5

FN9009.6

April 3, 2007

Start interval and 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.

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

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

from the equation above with:

temperature.

,

where

ΔI is the output inductor ripple current.

For an equation for the ripple current, see“Output Inductor

Selection” on page 7.

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 oversurrent setpoint by disabling the error

amplifier and drawing 20

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 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.

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 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.

represent numerous physical capacitors. Locate the ISL6520

for the MOSFETs’ gate and source connections from the

ISL6520 must be sized to handle up to 1A peak current.

FIGURE 1. OVERCURRENT OPERATION

TIME (5ms/DIV.)

OUTPUT INDUCTOR

5A/DIV.

CURRENT

I

PEAK

I

r

DS ON

()

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

=

(EQ. 3)

I

PEAK

I

OUT MAX

()

ΔI

()

2

----------

+

>

FIGURE 2. START UP SEQUENCE

500mV/DIV.

COMP/OCSET

1V/DIV.

TIME (2ms/DIV.)

LGATE

UGATE

PHASE

RETURN

ISL6520

FIGURE 3. PRINTED CIRCUIT BOARD POWER AND

GROUND PLANES OR ISLANDS

ISL6520