AAT2500 datasheet(14/26 Pages) ANALOGICTECH

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AAT2500

1MHz Step-Down Converter/LDO Regulator

2500.2006.05.1.16

Low Dropout Operation

For conditions where the input voltage drops to the

output voltage level, the converter duty cycle

increases to 100%. As 100% duty cycle is

approached, the minimum off-time initially forces

the high side on-time to exceed the 1MHz clock

cycle and reduce the effective switching frequency.

Once the input drops below the level where the out-

put can be regulated, the high side P-channel

MOSFET is turned on continuously for 100% duty

cycle. At 100% duty cycle, the output voltage tracks

the input voltage minus the IR drop of the high side

P-channel MOSFET RDS(ON).

Low Supply

The under-voltage lockout (UVLO) guarantees suf-

ficient VIN bias and proper operation of all internal

circuitry prior to activation.

Fault Protection

For overload conditions, the peak inductor current is

limited. Thermal protection disables switching when

the internal dissipation or ambient temperature

becomes excessive. The junction over-temperature

threshold is 140°C with 15°C of hysteresis.

Applications Information

Linear Regulator

Input and Output Capacitors: An input capacitor

is not required for basic operation of the linear reg-

ulator. However, if the AAT2500 is physically locat-

ed more than three centimeters from an input

power source, a CIN capacitor will be needed for

stable operation. Typically, a 1µF or larger capaci-

tor is recommended for CIN in most applications.

CIN should be located as closely to the device VIN

pin as practically possible.

An input capacitor greater than 1µF will offer supe-

rior input line transient response and maximize

power supply ripple rejection. Ceramic, tantalum,

or aluminum electrolytic capacitors may be select-

ed for CIN. There is no specific capacitor ESR

requirement for CIN. However, for 300mA LDO reg-

ulator output operation, ceramic capacitors are rec-

ommended for CIN due to their inherent capability

over tantalum capacitors to withstand input current

surges from low impedance sources such as bat-

teries in portable devices.

For proper load voltage regulation and operational

stability, a capacitor is required between OUT and

GND. The COUT capacitor connection to the LDO

regulator ground pin should be made as directly as

practically possible for maximum device perform-

ance. Since the regulator has been designed to

function with very low ESR capacitors, ceramic

capacitors in the 1.0µF to 10µF range are recom-

mended for best performance. Applications utilizing

Figure 1: AAT2500 Fixed Output.

Figure 2: AAT2500 with Adjustable Step-Down

Output and Enhanced Transient Response.

4.7

μF

VIN

59k

4.7

μF

10nF

VOUTLDO

VOUTBUCK

100pF

PGND

VCC

ENLDO

SGND

VLDO

OUT

GND

BYP

AAT2500

4.7

μF

VOUTBUCK

VIN

4.7

μF

10nF

VOUTLDO

PGND

VCC

ENLDO

SGND

VLDO

OUT

GND

BYP

AAT2500

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