REV. 0

ADP3170

–12–

higher efficiency, but increase the system cost. A Fairchild

good choice for both the low-side and high-side MOSFET.

With this choice, the high-side MOSFET dissipation is:

PR

I

VI

Q

f

I

VQ

f

Pm

A

A

nC

kHz

A

V

nC

kHz

W

HSF

DS ON HSF

HSF MAX

IN

L PEAK

G

MIN

G

IN

RR

MIN

HSF

=×

+

××

×

×

+

××

=×

+

××

×

×

+

××

=

()

(

)

()

.

.

.

2

2

2

614 7

5

28 6

50

183

21

5

100

183

2 04

Ω

(24)

where the second term represents the turn-off loss of the

MOSFET and the third term represents the turn-on loss due to

the stored charge in the body diode of the low-side MOSFET.

peak gate drive current provided by the ADP3170 is about 1 A. In

the low-side MOSFET at the valley of the inductor current.

The data sheet of the FDB7045L does not give that informa-

tion, so an estimated value of 100 nC is used. The estimate is

based on information found on the data sheets of similar devices.

The low-side MOSFET dissipation is:

PR

I

Pm

A

W

LSF

DS ON HSF

HSF MAX

LSF

=×

=×

=

()

(

)

.

2

2

618

1 94

Ω

(25)

Note that there are no switching losses in the low-side MOSFET.

Surface mount MOSFETs are preferred in CPU core converter

applications due to their ability to be handled by automatic

assembly equipment. The TO-263 package offers the power

handling of a TO-220 in a surface mount package. However,

this package still needs adequate copper area on the PCB to

help move the heat away from the package.

The junction temperature for a given area of two-ounce copper

can be approximated using:

TP

T

AD

A

JJ

=×

θ

(26)

assuming:

2

2

2

ambient temperature of 50

°C:

TC W

W

C

C

TC W

W

C

C

HSF

LSF

J

J

=×

=×

28

2 06

50

108

28

1 94

50

104

oo

o

oo

o

/.

/.

All of the above-calculated junction temperatures are safely

below the 175

°C maximum specified junction temperature of

the selected MOSFETs.

Power MOSFETs

Two external N-channel power MOSFETs must be selected for

use with the ADP3170, one for the main switch and one for the

synchronous switch. The main selection parameters for the power

requirement for the two power MOSFETs. When the ADP3170

is operating in continuous mode, the simplifying assumption can

be made that one of the two MOSFETs is always conducting

maximum duty ratio of the high-side FET is:

Df

t

DkHz

s

HSF MAX

MIN

OFF

HSF MAX

()

()

–

–(

.

)

%

=×

()

=×

=

1

1183

3 3

40

µ

(17)

The maximum duty ratio of the low-side (synchronous rectifier)

MOSFET is:

DD

LSF MAX

HSF MAX

()

()

–%

==

160

(18)

The maximum rms current of the high-side MOSFET is:

ID

II

I

vI

I

AA

A

A

A

HSF MAX

HSF MAX

L VALLEY

L VALLEY

L PEAK

L PEAK

HSF MAX

()

()

(

)

(

)

()

()

()

.

.(

.

.

)

.

.

=×

+×

=×

+×

+

=

22

22

3

04

17 4

17 4

28 6

28 6

3

14 7

(19)

The maximum rms current of the low-side MOSFET is:

ID

II

I

I

I

AA

A

A

A

LSF MAX

LSF MAX

L VALLEY

L VALLEY

L PEAK

L PEAK

HSF MAX

()

()

()

()

(

)

(

)

()

.

..

.

.

=×

+×

=×

+×

=

22

22

3

06

17 4

17 4

28 6

28 6

3

18

(20)

allowable dissipation. If 10% of the maximum output power is

allowed for MOSFET dissipation, the total dissipation will be:

PV

I

PV

A

W

D FET

OUT

OUT MAX

D FET

s

s

()

(

)

()

.

..

.

=×

×

=×

×

=

01

01 18

23

41

(21)

Allocating half of the total dissipation for the high-side MOSFET and

half for the low-side MOSFET, and assuming that the resis-

tive loss of the high-side MOSFET is one-third, and the switching

loss is two-thirds of its portion, the required maximum MOSFET

resistances will be:

R

P

I

W

A

m

DS ON HSF

D FETS

HSF MAX

()

()

()

.

.

=

×

=

×

=

3

41

314 7

6

2

2

Ω

(22)

R

P

I

W

A

m

DS ON LS

D FETS

LSF MAX

()

()

()

.

==

×

=

22

41

218

6

Ω

(23)

Note that there is a trade-off between converter efficiency and

cost. Larger MOSFETs reduce the conduction losses and allow