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ADP3171

Synchronous Buck Controller with Dual Linear Regulator Controllers

Analog Devices 

ADP3171

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 equa-

tion shows the relationship between the inductance, oscillator

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

and output voltages:

L = VOUT × tOFF

I L( RIPPLE )

(4)

For 2.5 A peak-to-peak ripple current, which corresponds to

approximately 50% of the 5 A full-load dc current in an inductor,

Equation 4 yields an inductance of:

L = 1.5V × 3.5 µs = 2.1 µH

2.5 A

A 1.7 µH inductor can be used, which gives a calculated ripple

current of 3 A at no load. The inductor should not saturate at

the peak current of 8 A and should be able to handle the sum of

the power dissipation caused by the average current of 5 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 Mu® from

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 magnetic

field in the core. The radiated EMI from the distributed mag-

netic 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 I gives some examples.

Table I. Magnetics Design References

Magnetic Designer Software

Intusoft (www.intusoft.com)

Designing Magnetic Components for High-Frequency DC-

DC Converters; by William T. McLyman, Kg Magnetics

ISBN 1-883107-00-08

Selecting a Standard Inductor

The companies listed in Table II can provide design consultation

and deliver power inductors optimized for high power applications

upon request.

Table II. Power Inductor Manufacturers

Coilcraft

(847) 639-6400

www.coilcraft.com

Coiltronics

(561) 752-5000

www.coiltronics.com

Sumida Electric Company

(510) 668-0660

www.sumida.com

Vishay-Dale

(203) 452-5664

www.vishay.com

RSENSE

The value of RSENSE is based on the required maximum output

current. The current comparator of the ADP3171 has a mini-

mum threshold of 69 mV. Note that this minimum value cannot

be used for the maximum specified nominal current, as head-

room is needed for ripple current and transients.

The current comparator threshold sets the peak of the inductor

current yielding a maximum output current, IO(MAX), which

equals the peak value less half of the peak-to-peak ripple cur-

rent. Solving for RSENSE allowing a 20% margin for overhead

and using the minimum current sense threshold of 69 mV yields:

RSENSE =

VCS (TH )( MIN )

IO( MAX )

+

IRIPPLE

2

= 69 mV = 10.6 mΩ

5A+ 3A

2

(5)

In this case, 7.5 m⍀ was chosen to provide ample headroom.

Once RSENSE has been chosen, the maximum output current at

the point where current limit is reached, IOUT(CL), can be calcu-

lated using the maximum current sense threshold of 87 mV:

IOUT (CL )

= VCS (TH )( MAX )

RSENSE

–

IL(RIPPLE )

2

(6)

IOUT (CL )

=

87 mV –

7.5 mΩ

3A

2

= 10.1 A

At output voltages below 450 mV, the current sense threshold is

reduced to 54 mV, and the ripple current is negligible. Therefore,

the worst-case dead short output current is reduced to:

IOUT (SC )

=

VCS (SC )

RSENSE

= 54 mV = 7.2 A

7.5 mΩ

(7)

To safely carry the current under maximum load conditions, the

sense resistor must have a power rating of at least:

PRSENSE = IO2 × RSENSE = 5 A2 × 7.5 mΩ = 188 mW

(8)

REV. 0

–7–

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