REV. 0

ADP3171

–9–

Power MOSFETs

Two external N-channel power MOSFETs must be selected for

use with the ADP3171, 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 ADP3171

is operating in continuous mode, the simplifying assumption

can be made that one of the two MOSFETs is always conduct-

the maximum duty ratio of the high-side FET is:

Df

t

D

kHz

s

HSF MAX

MIN

OFF

HSF MAX

()

()

–

–.

%

=+

()

=×

1

1

192

3 5

33

µ

(20)

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

MOSFET is:

DD

LSF MAX

HSF MAX

()

()

–%

==

167

(21)

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

ID

II

I

I

I

AA

A

A

A

HSF MAX

HSF MAX

L VALLEY

L VALLEY

L PEAK

L PEAK

HSF MAX

()

()

()

()

(

)

(

)

()

()

.

.( .

.

)

.

.

=×

+×

+

=×

+×

+

=

22

22

3

033

375

3 75

625

6 25

3

29

(22)

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

LSF MAX

()

()

()

(

)()

()

()

()

)

.

.( .

.

)

.

.

=×

+×

+

=×

+×

+

()

=

22

22

3

067

375

3 75

625

6 25

3

41

(23)

dissipation. If 10% of the maximum output power is allowed for

MOSFET dissipation, the total dissipation will be:

PV

I

PV

A

mW

D FETs

OUT

OUT MAX

D FETs

()

(

)

()

.

..

.

=×

×

=×

×

=

01

01 15

65

975

(24)

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

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

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

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

resistances will be:

R

P

I

mW

A

m

DS ON HSF

D FETs

HSF MAX

()

()

()

.

=

×

=

×

=

3

975

32 9

38

22

Ω

(25)

R

P

I

mW

A

m

DS ON LSF

D FETs

LSF MAX

()

()

()

.

=

×

=

×

=

2

975

24 1

29

22

Ω

(26)

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

Larger MOSFETs reduce the conduction losses and allow higher

efficiency, but increase the system cost. A Fairchild FDB6982

Ω nominal, 35 mΩ

Ω nominal, 22 mΩ

worst-case) is a good choice in this application.

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

PR

I

VI

Q

f

I

VQ

f

Pm

A

AnC

kHz

A

VnC

kHz

mW

HSF

DS ON HSF

HSF MAX

IN

L PEAK

G

MIN

G

IN

RR

MIN

HSF

=×

+

××

×

×

+×

×

=×

+ +×

×

×

+×

×

=

()

(

)

()

.

.

2

2

2

35

2 9

56 25

12

192

21

519

192

349

Ω

(27)

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. In the sec-

drive current provided by the ADP3171 is about 1 A. In the

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

data sheet of the FDS6982 shows a value of 19 nC for this

parameter.

The low-side MOSFET dissipation is:

PR

I

Pm

A

mW

LSF

DS ON LSF

LSF MAX

LSF

=×

=×=

()

(

)

.

2

2

22

4 1

370

Ω

(28)

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

In continuous inductor-current mode, the source current of the

and an amplitude of one-half of the maximum output current. To

prevent large voltage transients, a low ESR input capacitor sized

for the maximum rms current must be used. The maximum rms

capacitor current is given by:

II

D

D

IA

A

C RMS

O

HSF

HSF

C RMS

()

()

–

.– .

.

=×

=×

=

2

2

50 33

0 332 4

(29)

For a ZA-type capacitor with 1000

µF capacitance and 6.3 V

voltage rating, the ESR is 24 m

Ω and the maximum allowable

ripple current at 100 kHz is 2 A. At 105

°C, at least two such

capacitors should be connected in parallel to handle the calcu-

lated ripple current. At 50

°C ambient, however, a higher ripple

current can be tolerated, so one capacitor is adequate.

The ripple voltage across the input capacitor is:

VI

ESR

n

D

nC

f

VI

m

mF

kHz

mV

CRIPPLE

O

C

C

HSF MAX

CIN

MIN

CRIPPLE

O

()

()

()

.

=×

+

××









=×

+

××









=

24

1

033

11

192

26

Ω

(30)

Linear Regulators

The linear regulators provide a low-cost, convenient, and versatile

solution for generating moderate current supply rails. The maxi-

mum output load current is determined by the size and thermal

impedance of the external N-channel power MOSFET that is

placed in series with the supply and controlled by the ADP3171.

The output voltage is sensed at the LRFB

× pin and compared

to an internal reference voltage in a negative feedback loop that

keeps the output voltage in regulation. If the load is reduced or