5

completion of the OCP sampling and hold operation, the POR

function initiates the Soft Start operation.

Over Current Protection

The overcurrent function protects the converter from a shorted

to monitor the current. This method enhances the converter’s

efficiency and reduces cost by eliminating a current sensing

resistor.

The overcurrent function cycles the soft-start function in a

hiccup mode to provide fault protection. A resistor

Application diagram).

Immediately following POR, the ISL6431 initiates the

Overcurrent Protection sampling and hold operation. First,

the internal error amplifier is disabled. This allows an internal

20

ISL6431 then samples this voltage at the COMP pin. This

sampled voltage, which is referenced to the VCC pin, is held

internally as the Overcurrent Set Point.

When the voltage across the upper MOSFET, which is also

referenced to the VCC pin, exceeds the Overcurrent Set

Point, the overcurrent function initiates a soft-start sequence.

Figure 1 shows this operation with an overload condition. This

current is repeated with a 21ms period. Note that the inductor

current increases to over 14A during the Soft Start interval and

causes an overcurrent trip. The converter dissipates very little

power with this method. The measured input power for the

conditions of Figure 1 is only 0.25W.

The overcurrent function will trip at a peak inductor current

typical). The OC trip point varies mainly due to the

from the equation above with:

,

where

∆I is the output inductor ripple current.

For an equation for the ripple current see the section under

component guidelines titled ‘Output Inductor Selection’.

Soft Start

The POR function initiates the soft start sequence after the

overcurrent set point has been sampled. Soft start clamps the

error amplifier output (COMP pin) and reference input (non-

inverting terminal of the error amp) to the internally generated

Soft Start voltage. Figure 2 shows a typical soft start interval.

Initially the clamp on the error amplifier (COMP/OCSET pin)

controls the converter’s output voltage. The oscillator’s

triangular waveform is compared to the ramping error amplifier

voltage. This generates PHASE pulses of increasing width that

charge the output capacitor(s). With sufficient output voltage,

the clamp on the reference input controls the output voltage.

When the internally generated Soft Start voltage exceeds the

feedback (FB pin) voltage, the output voltage is in regulation.

This method provides a rapid and controlled output voltage rise.

Application Guidelines

Layout Considerations

As in any high frequency switching converter, layout is very

important. Switching current from one power device to another

can generate voltage transients across the impedances of the

interconnecting bond wires and circuit traces. These

interconnecting impedances should be minimized by using

wide, short printed circuit traces. The critical components

should be located as close together as possible, using ground

plane construction or single point grounding.

FIGURE 1. OVERCURRENT OPERATION

0A

4A

2A

6A

8A

10A

12A

14A

TIME (50

µs/DIV.)

I

PEAK

I

r

DS ON

()

-----------------------------------------------------

=

I

PEAK

I

OUT MAX

()

∆I

()

2

----------

+

>

0V

TIME (1ms/DIV.)

FIGURE 2. SOFT START INTERVAL

500mV/DIV.

ISL6431