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LT4430

4430fb

APPLICATIONS INFORMATION

Figure 7. Setting Overshoot Control Time

4430 F07

VIN

COC

IOC

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, tOC, is set by the formula:

tOC = (COC • 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 (COC) provide independent control of start-up

and short-circuit recovery response without compro-

mising small-signal frequency compensation. Choosing

the optimum COC 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 VIN bias generator topology

(please consult prior applications information section)

and optimizing frequency compensation under all normal

operating conditions. During this design phase, set COC

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 VIN = 48V and ILD =

20mA. Figure 8a demonstrates the power supply start-up

and short-circuit recovery behavior with no overshoot

control compensation (COC = 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.

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