ADP3192A datasheet(20/32 Pages) AD

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ADP3192A

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If the resulting ripple voltage is less than what is designed for,

the inductor can be made smaller until the ripple value is met.

This allows optimal transient response and minimum output

decoupling.

The smallest possible inductor should be used to minimize

the number of output capacitors. For this example, choosing a

320 nH inductor is a good starting point and gives a calculated

ripple current of 11 A. The inductor should not saturate at the

peak current of 35.5 A and should be able to handle the sum of

the power dissipation caused by the average current of 30 A in

the winding and core loss.

Another important factor in the inductor design is the dc

resistance (DCR), which is used for measuring the phase currents.

A large DCR can cause excessive power loss, though too small a

value can lead to increased measurement error. A good rule is

to have the DCR (RL) be about 1 to 1½ times the droop resistance

(RO). This example uses an inductor with a DCR of 1.4 mΩ.

Designing an Inductor

Once the inductance and DCR are known, the next step is to

either design an inductor or find a standard inductor that comes

as close as possible to meeting the overall design goals. It is also

important to have the inductance and DCR tolerance specified

to control the accuracy of the system. Reasonable tolerances

most manufacturers can meet are 15% inductance and 7% DCR

at room temperature. The first decision in designing the inductor

is choosing the core material. Several possibilities for providing

low core loss at high frequencies include the powder cores (from

Micrometals, Inc., for example, or Kool Mu® from MAGNETICS®)

and the gapped soft ferrite cores (for example, 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.

The best choice for a core geometry is a closed-loop type, such

as a potentiometer core (PQ, U, or E core) or toroid. A good

compromise between price and performance is a core with

a toroidal shape.

Many useful magnetics design references are available for

quickly designing a power inductor, such as

•

Intusoft Magnetics Designer Software

•

Designing Magnetic Components for High Frequency dc-dc

Converters

, by Colonel Wm. T. McLyman,

Kg Magnetics, Inc., ISBN 1883107008

Selecting a Standard Inductor

The following power inductor manufacturers can provide design

consultation and deliver power inductors optimized for high

power applications upon request:

•

Coilcraft®

•

Coiltronics®

•

Sumida Corporation®

CURRENT SENSE AMPLIFIER

Most designs require the regulator output voltage, measured at

the CPU pins, to drop when the output current increases. The

specified voltage drop corresponds to a dc output resistance (RO),

also referred to as a load line. The ADP3192A has the flexibility

of adjusting RO, independent of current-limit or compensation

components, and it can also support CPUs that do not require

a load line.

For designs requiring a load line, the impedance gain of the

CS amplifier (RCSA) must be to be greater than or equal to the load

line. All designs, whether they have a load line or not, should

keep RCSA ≥ 1 mΩ.

The output current is measured by summing the voltage across

each inductor and passing the signal through a low-pass filter.

This summer filter is the CS amplifier configured with resistors

RPH(X) (summers), and RCS and CCS (filter). The impedance gain

of the regulator is set by the following equations, where RL is the

DCR of the output inductors:

()

CSA

(6)

(7)

The user has the flexibility to choose either RCS or RPH(X). However,

it is best to select RCS equal to 100 kΩ, and then solve for RPH(X)

by rearranging Equation 6. Here, RCSA = RO = 1 mΩ because this

is equal to the design load line.

()

140

100

mΩ

CSA

Next, use Equation 7 to solve for CCS.

100

320

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