PRODUCT SPECIFICATION

FAN6520A

REV. 1.0.2 8/26/04

11

Power MOSFET Selection

For more information on MOSFET selection for synchro-

nous buck regulators, refer to: AN-6005: Synchronous Buck

MOSFET Loss Calculations.

This Fairchild app note is located at:

http://www.fairchildsemi.com/an/AN/AN-6005.pdf

In typical applications, the FAN6520A converter's output

voltage is low with respect to its input voltage, therefore the

lower MOSFET (Q2) is conducting the full load current for

most of the cycle. Therefore choose a MOSFET for Q2

In contrast, the high-side MOSFET (Q1) has a much shorter

duty cycle, and its conduction loss will therefore have less of

an impact. Q1, however, sees most of the switching losses,

so Q1’s primary selection criteria should be gate charge.

High-Side Losses

Figure 10 shows a MOSFET’s switching interval, with the

upper graph being the voltage and current on the Drain to

constant current charging the gate. The x-axis, therefore, is

specified or can be derived from the MOSFET’s datasheet.

Assuming switching losses are about the same for both the

rising edge and falling edge, Q1’s switching losses, occur

during the shaded time when the MOSFET has voltage

across it and current through it.

These losses are given by:

where:

(Figure 10).

constant current assumption for the driver to simplify the

Figure 11. Drive Equivalent Circuit

the gate charge required to get the MOSFET to its threshold

as high as 20V in a typical portable application. Care should

also be taken to include the delivery of the MOSFET’s gate

for the FAN6520A:

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P

SW

V

DS

I

L

×

2

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

2

×

t

s

×



 F

SW

=

(14)

P

COND

V

OUT

V

IN

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



 I

OUT

2

×

R

DS ON

()

×

=

(15)

V

SP

t1

t2

t3

4.5V

t4

t5

Q

G(SW)

V

DS

I

D

Q

GS

Q

GD

V

TH

V

GS

C

ISS

C

GD

C

ISS

C

GD

R

D

R

GATE

HDRV

5V

SW

VIN

G

t

s

Q

GSW

()

I

DRIVER

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

Q

GSW

()

VCC

V

SP

–

R

DRIVER

R

GATE

+

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





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

≈≈

(16)