Datasheet

LT4430
20
4430fc
For more information www.linear.com/LT4430
applicaTions inForMaTion
Next, increase C
OC
’s value. Either use a capacitor substitu-
tion box or solder each new value into the circuit. Monitor
the
start-up and short-circuit recovery waveforms. Note
any changes. Figures 8b to 8e illustrate what happens as
C
OC
increases. In general, overshoot decreases as C
OC
increases.
C
OC
= 0.0168µF in Figure 8b begins to affect loop dynam-
ics, but start-up still exhibits about 1.5V of overshoot.
Short-cir
cuit recovery is considerably more damped. C
OC
= 0.022µF in Figure 8c damps start-up overshoot to 0.5V
and short-circuit recovery remains similar to that of Figure
8b. C
OC
= 0.033µF in Figure 8d provides under 100mV
of overshoot and short-circuit recovery is slightly more
damped. C
OC
= 0.047µF in Figure 8e achieves zero over-
shoot at the expense of additional damping and delay time
in
short-circuit recovery. In this example, C
OC
= 0.033µF
provides the best value for both start-up and short-circuit
recovery. Figure 8f provides an expanded scale of the
waveforms. After a C
OC
value is selected, check start-up
and short-circuit recovery over the V
IN
supply range and
with higher output load conditions. Modify the value as
necessar
y.
Start-up and short-circuit recovery waveforms for various
designs will differ from the photos shown in this example.
Factors affecting these waveforms include the isolated
topology chosen, the primary-side and secondary-side
bias circuitry and input/output conditions. For instance,
in many isolated power supplies, a winding on the main
power transformer bootstraps the supply voltage for the
primary-side control circuitry. Under short-circuit condi
-
tions, the primary-side control circuitry’s supply voltage
collapses,
generating a restart cycle. Recovery from
short-circuit is therefore identical to start-up. In the flyback
example discussed, the primary-side control circuitry is
always active. Switching never stops in short-circuit. The
LT4430 error amplifier COMP pin changes from its low
clamp level to its higher regulating value during start-up
and changes from its high clamp level to its lower regulat
-
ing point during short-circuit recovery. This large-signal
behavior explains the observed difference in the start-up
versus short-circuit recovery waveforms.
A final point of discussion involves the chosen C
OC
value.
LT C recommends that the designer use a value that con-
trols overshoot
to the acceptable level, but is not made
overly large. The temptation arises to use the overshoot
control
function as a power supply “soft-start” feature.
Larger values of C
OC
, above what is required to control
overshoot, do result in smaller dV/dt rates and longer
start-up times. However, large values of C
OC
may stall the
feedback loop during start-up or short-circuit recovery,
resulting in an extended period of time that the output
voltage “flatspots”. This voltage shelf may occur at an
intermediate value of output voltage, promoting anomalous
behavior with the powered load circuitry. If this situation
occurs with the desired C
OC
value, solutions may require
circuit modifications. In particular, bias supply holdup
times are a prime point of concern as switching stops
during these output voltage flatspots. As a reminder,
the purpose of this LT4430 circuitry is to control and
prevent excessive output voltage overshoot that would
otherwise induce damage or destruction, not to control
power supply timing, sequencing, etc. It is ultimately the
user’s responsibility to define the acceptance criteria for
any waveforms generated by the power supply relative to
overall system requirements.