REV. A

ADP3158/ADP3178

–9–

current. The current comparators of the ADP3158 and ADP3178

have a minimum current limit threshold of 69 mV. Note that the

69 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

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

calculated as:

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 ADP3158 and ADP3178, one for the main switch

and an identical one for the synchronous switch. The main

selection parameters for the power MOSFETs are the threshold

The minimum input voltage dictates whether standard threshold

requirement for the two power MOSFETs. When the ADP3158

and ADP3178 are operating in continuous mode, the simplifying

assumption can be made that one of the two MOSFETs is always

1.65 V, the 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

226

(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

22

113

88

15

(17)

R

P

I

W

A

m

DS ON LSF

LSF

LSF

()

.

.

≤=

=

Ω

22

113

10 8

10

(18)