REV. 0

ADP3164

–12–

ment for the power MOSFETs. When the ADP3164 is operating

in continuous mode, the simplifying assumption can be made

that in each phase one of the two MOSFETs is always conducting

the duty ratio of the high-side MOSFET is:

D

V

V

V

V

HSF

OUT

IN

1 475

12

12 3

.

. %

(16)

The duty ratio of the low-side (synchronous rectifier) MOSFET is:

DD

LSF MAX

HSF MAX

()

()

. %

187 7

(17)

The maximum rms current of the high-side MOSFET during

normal operation is:

I

I

n

D

I

I

AA

A

A

HSF MAX

O

HSF

L RIPPLE

O

()

()

.

.

.

1

3

80

4

0 123

1

10 8

380

702

2

2

2

2

(18)

The maximum rms current of the low-side MOSFET during

normal operation is:

II

D

D

AA

LSF MAX

HFS M AX

LSF

HSF

()

(

)

.

.

.

.

702

0 877

0 123

18 75

(19)

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

MOSFET dissipation, the total dissipation in the eight MOSFETs

of the 4-phase converter will be:

PV

I

PV

A

W

FET TOTAL

MIN

O

FET TOTAL

()

()

.

..

.

01

0 1 1 3845

80

11 08

(20)

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

MOSFETs and half for the four low-side MOSFETs, and

assuming that the resistive and switching losses of the high-side

MOSFETs are equal, the required maximum MOSFET resis-

tances will be:

R

P

nI

R

W

A

m

DS ON HSF

FET TOTAL

HSF MAX

DS ON HSF

()

()

()

()

.

.

4

11 08

44

7 02

14

2

2

(21)

and:

R

P

nI

R

W

A

m

DS ON LSF

FET TOTAL

LSF MAX

DS ON LSF

()

()

()

()

.

.

.

2

11 08

2

4 18 75

394

2

2

(22)

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

nominal, 5.6 m

worst-case) for the low-side are good choices.

The high-side MOSFET dissipation is:

PR

I

VI

Q

f

I

VQ

f

Pm

A

V

A

nC

kHz

A

V

nC

kHz

W

HSF

DS ON HSF

HSF MAX

IN

L PK

G

SW

G

IN

RR

SW

HSF

()

(

)

()

.

.

2

2

2

10

7 02

12

26

35

200

21

12

150

200

1 95

(23)

Where the first term is the conduction loss of the MOSFET, 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-

gate drive current provided by the ADP3414 is about 1 A. In

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

data sheet of the FDB8030L does not give that information, so

an estimated value of 150 nC is used. This estimate is based on

information found on data sheets of similar devices. In both

The worst-case low-side MOSFET dissipation is:

PR

I

Pm

A

W

LSF

DS ON LSF

LSF MAX

LSF

()

(

)

..

.

2

2

5 6

18 75

1 97

(24)

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

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

high-side MOSFET is approximately a square wave with a duty

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:

I

I

n

nD

nD

I

A

A

C RMS

O

HSF

HSF

C RMS

()

()

–(

)

.

(

.

)

2

2

80

4

4

0 123

4

0 123

10

(25)

Note that the capacitor manufacturer’s ripple current ratings are

often based on only 2000 hours of life. This makes it advisable

to further derate the capacitor, or to choose a capacitor rated at

a higher temperature than required. Several capacitors may be

placed in parallel to meet size or height requirements in the

design. In this example, the input capacitor bank is formed by

three 270 F, 16 V OS-CON capacitors with a ripple current

rating of 4.4 A each.

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