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ADP3161

4-Bit Programmable 2-Phase Synchronous Buck Controller

Analog Devices 

ADP3161

CT Selection—Choosing the Clock Frequency

The ADP3161 uses a fixed-frequency control architecture. The

frequency is set by an external timing capacitor, CT. The value of

CT for a given clock frequency can be selected using the graph in

TPC 2.

The clock frequency determines the switching frequency, which

relates directly to switching losses and the sizes of the inductors

and input and output capacitors. A clock frequency of 400 kHz

sets the switching frequency of each phase, fSW, to 200 kHz,

which represents a practical trade-off between the switching

losses and the sizes of the output filter components. From

TPC 2, for 400 kHz the required timing capacitor value is 150 pF.

For good frequency stability and initial accuracy, it is recom-

mended to use a capacitor with low temperature coefficient and

tight tolerance, e.g., an MLC capacitor with NPO dielectric and

with 5% or less tolerance.

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. In a two-phase converter a practi-

cal value for the peak-to-peak inductor ripple current is under

50% of the dc current in the same inductor. A choice of 46%

for this particular design example yields a total peak-to-peak output

ripple current of 23% of the total dc output current. The follow-

ing equation shows the relationship between the inductance,

oscillator frequency, peak-to-peak ripple current in an inductor

and input and output voltages.

L = (VIN –VAVG ) ×VAVG

VIN × fSW × IL(RIPPLE )

(1)

For 6 A peak-to-peak ripple current, which corresponds to

just under 50% of the 13 A full-load dc current in an induc-

tor, Equation 1 yields an inductance of

L = (5V – 1.780V ) × 1.780V = 955 nH

5V × 400 kHz/2 × 6 A

A 1 µH inductor can be used, which gives a calculated ripple

current of 5.7 A at no load. The inductor should not saturate at

the peak current of 18.7 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.

The output ripple current is smaller than the inductor ripple

current due to the two phases partially canceling. This can be

calculated as follows:

IO∆

=

2 ×VAVG

VIN

(VIN – 2 ×VAVG )

× L × fOSC

=

2 × 1.780V × (5V – 2 × 1.780V ) = 2.6 A

(2)

5V × 1 µH × 400 kHz

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-Mµ® from

Magnetics) 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 mag-

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

slugs, provide lower cost but do not have a focused magnetic

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.

There are many useful references for quickly designing a power

inductor. Table II gives some examples.

Table II. Magnetics Design References

Magnetic Designer Software

Intusoft (http://www.intusoft.com)

Designing Magnetic Components for High-Frequency DC-DC

Converters

McLyman, Kg Magnetics

ISBN 1-883107-00-08

Selecting a Standard Inductor

The companies listed in Table III can provide design consul-

tation and deliver power inductors optimized for high power

applications upon request.

Table III. Power Inductor Manufacturers

Coilcraft

(847) 639-6400

http://www.coilcraft.com

Coiltronics

(561) 752-5000

http://www.coiltronics.com

Sumida Electric Company

(408) 982-9660

http://www.sumida.com

COUT Selection—Determining the ESR

The required equivalent series resistance (ESR) and capacitance

drive the selection of the type and quantity of the output capaci-

tors. The ESR must be small enough to contain the voltage

deviation caused by a maximum allowable CPU transient cur-

rent within the specified voltage limits, giving consideration also

to the output ripple and the regulation tolerance. The capaci-

tance must be large enough that the voltage across the capacitor,

which is the sum of the resistive and capacitive voltage deviations,

does not deviate beyond the initial resistive deviation while the

inductor current ramps up or down to the value corresponding

to the new load current. The maximum allowed ESR also repre-

sents the maximum allowed output resistance, ROUT.

REV. 0

–7–

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