7

FN9016.5

March 28, 2007

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 ISL6520A

for the MOSFETs’ gate and source connections from the

ISL6520A must be sized to handle up to 1A peak current.

Figure 4 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 20

μA. Provide local V

CC

decoupling between VCC and GND pins. Locate the

PHASE pins. All components used for feedback

compensation should be located as close to the IC a

practical.

Feedback Compensation

Figure 5 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

peak-to-peak oscillator voltage

Modulator Break Frequency Equations

The compensation network consists of the error amplifier

a closed loop transfer function with the highest 0dB crossing

180 degrees. The equations below relate the compensation

locating the poles and zeros of the compensation network:

LGATE

UGATE

PHASE

RETURN

ISL6520A

FIGURE 3. PRINTED CIRCUIT BOARD POWER AND

GROUND PLANES OR ISLANDS

FIGURE 4. PRINTED CIRCUIT BOARD SMALL SIGNAL

LAYOUT GUIDELINES

+5V

ISL6520A

COMP/OCSET

GND

VCC

BOOT

PHASE

+5V

FIGURE 5. VOLTAGE-MODE BUCK CONVERTER

COMPENSATION DESIGN

REFERENCE

ESR

ΔV

OSC

ERROR

AMP

PWM

DRIVER

(PARASITIC)

+

-

REFERENCE

COMP

FB

ISL6520A

COMPARATOR

DRIVER

DETAILED COMPENSATION COMPONENTS

PHASE

+

-

+

-

OSC

F

LC

1

2

π x L

O

x C

O

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

=

F

ESR

1

2

π x ESR x C

O

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

=

(EQ. 4)

ISL6520A