QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
1
LT1952
DESCRIPTION
Demonstration circuit 968A-A is isolated input to
high current output 1/8th Brick footprint con-
verter featuring the LT®1952 switching control-
ler. The DC968 converts isolated 36V to 75V
input to 3.3V output and provides over 30A of
output current. The converter operates at
300kHz with efficiency greater than 91%. The
DC968 can be easily modified to generate out-
put voltages in the range from 0.6V to 28V. The
output currents are limited by total output power
of up to 200W. The other available versions of
DC968A are:
DC968A-B 18-36Vin to 5V@20A
DC968A-C 36-75Vin to 12V@12A
DC968A-D 36-75Vin to 28V@4A
Also, the DC968 can be modified for other input
voltages like 8V-36V, 18V-75V, 44V-52V, and so
on. The wider input voltage range will decrease
the converter efficiency. Therefore, narrow input
voltage range will be more desirable.
The DC968 circuit features soft-start which pre-
vents output voltage overshoot on startup or
when recovering from overload condition.
The DC968 has precise over-current protection
circuit that allows for continuous operation un-
der short circuit conditions. The low power dis-
sipation 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 avail-
able. 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 30A 36V
Maximum Input Voltage IOUT = 0A to 30A 75V
VOUT VIN = 36V to 75V, IOUT = 0A to 30A 3.3V ±3%
Typical Output Ripple VOUT VIN = 36V to 75V, IOUT = 0A to 30A 50mVP–P
Nominal Switching Frequency 300kHz
QUICK START PROCEDURE
Demonstration circuit 968 is easy to set up to
evaluate the performance of LT1952 circuit. Re-
fer to Figure 1 for proper measurement
equipment setup and follow the procedure be-
low:
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 output voltage ripple by
touching the probe tip directly across the Vin or
Vout and GND terminals. See Figure 2. for
proper scope probe technique.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
2
1.
With power off, connect the input power sup-
ply to Vin and GND. Make sure that the input
power supply 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 75V.
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 estab-
lished, adjust the load within the operating
range and observe the output voltage regula-
tion, ripple voltage, efficiency and other pa-
rameters.
5.
The DC968 is equipped with an output ca-
pacitor CSYS (470uF) that approximates
typical system rail capacitance. If system
board already has capacitance of similar
value CSYS can be removed.
Figure 1. Proper Measurement Equipment Setup
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
3
Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
CHANGING THE OUTPUT VOLTAGE
To set the output voltage lower than 3.3V,
change the bottom voltage divider resistor
connected to LT4430 FB pin (see the sche-
matic on page 5). For example, to get 2.5V
output, change R38 resistor value to 26.1k.
To get higher than 3.3V output voltages trans-
former with higher turns ratio will be required.
Also, output MOSFETs with higher voltage rat-
ings may be required. Please contact LTC fac-
tory for details.
PRIMARY MOSFET DRIVER LTC4440
The DC968 has an LTC4440 MOSFET driver
U2. The LTC4440 can be disabled if the effi-
ciency is not important. If U2 is disabled, the
DELAY pin of LT1952 should be readjusted to
optimize the efficiency. Please contact LT fac-
tory for assistance.
OPTIONAL RESET CIRCUIT
The DC968 is equipped with an optional ac-
tive reset circuit that is disabled on DC968A-
A. In most applications the circuit is not re-
quired because there is no benefit from active
reset. However, if the active reset is required,
adding few resistors can enable the circuit.
Please consult LT factory for assistance.
DC968-A 3.3V @30A Output Efficiency
84%
85%
86%
87%
88%
89%
90%
91%
92%
5 15 25 35
Iout [A]
Efficiency
Figure 3. High efficiency of DC968A-A al-
lows the board to be used in thermally
critical applications
OUTPUT LOAD STEP RESPONSE
The load step response of DC968A-A is very
fast even though relatively small amount of
output capacitance is present (200uF ceramic
and 470uF electrolytic). This is thanks to fast
error amplifier of LT4430, optimal amount of
current slope compensation of LT1952, fast
opto coupler and fast error amplifier of
LT1952. If higher load steps need to be han-
dled more output capacitance can be added in
order to keep the voltage transients at the de-
sired level. The load step transients are
shown in Figure 4.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
4
Figure 4. Fast transient response of DC968
is superior to many competing power
modules without the additional output ca-
pacitors.
