QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C
ISOLATED DC/DC POWER CONVERTER
LT1952
DESCRIPTION
Demonstration circuit 968A-C is isolated input to high
current output 1/8th Brick footprint converter featur-
ing the LT
®
1952 switching controller. The DC968-C
converts isolated 36V to 75V input to 12V output and
provides over 12A of output current. The converter
operates at 300kHz with efficiency up to 95%. With
proper amount of airflow, the DC968-C converter can
generate up to 15A of output current. The DC968 can
be easily modified to generate output voltages in the
range from 0.6V to 28V. The output currents are lim-
ited by total output power of up to 200W. The other
available versions of DC968A are:
DC968A-A 36-75Vin to 3.3V@30A
DC968A-B 18-36Vin to 5V@20A
DC968A-D 36-75Vin to 28V@4A
Also, the DC968 can be modified for other input volt-
ages like 8V-36V, 18V-75V, 44V-52V, and so on. The
wider input voltage range will decrease the converter
Table 1.
Performance Summary
PARAMETER
CONDITION
VALUE
efficiency. Therefore, narrow input voltage range will
be more desirable.
The DC968 circuit features soft-start which prevents
output voltage overshoot on startup or when recover-
ing from overload condition.
The DC968 has precise over-current protection circuit
that allows for continuous operation under short cir-
cuit 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
Minimum Input Voltage
Maximum Input Voltage
VOUT
Typical Output Ripple VOUT
Nominal Switching Frequency
IOUT = 0A to 12A
IOUT = 0A to 12A
VIN = 36V to 72V, IOUT = 0A to 12A
VIN = 36V to 72V, IOUT = 0A to 12A
36V
72V
12V ±3%
100mVP–P
300kHz
QUICK START PROCEDURE
Demonstration circuit 968 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-
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C
ply has sufficient current rating at minimum input
voltage for the required output load.
2.
Turn on the power at the input.
ISOLATED DC/DC POWER CONVERTER
4.
Once the proper output voltage is established, ad-
NOTE:
Make sure that the input voltage does not
exceed 75V.
3.
Check for the proper output voltage.
just the load within the operating range and ob-
serve the output voltage regulation, ripple voltage,
efficiency and other parameters.
5.
The DC968 is equipped with an output capacitor
Vout = 12V, +/-3%.
If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
CSYS (150uF) 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
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C
ISOLATED DC/DC POWER CONVERTER
Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
CHANGING THE OUTPUT VOLTAGE
To set the output voltage lower than 12V, change
the bottom voltage divider resistor connected to
LT4430 FB pin (see the
simplified schematic
on
page 5). For example, to get 9V output, change R38
resistor value to 24.9k. However, keep in mind that
changing the transformer as well may increase the
efficiency. The schematic on page 6 shows all of the
components on the PCB. The schematic on page 6
can be used as a reference if optional circuits need
to be implemented.
To get higher than 12V output voltages transformer
with higher turns ratio may be required. Also, out-
put MOSFETs with higher voltage ratings may be
required. Please contact LTC factory for details.
PRIMARY MOSFET DRIVER LTC4440
The DC968 has an LTC4440 MOSFET driver U2. The
LTC4440 can be disabled if the efficiency is not im-
portant. If U2 is disabled, the DELAY pin of LT1952
should be readjusted to optimize the efficiency.
Please contact LT factory for assistance.
OPTIONAL RESET CIRCUIT
The DC968 is equipped with an optional active reset
circuit that is disabled on DC968A-C. In most appli-
cations the circuit is not required 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.
96%
95%
Efficiency
94%
93%
92%
91%
90%
0
LT1952 DC968-C 12V @12A output
2
4
6
8
Iout [A]
10
12
14
Figure 3. High efficiency of DC968A-C allows the
board to be used in thermally critical applications
with outputs up to 15A.
