REV. A

ADP3158/ADP3178

–7–

CT Selection for Operating Frequency

The ADP3158 and ADP3178 use a constant off-time architecture

time the high-side N-channel MOSFET switch turns on, the volt-

age across CT is reset to 0 V. During the off-time, CT is charged

by a constant current of 150

µA. Once CT reaches 3.0 V, a new

on-time cycle is initiated. The value of the off-time is calculated

using the continuous-mode operating frequency. Assuming a

age of 1.7 V, the corresponding off-time is:

t

V

Vf

t

V

V

kHz

s

OFF

OUT

IN

NOM

OFF

= 







×

=−









×=

1

1

1

17

5

1

200

33

–

.

.

µ

(1)

The timing capacitor can be calculated from the equation:

C

tI

V

sA

V

pF

T

OFF

CT

TTH

=

×

=

µ×

µ ≈

()

.

3 3

150

3

150

(2)

(3)

f

t

VI

R

R

R

V

VI

R

R

R

R

MIN

OFF

IN

O MAX

DS ON HSF

SENSE

L

OUT

IN

O MAX

DS ON HSF

SENSE

L

DS ON LSF

=×

×+

+

×+

+

1

–

() –

–

(

–

)

()

(

)

(

)

()

()

)

The converter only operates at the nominal operating frequency

due to the parasitic voltage drops across the power devices. The

195 kHz (see Equation 3), where:

(estimated value: 14 m

Ω)

(estimated value: 6 m

Ω)

(estimated value: 4 m

Ω)

(estimated value: 3 m

Ω)

Inductance Selection

The choice of inductance determines the ripple current in the

inductor. Less inductance leads to more ripple current, which

increases the output ripple voltage and the conduction losses in

the MOSFETs, but allows using smaller-size inductors and, for

a specified peak-to-peak transient deviation, output capacitors

with less total capacitance. Conversely, a higher inductance means

lower ripple current and reduced conduction losses, but requires

larger-size inductors and more output capacitance for the same

peak-to-peak transient deviation. The following equation shows

the relationship between the inductance, oscillator frequency,

peak-to-peak ripple current in an inductor and input and

output voltages.

L

Vt

I

OUT

OFF

L RIPPLE

=

×

()

(4)

For 4 A peak-to-peak ripple current, which corresponds to

approximately 25% of the 15 A full-load dc current in an inductor,

Equation 4 yields an inductance of

L

Vs

A

H

=

×µ =µ

17

33

4

14

..

.

A 1.5

µH inductor can be used, which gives a calculated ripple

current of 3.8 A at no load. The inductor should not saturate at

the peak current of 17 A and should be able to handle the sum

of the power dissipation caused by the average current of 15 A

in the winding and the core loss.

Designing an Inductor

Once the inductance is known, the next step is either to design an

inductor or find a standard inductor that comes as close as

possible to meeting the overall design goals. The first decision

in designing the inductor is to choose the core material. There

are several possibilities for providing low core loss at high frequen-

cies. Two examples are the powder cores (e.g., Kool-M

Magnetics, Inc.) and the gapped soft ferrite cores (e.g., 3F3 or 3F4

from Philips). Low frequency powdered iron cores should be

avoided due to their high core loss, especially when the inductor

value is relatively low and the ripple current is high.

Two main core types can be used in this application. Open

magnetic loop types, such as beads, beads on leads, and rods

and slugs, provide lower cost but do not have a focused mag-

netic field in the core. The radiated EMI from the distributed

magnetic field may create problems with noise interference in

the circuitry surrounding the inductor. Closed-loop types, such

as pot cores, PQ, U, and E cores, or toroids, cost more, but

have much better EMI/RFI performance. A good compromise

between price and performance are cores with a toroidal shape.