10

FN9064.1

April 8, 2005

Application Guidelines

Soft-Start Interval

Initially, the soft-start function clamps the error amplifier’s output

of the PWM converter. This generates PHASE pulses of

increasing width that charge the output capacitor(s). The

resulting output voltages start-up as shown in Figure 6.

The soft-start function controls the output voltage rate of rise

to limit the current surge at start-up. The soft-start interval

and the surge current are programmed by the soft-start

increases the peak surge current. Using the recommended

0.1

µF soft start capacitors ensure all output voltages ramp

up to their set values in a quick and controlled fashion, while

meeting the system timing requirements.

Shutdown

The PWM output does not switch until the soft-start voltage

the reference on each linear’s amplifier is clamped to the

soft-start voltage. Holding the SS24 pin low (with an open

drain or open collector signal) turns off regulators 1, 2 and 3.

Regulator 4 (MCH) will simply drop its output to the

intermediate soft-start level. This output is not allowed to

violate the 2V maximum potential gap to the ATX 3.3V

output.

Layout Considerations

MOSFETs switch very fast and efficiently. The speed with

which the current transitions from one device to another

causes voltage spikes across the interconnecting

impedances and parasitic circuit elements. The voltage

spikes can degrade efficiency, radiate noise into the circuit,

and lead to device over-voltage stress. Careful component

layout and printed circuit design minimizes the voltage

spikes in the converter. Consider, as an example, the turn-off

transition of the upper MOSFET. Prior to turn-off, the upper

MOSFET was carrying the full load current. During the turn-

off, current stops flowing in the upper MOSFET and is picked

up by the lower MOSFET or Schottky diode. Any inductance

in the switched current path generates a large voltage spike

during the switching interval. Careful component selection,

tight layout of the critical components, and short, wide circuit

traces minimize the magnitude of voltage spikes.

There are two sets of critical components in a DC-DC

converter using an ISL6524A controller. The switching

power components are the most critical because they switch

large amounts of energy, and as such, they tend to generate

equally large amounts of noise. The critical small signal

components are those connected to sensitive nodes or

those supplying critical bypass current.

The power components and the controller IC should be

placed first. Locate the input capacitors, especially the high-

frequency ceramic de-coupling capacitors, close to the

power switches. Locate the output inductor and output

capacitors between the MOSFETs and the load. Locate the

PWM controller close to the MOSFETs.

The critical small signal components include the bypass

these components close to their connecting pins on the

control IC. Minimize any leakage current paths from any SS

node, since the internal current source is only 28

µA.

A multi-layer printed circuit board is recommended. Figure

10 shows the connections of the critical components in the

could represent numerous physical capacitors. Dedicate one

solid layer for a ground plane and make all critical

TABLE 1. OUT1 OUTPUT VOLTAGE PROGRAM

PIN NAME

NOMINAL

DACOUT

VOLTAGE

VID3

VID2

VID1

VID0

VID25

01000

1.050

01000

1.050

01001

1.075

00110

1.100

00111

1.125

00100

1.150

00101

1.175

00010

1.200

00011

1.225

00000

1.250

00001

1.275

11110

1.300

11111

1.325

11100

1.350

11101

1.375

11010

1.400

11011

1.425

11000

1.450

11001

1.475

10110

1.500

10111

1.525

10100

1.550

10101

1.575

10010

1.600

10011

1.625

10000

1.650

10001

1.675

01110

1.700

01111

1.725

01100

1.750

01101

1.775

01010

1.800

01011

1.825

NOTE: 0 = connected to GND, 1 = open or connected to 3.3V

through pull-up resistors