Soft Start Function - Linear Technology 968A-D Quick Start Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D
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.
Figure 5. The LT4430 opto coupler driver pro-
duces monotonic output voltage rise at startup
without output voltage overshoot.
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ISOLATED DC/DC POWER CONVERTER
DEBUGGING AND TESTING
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.
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.
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