AAT2550 datasheet(23/35 Pages) ANALOGICTECH

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Step-Down Converter

Applications Information

Inductor Selection

The step-down converter uses peak current mode

control with slope compensation to maintain stability

for duty cycles greater than 50%. The output induc-

tor value must be selected so the inductor current

down slope meets the internal slope compensation

requirements. The internal slope compensation for

the adjustable and low-voltage fixed versions of the

AAT2550 is 0.24A/µsec. This equates to a slope

compensation that is 75% of the inductor current

down slope for a 1.5V output and 4.7µH inductor.

This is the internal slope compensation for the

adjustable (0.6V) version or low-voltage fixed ver-

sions. When externally programming the 0.6V ver-

sion to 2.5V, the calculated inductance is 7.5µH.

In this case, a standard 6.8µH value is selected.

For high-voltage fixed versions (

≥2.5V), m = 0.48A/

µsec. Table 5 displays inductor values for the

AAT2550 fixed and adjustable options.

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

saturation characteristics. The inductor should not

show any appreciable saturation under normal load

conditions. Some inductors may meet the peak and

average current ratings yet result in excessive loss-

es 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 Sumida 4.7µH CDRH3D16 series inductor has

a 105m

Ω DCR and a 900mA DC current rating. At

full load, the inductor DC loss is 38mW, which gives

a 4% loss in efficiency for a 600mA, 1.5V output.

Input Capacitor

Select a 4.7µF to 10µF X7R or X5R ceramic capac-

itor for the input. To estimate the required input

capacitor size, determine the acceptable input rip-

ple level (VPP) and solve for C. The calculated

value varies with input voltage and is a maximum

when VIN is double the output voltage.

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 5.0V DC applied

is actually about 6µF.

IN(MIN) =

⎛⎞

- ESR

4 · F

⎝⎠

⎛⎞

· 1 -

for V

IN = 2 · VO

⎝⎠

⎛⎞

· 1 -

⎝⎠

IN =

⎛⎞

- ESR

⎝⎠

0.75

⋅ V

L =

≈ 3

⋅ V

= 3

⋅ 2.5V = 7.5μH

0.75

⋅ V

0.24A

μsec

0.75

⋅ V

m =

= 0.24

0.75

⋅ 1.5V

4.7

μH

μsec

AAT2550

Total Power Solution for Portable Applications

2550.2006.07.1.0

Table 5: Inductor Values.

Configuration

Output Voltage

Inductor

0.6V Adjustable With

1V, 1.2V

2.2µH

External Feedback

1.5V, 1.8V

4.7µH

2.5V, 3.3V

6.8µH

Fixed Output

0.6V to 3.3V

4.7µH

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