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ISL6431

Advanced Pulse Width Modulation (PWM) Controller for Broadband Applications

Renesas Electronics 

ISL6431

causes an over-current trip. The converter dissipates very little

power with this method. The measured input power for the

conditions of Figure 1 is only 1.5W.

OUTPUT INDUCTOR

CURRENT

5A/DIV.

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. The entire startup sequence

typically take about 11ms.

COMP/OCSET

1V/DIV.

VOUT

500mV/DIV.

TIME (5ms/DIV.)

FIGURE 1. OVERCURRENT OPERATION

The over-current function will trip at a peak inductor current

(IPEAK) determined by:

IPEAK = I--O-----C----S----E-r--D-T---S--x----O-R----N-O----C-----S----E---T--

where IOCSET is the internal OCSET current source (20A

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

rDS(ON) variations. To avoid over-current tripping in the normal

operating load range, find the ROCSET resistor from the

equation above with:

1. The maximum rDS(ON) at the highest junction temperature.

2. The minimum IOCSET from the specification table.

3. Determine IPEAK for

IPEAK

>

IOUTMAX

+

-------I---

2

,

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 start up interval where the

COMP/OCSET pin has been released from a grounded (system

shutdown) state. Initially, the COMP/OCSET is used to sample

the overcurrent setpoint by disabling the error amplifier and

drawing 20A through ROCSET. Once the over-current level has

been sampled, the soft start function is initiated. The clamp on the

error amplifier (COMP/OCSET pin) initially controls the

converter’s output voltage during soft start. 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). When the internally generated

FN9018 Rev 1.00

Jun 2002

TIME (2ms/DIV.)

FIGURE 2. START UP SEQUENCE

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.

VIN

ISL6431

UGATE

Q1

PHASE

Q2

LGATE

LO

VOUT

CIN

CO

RETURN

FIGURE 3. PRINTED CIRCUIT BOARD POWER AND

GROUND PLANES OR ISLANDS

Figure 3 shows the critical power components of the converter. To

minimize the voltage overshoot, the interconnecting wires

indicated by heavy lines should be part of a ground or power

plane in a printed circuit board. The components shown in Figure

3 should be located as close together as possible. Please note

that the capacitors CIN and CO may each represent numerous

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