L4
10uH
R53
200R
R2
560k
C8
100p
1
6
* Please change the ref designator to reflect the symbol.
Demo Board DC1317A-F schematic
NOTE:
This schematic shows only the components required for operation of -F version
of DC1317A demo board.
All optional components of DC1317A demo board have been removed.
Also, all zero-ohm resistors have been replaced with wires.
Please consult the full DC1317A-F schematic to decide if any of the optional
components should be included in your design.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F
ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER
7

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-F
ACTIVE RESET ISOLATED 9-36V INPUT TO 3.3V @20A DC/DC POWER CONVERTER
8

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