REV. A

ADP3154

–8–

The actual short-circuit current is less than the above calculated

output voltage drops below 1 V. The relationship between the

off-time and the output voltage is:

t

CV

V

k

A

OFF

T

O

≈

×

+

1

360

2

Ω

µ

With a short circuit across the output, the off-time will be about

70

µs. During that time the inductor current gradually decays.

The amount of decay depends on the L/R time constant in the

output circuit. With an inductance of 3.3

µH and total resis-

tance of 22 m

Ω, the time constant will be 108 µs. This yields an

average short-circuit current of about 20 A. To safely carry the

short-circuit current, the sense resistor must have a power rating

of at least 20 A

2

× 5.0 mΩ = 20 W.

Current Transformer Option

An alternative to using a low value and high power current sense

resistor is to reduce the sensed current by using a low cost cur-

rent transformer and a diode. The current can then be sensed

with a small-size, low cost SMT resistor. Using a transformer

with one primary and 50 secondary turns reduces the worst-case

resistor dissipation to a few mW. Another advantage of using

this option is the separation of the current and voltage sensing,

which makes the voltage sensing more accurate.

Power MOSFETs

Two external N-channel power MOSFETs must be selected for

use with the ADP3154, one for the main switch, and an identi-

cal one for the synchronous switch. The main selection param-

The minimum input voltage dictates whether standard threshold

requirement for the two power MOSFETs. When the ADP3154

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:

× t

× 3.0 µs) = 46%

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

FET is:

The maximum rms current of the high side FET is:

0.5

= 11.6 A rms

The maximum rms current of the low side FET is:

0.5

= 12.5 A rms

Inductor Selection

The minimum inductor value can be calculated from ESR, off-

time, dc output voltage and allowed peak-to-peak ripple voltage

using the following equation:

L

Vt

R

V

Vs

m

mV

H

MIN

O OFF

E MAX

RIPPLE p p

1

20

3

5 3

10

32

==

××

=

−

()

,

..

.

µ

µ

Ω

The minimum inductance gives a peak-to-peak ripple current of

The inductor peak current in normal operation is:

The inductor valley current is:

The inductor for this application should have an inductance

of 3.3

µH at full load current and should not saturate at the

worst-case overload or short circuit current at the maximum

specified ambient temperature.

Tips for Selecting the Inductor Core

Ferrite designs have very low core loss, so the design should

focus on copper loss and on preventing saturation. Molypermalloy,

or MPP, is a low loss core material for toroids, and it yields the

smallest size inductor, but MPP cores are more expensive than

ferrite cores or the Kool M

The minimum capacitance of the output capacitor is determined

from the requirement that the output be held up while the in-

ductor current ramps up (or down) to the new value. The mini-

mum capacitance should produce an initial dv/dt that is equal

(but opposite in sign) to the dv/dt obtained by multiplying the

di/dt in the inductor and the ESR of the capacitor:

C

II

Rdi dt

AA

mV

H

F

MIN

OMAX

OMIN

E

()

=

−

()

×

()

=µ

–.

/

.

./ .

0 8

17

1

0 8

52 0

3 0

3840

Ωµ

In the above equation the value of di/dt is calculated as the

connected 2200

µF capacitors have a total capacitance of

4400

µF, so the minimum capacitance requirement is met with

ample margin.

The current comparator of the ADP3154 has a threshold range

that extends from 0 mV to 125 mV (minimum). Note that the

full 125 mV range cannot be used for the maximum specified

nominal current, as headroom is needed for current ripple and

transients.

The current comparator threshold sets the peak of the inductor

the peak value less half of the peak-to-peak ripple current. Solv-

the minimum current sense threshold of 125 mV yields:

Ω

Ω) = 29 A