IRF7401 datasheet(14/22 Pages) AD

14 / 22 page

REV.

–14–

ADN8830

Inductor Selection

In addition to the external transistors, the PWM amplifier requires

an inductor and a capacitor at its output to filter the switched

output waveform. Proper inductor selection is important to

achieve the best efficiency. The duty cycle of the PWM sets the

OUT A output voltage and is

OUT A

(22)

The average current through the inductor is equal to the TEC

current. The ripple current through the inductor,

IL, varies

with the duty cycle and is equal to

ΔI

CLK

××

()

1–

(23)

where fCLK is the clock frequency as set by the resistor RFREQ at

Pin 26 or an external clock frequency. Refer to the Setting the

Switching Frequency section for more information. Selecting a

faster switching frequency or a larger value inductor will reduce

the ripple current through the inductor. The waveform of the

inductor current is shown in Figure 13.

TIME

ITEC

ΔIL

fCLK

T =

Figure 13. Current Waveform Through Inductor

It is important to select an inductor that can tolerate the maxi-

mum possible current that could pass through it. Most TECs

are specified with a maximum voltage and current for proper

and reliable operation. The maximum instantaneous inductor

current can be found as

L MAX

TEC MAX

(24)

where

IL can be found from Equation 23 with the appropriate

duty cycle calculated from Equation 22 with OUT A = VTEC, MAX.

Design Example 3

A TEC is specified with a maximum current of 1.5 A and maxi-

mum voltage of 2.5 V. The ADN8830 will be operating from a

3.3 V supply voltage with a 200 kHz clock and a 4.7

μH inductor.

The duty cycle of the PWM amplifier at 2.5 V is calculated to be

75.8%. Using Equation 23, the inductor ripple current is found

to be 664 mA. From Equation 24, the maximum inductor current

will be 1.82 A and should be considered when selecting the

inductor. Notice that increasing the clock frequency to 1 MHz would

reduce IL, MAX to 1.56 A.

Design Example 4

Using the same TEC as above, the ADN8830 will be powered

from 5.0 V instead. Here, the duty cycle is 50%, which happens

to be the worst-case duty cycle for inductor current ripple. Now

DIL equals 1.33 A with a 200 kHz clock, and IL, MAX is 2.83 A.

Reducing the inductor ripple current is another compelling

reason to operate the ADN8830 from a 3.3 V supply instead.

Table II lists some inductor manufacturers and part numbers

along with some key specifications. The column IMAX refers to the

maximum current at which the inductor is rated to remain linear.

Although higher currents can be pushed through the inductor,

efficiency and ripple voltage will be dramatically degraded.

This is by no means a complete list of manufacturers or inductors

that can be used in the application. More information on these

inductors is available at their websites. Note the trade-offs

between inductor height, maximum current, and series resistance.

Smaller inductors cannot handle as muèH current and therefore

require higher clock speeds to reduce their ripple current. They

also have higher series resistance, which can lower the overall

efficiency of the ADN8830.

PWM Output Filter Requirements

The switching of Q1 and Q2 creates a pulse width modulated

(PWM) square wave from 0 V to VDD. This square wave must

be filtered sufficiently to create a steady voltage that will drive

the TEC. The ripple voltage across the TEC is a function of the

inductor ripple current, the L-C filter cutoff frequency, and the

equivalent series resistance (ESR) of the filter capacitor. The

equivalent circuit for the PWM side is given in Figure 14.

Table II. Partial List of Inductors and Key Specifications

Inductance ( H)

IMAX (A)

RS, TYP (m )Height (mm)

Part Number

Manufacturer

Website

4.71.1

200

LPO1704-472M

Coilcraft

www.coilcraft.com

4.71.59

A918CY-4R7M

Toko

www.toko.com

4.73.948

2.8

UP2.8B-4R7

Cooper

www.cooperet.com

4.71.5

DO1608C-472

Coilcraft

www.coilcraft.com

4.71.32

CDRH4D28 4R7

Sumida

www.sumida.com

4.77.512

4.5

892NAS-4R7M

Toko

www.toko.com

4.7

5.418

5.2

DO3316P-472

Coilcraft

www.coilcraft.com

2.780

2.8

UP2.8B-100

Cooper

www.cooperet.com

DO5022P-153HC

Coilcraft

www.coilcraft.com

4.586

7.1

DO5022P-473

Coilcraft

www.coilcraft.com

*Recommend inductor in typical application circuit Figure 1.

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