OUTPUT LOAD STEP RESPONSE
The load step response of DC968A-C is very fast
even though relatively small amount of output ca-
pacitance is present (33uF ceramic and 150uF elec-
trolytic). 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.
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C
DEBUGGING AND TESTING
ISOLATED DC/DC POWER CONVERTER
The DC968 can easily be tested and debugged by
powering the bias circuit with a separate power
source. 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
activated without the main primary power being
applied to +Vin.
Figure 4. Fast transient response of DC968 is su-
perior to many competing power modules without
the additional output capacitors.
SOFT START FUNCTION
The DC968 features LT4430 opto coupler driver that
has soft start function which produces 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.
To activate the secondary side MOSFET driver
(LTC3900) connect a 10V, 100mA power source to
collector of Q9 via 1A, 40V diode. That way the cur-
rent coming from DC968 bias supply will not be
able to flow back to 10V supply once the converter
is running.
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 correct de-
lay from turn on of FG and gate of Q1 is usually in
the range from -100ns to +100ns. By slowly in-
creasing the +Vin from 0V to 48V the output voltage
and input current can be monitored. The input cur-
rent should not exceed 100mA without the output
load. If one of the MOSFETs is damaged, the input
current will exceed 100mA.
Figure 5. The LT4430 opto coupler driver pro-
duces monotonic output voltage rise at startup
without output voltage overshoot.
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C
+Vin
+Vin
2.2R
R1
+Vb
PZTA42
+Vr2
Cin1
Cin2
Cin3
2.2uF, 100V
R11
82k
D5
PDZ10B
D3
BAS516
BAS516
6
10
Q2
HAT2244
Q3
HAT2244
Q10
L2
Vu1
DO1607B-155ML-C
D2
PA1393.302
L1
+Vo1
ISOLATED DC/DC POWER CONVERTER
7
+Vout
1
36V to 72V
T1
-Vin
PA0423
FG
+Vin
2
CG
Co1
33uF, 16V
Csys
150uF, 16V, APXE
+
GND
R5
10R
Vu1
C3
1u
C21
220p
D1
R13
22K
U1
LT1952-1
R14
33K
R15
1
2
BAS516
16
15
14
13
12
11
10
9
R23
1
2
3
R3
57
U2
LTC4440
Vcc
GND
Inp
Boost
TG
TS
6
5
4
Q1
PHM21NQ15
5
L3
DO1608C-684ML-C
R8
2.2R
R16
47k
R18
10k
1
2
CG
3
4
C14
1uF
D9
PDZ7.5B
C2
820p
D7
BAS521
Rcs1
0.010R
D6
BAS521
C5
0.22u, 25V
U5
LTC3900
CS+
Sync
8
7
6
5
FG
R17
560R
C7
220p
Sout
C8
100p
T2A
PE-68386
Comp
FB=1.23V
Rosc
Sync
MaxDC
Vr=2.5V
SD
GND
R33
Sout
Vin
Out
PGND
Delay
OC
Isense
Blank
Sout
BAT760
Vfb
Q12
BC857BF
R21
10k
CS- Timer
CG
Vcc
R19
38.3K
GND
FG
Vu1
Q9
BCX55
Vaux
R9
47R
Cu1
4.7u
R10
1k
133k
3
4
D10
C9
100p
2k
C12
470p
C10
0.1u
R22
13.3k
5
6
7
8
R20
68.1k
C11
2.2n
R26
220R
Vu1
D11
BAS516
U4
LT4430
R27
270R
Opto
Comp
FB
R38
18.2K
6
5
4
C18
10p
C17
3.3n
R34
8.45k
R35
348K
+Vo1
Vfb
U6
PS2801-1
Vaux
R31
10R
C16
.22uF
442k
442k
R32
R28
R24
82K
C13
1uF
C23
1uF
R30
1.2k
1
2
3
Vcc
GND
SS
R29
22K
158K
+Vr2
+Vb
C15
2.2nF, 2kV
C19
1uF
C24
10p
5
