AAT2513 datasheet(12/22 Pages) ANALOGICTECH

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AAT2513

Dual 600mA Step-Down

Converter with Synchronization

2513.2007.04.1.1

Table 1 displays the suggested inductor values for

the AAT2513.

Manufacturer's specifications list both the inductor

DC current rating, which is a thermal limitation, and

the peak current rating, which is determined by the

inductor's saturation characteristics. The inductor

should not show any appreciable saturation under

all normal load conditions. Some inductors may

meet the peak and average current ratings yet

result in excessive losses due to a high DCR.

Always consider the losses associated with the

DCR and its effect on the total converter efficiency

when selecting an inductor.

The 2.2uH CDRH2D11 series inductor selected

from Sumida has a 98m

Ω DCR and a 1.27A DC

current rating. At full load the inductor DC loss is

35mW which corresponds to a 3.2% loss in effi-

ciency for a 600mA, 1.8V output.

Input Capacitor

A key feature of the AAT2513 is that the funda-

mental switching frequency ripple at the input can

be reduced by operating the two converters 180°

out of phase. This reduces the input ripple by

roughly half, reducing the required input capaci-

tance. An X5R ceramic input capacitor as small as

1µF is often sufficient. To estimate the required

input capacitor size, determine the acceptable

input ripple level (V

PP) and solve for C. The calcu-

lated value varies with input voltage and is a maxi-

mum when V

IN is double the output voltage.

This equation provides an estimate for the input

capacitor required for a single channel.

The equation below solves for the input capacitor

size for both channels. It makes the worst case

assumption that both converters are operating at

50% duty cycle with in phase synchronization.

Because the AAT2513 channels will generally

operate at different duty cycles the actual ripple will

vary and be less than the ripple (V

PP) used to solve

for the input capacitor in the above equation.

Always examine the ceramic capacitor DC voltage

coefficient characteristics when selecting the prop-

er value. For example, the capacitance of a 10µF

6.3V X5R ceramic capacitor with 5V DC applied is

actually about 6µF.

The maximum input capacitor RMS current is:

The input capacitor RMS ripple current varies with

the input and output voltage and will always be less

than or equal to half of the total DC load current of

both converters combined.

O1(MAX) + IO2(MAX)

RMS(MAX)

⎛⎞

RMS = IO1 ·

· 1 -

+ I

O2 ·

· 1 -

⎝⎠

⎛⎞

⎝⎠

⎛⎞

⎝⎠

⎛⎞

⎝⎠

IN =

⎛⎞

- ESR

4 · F

⎝⎠

O1 + IO2

⎛⎞

⋅ 1 -

⎝⎠

IN =

⎛⎞

- ESR

⋅ F

⎝⎠

Table 1: Inductor Values.

Configuration

Output Voltage

Inductor

Slope Compensation

0.6V adjustable

0.6V-2.0V

2.2µH

with external

2.5V

3.3µH

0.6A/µs

resistive divider

3.3V

4.7µH

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