Datasheet

LT4430
19
4430fc
For more information www.linear.com/LT4430
applicaTions inForMaTion
Figure 7. Setting Overshoot Control Time
4430 F07
V
IN
OC
C
OC
I
OC
8.5µA
Setting Overshoot Control Time
Figure 7 shows how to calculate the overshoot time by
connecting a capacitor from the OC pin to GND.
The overshoot control time, t
OC
, is set by the formula:
t
OC
= (C
OC
• 0.6V)/8.5µA
The OC pin requires a minimum capacitor of 100pF due to
stability requirements with the overshoot control amplifier.
This yields a minimum time of 7µs which is generally on
the order of a few cycles of the switching regulator. Us
-
ing the
minimum capacitor value results in no influence
on
start-up characteristics. Larger OC capacitor values
increase the overshoot control time and only increase the
amplifier stability. Do not modulate the overshoot control
time by externally increasing the OC charging current or
by externally driving the OC pin.
Choosing the Overshoot Control (OC) Capacitor Value
As discussed in the frequency compensation section,
the designer enjoys considerable freedom in setting the
feedback loop’s pole and zero locations for stability. Dif
-
ferent pole and zero combinations can produce the same
gain
-phase characteristics, but result in noticeably different
large-signal responses. Choosing frequency compensation
values that optimize both small-signal and large-signal
responses is difficult. Compromise values often result.
Power supply start
-up and short-circuit recovery are the
worst-case
large signal conditions. Input voltage and
output load characteristics heavily influence power supply
behavior as it attempts to bring the output voltage into
regulation. Frequency compensation values that provide
stable response under normal operating conditions can
allow severe output voltage overshoot to occur during
start-up and short-circuit recovery conditions. Large
overshoot often results in damage or destruction to the
load circuitry being powered, not a desirable trait.
The LT4430’s overshoot control circuitry plus one external
capacitor (C
OC
) provide independent control of start-up
and short-circuit recovery response without compro-
mising small
-signal
frequency compensation. Choosing
the optimum C
OC
value is a straightforward laboratory
procedure. The following description and set of pictures
explain this procedure.
Before choosing a value for the OC pin capacitor, complete
the remainder of the power supply design. This process
includes evaluating the chosen V
IN
bias generator topology
(please consult prior applications information section)
and optimizing frequency compensation under all normal
operating conditions. During this design phase, set C
OC
to its minimum value of 100pF. This ensures negligible
interaction from the overshoot control circuitry. Once these
steps are complete, construct a
test setup that monitors
start-up and short-circuit recovery waveforms. Perform this
testing with the output lightly loaded. Light load, following
full slew operation, is the worst-case as the feedback loop
transitions from full to minimal power delivery.
As an example, refer to the schematic on the last page
illustrating the 5V, 2A isolated flyback converter. All of
the following photos are taken with V
IN
= 48V and I
LD
=
20mA. Figure 8a demonstrates the power supply start-up
and short-circuit recovery behavior with no overshoot
control compensation (C
OC
= 100pF minimum). The 5V
output overshoots by several volts on both start-up and
short-circuit recovery due to the conservative nature of
the small-signal frequency compensation values.