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

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-B
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.
ISOLATED DC/DC POWER CONVERTER
DEBUGGING AND TESTING
The DC968 can easily be tested and debugged by
powering the bias circuit separately from the main
power circuit. To place DC968 into debug mode re-
move 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) 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 cur-
rent can be monitored. The input current 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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