SOFT START FUNCTION
The DC968 features LT4430 opto coupler
driver that has soft start function which pro-
duces monotonic 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.
Figure 5. The LT4430 opto coupler driver
produces monotonic output voltage rise at
startup without output voltage overshoot.
DEBUGGING AND TESTING
The DC968 can easily be tested and de-
bugged by powering the bias circuit sepa-
rately from the main power circuit. To place
DC968 into debug mode remove the resistor
R1 and connect 48V, 100mA power source to
+Vb node (right side of R1). By doing this, the
primary PWM controller LT1952 can be acti-
vated without the main primary power being
applied to +Vin.
To activate the secondary side MOSFET
driver (LTC3900) diode OR a 10V, 100mA
power source into collector of Q9.
Once the primary and secondary controllers
are running the MOSFET gate timing can be
checked.
If the MOSFET gate timing is correct the main
power input can be applied to +Vin. The delay
from turn on of FG and gate of Q1 should be
in the range from -100ns to +100ns. By slowly
increasing the +Vin from 0V to 48V output
voltage and input current can be monitored.
The input current should not exceeded 100mA
without the output load. If one of the MOS-
FETs is damaged, the input current will ex-
ceed 100mA.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
5
C2
1n, 200V
Rcs1
0.015R
Cin1
+Vin
R29
22.1K
Cu1
1uF
R23 910
R24
39K
Vu1
R15 115k
R20
72.8K
C10
0.1u
Vu1
Q1
D10
BAT760
C15
2.2nF, 2kV
R28
442k
R22
13.3k
PHM21NQ15
2
5
PA0861.004
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
Sout
R33
158k
C13
.47u
SS
SS
R26
220R
R30
1.2k
R14
33K
C23
1uF
Vfb
Vfb
R13
22K
R42 0R
Co1
100uF, X7R
+Vo1
R17
560R
Co2
T2A
PE-68386
Vaux
C14
1u, 10V
C12
470p
R19
38.3K
Vaux
CS-
2
CS+
1
CG
3
Vcc
4FG 5
GND 6
Timer 7
Sync 8
U5
LTC3900
R21
10k
R27
270R
R38
18.2K
R35
82.5k
R34
1.96k
C19
1uF, X5R
C16
.22u, 10V
C17
15n
+Vo1
Vaux
R31
10R
SS
3
Vcc
1
FB 4
Opto 6
GND
2Comp 5
U4
LT4430
C18
10p
R18
10k
Cin3
2.2uF, 100V
+Vin
Cin2 D5
PDZ10B
Vu1
D2
BAS516
R11
82k
1
6
-Vin
+Vin
36V to 72V
R1
2.2R
R32
332k
L1
PA1671.650
Q2
HAT2165
Q3
HAT2165
7
10
R16
33k
C5
0.22u, 25V
D6
B0540W
Q9
BCX55
R10
1k
D9
PDZ7.5B
R8
2.2R
FG
FG
CG
CG
C3
1u
R5
10R
Q12
BC857BF
D3
BAS516
Q10
PZTA42
+Vb
+Vb
L2
1.5mH
U6
PS2801-1
T1
Inp
3
Vcc
1
TS 4
Boost 6
GND
2TG 5
U2
LTC4440
Vu1
D11
BAS516
GND
+Vout
C7
220p
Sout
C8
100p
C6
0.1u
D7
B0540W
C4
0.1u
D8
B0540W
+
Csys
470uF, 6.3V
C24
47p
C9
100p
+Vr2
+Vr2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A
ISOLATED DC/DC POWER CONVERTER
6
