AAT2500 datasheet(15/26 Pages) ANALOGICTECH

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the exceptionally low output noise and optimum

power supply ripple rejection should use 2.2µF or

greater for COUT. In low output current applications,

where output load is less than 10mA, the minimum

value for COUT can be as low as 0.47µF.

Equivalent Series Resistance: ESR is a very

important characteristic to consider when selecting a

capacitor. ESR is the internal series resistance asso-

ciated with a capacitor that includes lead resistance,

internal connections, size and area, material compo-

sition, and ambient temperature. Typically, capacitor

ESR is measured in milliohms for ceramic capaci-

tors and can range to more than several ohms for

tantalum or aluminum electrolytic capacitors.

Bypass Capacitor and Low Noise

Applications

A bypass capacitor pin is provided to enhance the

low noise characteristics of the LDO. The bypass

capacitor is not necessary for operation; however,

for best device performance, a small ceramic

capacitor in the range of 470pF to 10nF should be

placed between the bypass pin (BYP) and the

device ground pin (GND). To practically realize the

highest power supply ripple rejection and lowest

output noise performance, it is critical that the

capacitor connection between the BYP pin and

GND pin be direct and PCB traces should be as

short as possible.

DC leakage on this pin can affect the LDO regula-

tor output noise and voltage regulation perform-

ance. For this reason, the use of a low leakage,

high quality ceramic (NPO or C0G type) or film

capacitor is highly recommended.

Step-Down Converter

Inductor Selection: The step-down converter

uses peak current mode control with slope com-

pensation to maintain stability for duty cycles

greater than 50%. The output inductor value must

be selected so the inductor current down slope

meets the internal slope compensation require-

ments. The internal slope compensation for the

adjustable and low-voltage fixed versions of the

AAT2500 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 10µH value is selected.

For high-voltage fixed versions (2.5V and above),

m = 0.48A/µsec. Table 1 displays inductor values

for the AAT2500 fixed and adjustable options.

0.75 ⋅ VO

L =

≈ 3

⋅ VO

= 3

⋅ 2.5V = 7.5μH

0.75 ⋅ VO

0.24A

μsec

0.75 ⋅ VO

m =

= 0.24

0.75 ⋅ 1.5V

4.7μH

μsec

AAT2500

1MHz Step-Down Converter/LDO Regulator

2500.2006.05.1.16

Table 1: Inductor Values.

Configuration

Output Voltage

Inductor

Slope Compensation

0.6V Adjustable With

0.6V to 2.0V

4.7µH

0.24A/µsec

External Resistive Divider

2.5V

10µH

0.24A/µsec

Fixed Output

0.6V to 2.0V

4.7µH

0.24A/µsec

2.5V to 3.3V

4.7µH

0.48A/µsec

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