ADP3152

–8–

REV. 0

The output filter capacitor must have an ESR of less than

5.9 m

Ω. One can use, for example, six FA type capacitors from

Panasonic, with 2700

µF capacitance, 10 V voltage rating, and

34 m

Ω ESR. The six capacitors have a total typical ESR of

~ 5 m

Ω when connected in parallel.

Inductor Selection

The minimum inductor value can be calculated from ESR, off

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

=

=

2.8

× 2.2 µ× 5.9 m

14 m

= 2.6 µH

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 2.6

µH at full load current and should not saturate at the

worst-case overload or short circuit current at the maximum

specified ambient temperature. A suitable inductor is the

CTX12-13855 from Coiltronics, which is 4.4

µH at 1 A and

about 2.5

µH at 14.2 A.

Tips for Selecting 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

lowest cost core is made of powdered iron, for example the #52

material from Micrometals, Inc., but yields the largest size

inductor.

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 which 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.

=

=

14.2–0.8

5.9 m (2.2 / 4. 4

µH )

= 4.5 mF

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

divided by the maximum inductance (4.4

µH) of the CTX12-

13855 inductor from Coiltronics. The parallel-connected six

2700

µF/10 V FA series capacitors from Panasonic have a total

capacitance of 16,200

µF, so the minimum capacitance require-

ment is met with ample margin.

The current comparator of the ADP3152 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, tran-

sients and inductor core saturation.

The current comparator threshold sets the peak of the inductor

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

ADP3152 and in the external component values yields:

Ω

A practical solution is to use three 20 m

Ω resistors in parallel,

with an effective resistance of about 6.7 m

Ω.

Ω) = 21.5 A

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:

≈

×1V

360 k

Ω

+ 2 µA

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

70

µs. During that off time the inductor current gradually de-

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

the output circuit. With an inductance of 2.5

µH and total resis-

tance of 23 m

Ω, the time constant will be 108 µs, which yields a

valley current of 11.3 A and an average short-circuit current of

about 16.3 A. To safely carry the short-circuit current, the sense

resistor must have a power rating of at least 16.3 A

2

× 6.8 mΩ =

1.8 W.

Current Transformer Option

An alternative to using 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. If we use a transformer

with one primary and 50 secondary turns, the worst-case resistor

dissipation is reduced to a fraction of a mW. Another advantage

of using this option is the separation of the current and voltage

sensing, which makes the voltage sensing more accurate.

Power MOSFET

Two external N-channel power MOSFETs must be selected for

use with the ADP3152, one for the main switch, and an identi-

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