REV. B

–10–

ADP3160/ADP3167

current. The current comparator of the ADP3160 has a mini-

mum current limit threshold of 142 mV. Note that the 142 mV

value cannot be used for the maximum specified nominal current,

as headroom is needed for ripple current and tolerances.

The current comparator threshold sets the peak of the inductor

twice the peak inductor current value less half of the peak-to-

peak inductor ripple current. From this the maximum value of

R

V

I

I

mV

AA

m

SENSE

CS CL

MIN

O

LRIPPLE

£

+

=

+

=W

()(

)

()

..

.

22

142

26 7

6 1

43

(6)

In this case, 4 m

W was chosen as the closest standard value.

the maximum current sense threshold of 172 mV:

I

V

R

I

mV

m

AA

OUT CL

CS CL

MAX

SENSE

L RIPPLE

()

()(

)

()

–

–.

.

=¥

=

¥

W

=

2

2

172

4

12 2

73 8

(7)

At output voltages below 425 mV, the current sense threshold is

reduced to 95 mV, and the ripple current is negligible. There-

fore, at dead short the output current is reduced to:

I

mV

m

A

OUT SC

()

.

=¥

=

2

95

4

47 5

W

(8)

To safely carry the current under maximum load conditions, the

sense resistor must have a power rating of at least:

PI

R

R

SENSE RMS

SENSE

SENSE

=¥

()

2

(9)

where:

I

I

n

V

V

SENSE RMS

O

OUT

IN

()

2

2

=¥

¥

h

(10)

In this formula, n is the number of phases, and

is the converter

efficiency, in this case assumed to be 85%. Combining Equations 9

and 10 yields:

P

AV

V

mmW

R

SENSE

=¥

¥

¥W =

53 4

2

17

085 12

4

950

2

..

.

Power MOSFETs

In the standard 2-phase application, two pairs of N-channel

power MOSFETs must be used with the ADP3160 and

ADP3412, one pair as the main (control) switches and the

other pair as the synchronous rectifier switches. The main

age to the ADP3412) dictates whether standard threshold or

recommended.

ment for the power MOSFETs. When the ADP3160 is operating

in continuous mode, the simplifying assumption can be made

that in each phase one of the two MOSFETs is always conduct-

D

V

V

HSF

OUT

IN

== 13 3

.%

(11)

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

DD

LSF

HSF

==

186 7

–. %

(12)

The maximum rms current of the high-side MOSFET during

normal operation is:

I

I

D

I

I

A

HSF MAX

O

HSF

LRIPPLE

O

()

()

.

=¥ +

¥

Ê

ËÁ

ˆ

¯˜

=

2

1

3

98

2

2

(13)

The maximum rms current of the low-side MOSFET during

normal operation is:

II

D

D

A

LSF MAX

HSF MAX

LSF

HSF

()

()

== 25

(14)

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

MOSFET dissipation, the total dissipation in the four MOSFETs

of the 2-phase converter will be:

PV

I

PV

A

W

MOSFET TOTAL

MIN

O

MOSFET TOTAL

()

()

.

..

.

.

=¥

¥

=¥

¥

=

01

01 157

534

84

(15)

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

MOSFETs and half for the pair of low-side MOSFETs, and

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

MOSFET are equal, the required maximum MOSFET resis-

tances will be:

R

P

I

R

W

A

m

DS ON HS MAX

MOSFET TOTAL

HSF MAX

DS ON HS MAX

()

(

)

()

()

()

(

)

.

(.

)

=

¥

=

¥

=W

8

84

89 8

11

2

2

(16)

R

P

I

R

W

A

m

DS ON LS MAX

MOSFET TOTAL

LSF MAX

DS ON LS MAX

()

(

)

()

()

()

(

)

.

()

.

=

¥

=

¥

=W

4

84

425

34

2

2

(17)

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. If efficiency is

W

nominal, 10 m

W worst case) for the high-side and a Fairchild

W nominal, 5.6 mW worst case)

for the low-side are good choices. The high-side MOSFET

dissipation is:

PR

I

VI

Q

f

I

VQ

f

HSF

DS ON HS

HFS MAX

IN

L PK

G

SW

G

IN

RR

SW

=¥

¥¥

¥

¥

+¥

¥

()

()

()

()

2

2

(18)

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.