PRODUCT SPECIFICATION

FAN6520A

REV. 1.0.2 8/26/04

7

Figure 4. Soft-Start Interval

The FAN6520A incorporates a MOSFET shoot-through

protection method which allows a converter to both sink and

source current. Care should be exercised when designing a

converter with the FAN6520A when it is known that the

converter may sink current.

When the converter is sinking current, it is behaving as a

boost converter that is regulating its input voltage. This

means that the converter is boosting current into the VCC

rail, which supplies the bias voltage to the FAN6520A.

If this current has nowhere to go—such as to other distrib-

uted loads on the VCC rail, through a voltage limiting pro-

tection device, or other methods—the capacitance on the

VCC bus will absorb the current. This situation will allow

the voltage level of the VCC rail to increase. If the voltage

level of the rail is boosted to a level that exceeds the maxi-

mum voltage rating of the FAN6520A, then the IC will expe-

rience an irreversible failure and the converter will no longer

be operational. Ensure that there is a path for the current to

follow other than the capacitance on the rail to prevent this

failure mode.

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 imped-

ances of the interconnecting bond wires and circuit traces.

Use wide, short-printed circuit traces to minimize these

interconnecting impedances. The critical components should

be located as close together as possible, using ground plane

construction or single point grounding.

Figure 5. Printed Circuit Board Power and

Ground Planes or Islands

Figure 5 shows the critical power components of the con-

verter. To minimize the voltage overshoot, the interconnect-

ing wires indicated by heavy lines should be part of a ground

or power plane in a printed circuit board. The components

shown in Figure 5 should be located as close together as

each represent numerous physical capacitors. Locate the

FAN6520A within two inches of the Q1 and Q2 MOSFETs.

The circuit traces for the MOSFETs’ gate and source con-

nections from the FAN6520A must be sized to handle up

to 1A peak current.

Figure 5 shows the circuit traces that require additional

layout consideration. Use single point and ground plane

construction for the circuits shown. Minimize any leakage

current paths on the COMP/OCSET pin and locate the

the internal current source is only 20µA. Provide local VCC

decoupling between VCC and GND pins. Locate the capaci-

tor, CBOOT as close as practical to the BOOT and PHASE

pins. All components used for feedback compensation

should be located as close to the IC as practical.

Figure 6. PC Board Small Signal Layout Guidelines

Q2

LDRV

SW

HDRV

Q1

Vin

FAN

6520A

Q2

VCC

SW

BOOT

Q1

L

OUT

C

OUT

GND

C

BOOT

+5V

COMP/OCSET

+5V

Vin