REV. A

ADP3159/ADP3179

–9–

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

L RIPPLE

≤

+

=

+

=Ω

()(

)

()

.

2

69

15

1 9

4

(8)

In this case, 4 m

Ω was chosen as the closest standard value.

the maximum current sense threshold of 87 mV:

I

V

R

I

mV

m

A

A

OUT CL

CS CL

MAX

SENSE

L RIPPLE

()

()(

)

(

)

–

–

.

=

=

Ω

≈

2

87

4

38

2

20

(9)

At output voltages below 450 mV, the current sense threshold is

reduced to 54 mV, and the ripple current is negligible. There-

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

I

mV

m

A

OUT SC

()

.

=

Ω

=

54

4

13 5

(10)

To safely carry the current under maximum load conditions, the

sense resistor must have a power rating of at least:

PI

R

A

m

W

R

O

SENSE

SENSE

=×

=

×

Ω =

()

(

)

.

22

20

4

1 6

(11)

Power MOSFETs

Two external N-channel power MOSFETs must be selected for

use with the ADP3159, one for the main switch and an identical

one for the synchronous switch. The main selection parameters

The minimum input voltage dictates whether standard threshold

requirement for the two power MOSFETs. When the ADP3159

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

D

kHz

s

HSF MAX

MIN

OFF

HSF MAX

()

()

– ()

–

.%

=×

=×

µ

1

1

195

3 3

36

(12)

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

MOSFET is:

DD

LSF MAX

HSF MAX

()

()

–

%

==

154

(13)

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

ID

II

I

I

I

AA

A

A

A rms

RMSHSF

HSF MAX

L VALLEY

L VALLEY

L PEAK

L PEAK

RMSHSF

=×

+×

+

=×

+×+

=

()

()

()

(

)

(

)

()

%

.(

.

.

)

.

.

22

22

3

36

13 1

13 1

16 1

16 1

3

88

(14)

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

ID

II

I

I

I

AA

A

A

A rms

RMSLSF

LSF MAX

L VALLEY

L VALLEY

L PEAK

L PEAK

RMSLSF

=×

+×

+

=×

+×+

=

()

()

()

(

)

(

)

%

.(

.

.

)

.

.

22

22

3

54

13 1

13 1

16 1

16 1

3

10 8

(15)

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

MOSFET dissipation, the total dissipation will be:

PV

I

W

D FETs

OUT

OUT MAX

()

(

)

..

=×

×

=

01

2 26

(16)

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

and half for the low-side MOSFET and assuming that switching

losses are small relative to the dc conduction losses, the required

minimum MOSFET resistances will be:

R

P

I

W

A

m

DS ON HSF

HSF

HSF

()

.

.

≤=

=

Ω

2

2

113

88

15

(17)

R

P

I

W

A

m

DS ON LSF

LSF

LSF

()

.

.

≤=

=

Ω

2

2

113

10 8

10

(18)

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 not a

10 m

Ω nominal, 16 mΩ worst-case) for the high-side and a

Ω nominal,

10 m

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

The high-side MOSFET dissipation is:

PI

R

VI

Q

f

I

PA

m

V

A

nC

kHz

A

W

DHSF

RMSHSF

DS ON

IN

L PEAK

G

MIN

G

DHSF

=×

+

××

×

×

=×

Ω +

××

×

×

=

2

2

2

88

16

5

15

70

195

21

175

()

()

..

(19)

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

the ADP3159 is about 1 A.

The low-side MOSFET dissipation is:

PI

R

PA

m

W

DLSF

RMSLSF

DS ON

DLSF

=×

=×

Ω =

2

2

10 8

10

1 08

()

..

(20)

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