5

FN9083.2

January 20, 2005

rail, which supplies the bias voltage to the ISL6520B. If there

is nowhere for this current to go, such as to other distributed

will absorb the current. This situation will allow voltage level

boosted to a level that exceeds the maximum voltage rating

of the ISL6520B, then the IC will experience an irreversible

failure and the converter will no longer be operational.

Ensuring that there is a path for the current to follow other

than the capacitance on the rail will 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 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 2 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 2 should be located as close together as possible.

represent numerous physical capacitors. Locate the ISL6520B

for the MOSFETs’ gate and source connections from the

ISL6520B must be sized to handle up to 1A peak current.

Figure 3 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/SD pin and locate the resistor,

current source is only 20

as close as practical to the BOOT and PHASE pins. All

components used for feedback compensation should be

located as close to the IC a practical.

Feedback Compensation

Figure 4 highlights the voltage-mode control loop for a

synchronous-rectified buck converter. The output voltage

the oscillator (OSC) triangular wave to provide a

pulse-width modulated (PWM) wave with an amplitude of

The modulator transfer function is the small-signal transfer

LGATE

UGATE

PHASE

RETURN

ISL6520B

FIGURE 2. PRINTED CIRCUIT BOARD POWER AND

GROUND PLANES OR ISLANDS

FIGURE 3. PRINTED CIRCUIT BOARD SMALL SIGNAL

LAYOUT GUIDELINES

+5V

ISL6520B

GND

VCC

BOOT

PHASE

FIGURE 4. VOLTAGE-MODE BUCK CONVERTER

COMPENSATION DESIGN

REFERENCE

ESR

ERROR

AMP

PWM

DRIVER

(PARASITIC)

+

-

REFERENCE

COMP/SD

FB

ISL6520B

COMPARATOR

DRIVER

DETAILED COMPENSATION COMPONENTS

PHASE

+

-

+

-

OSC

ISL6520B