AAT2500 datasheet(17/26 Pages) ANALOGICTECH

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In applications where the input power source lead

inductance cannot be reduced to a level that does

not affect the converter performance, a high ESR

tantalum or aluminum electrolytic should be placed

in parallel with the low ESR, ESL bypass ceramic.

This dampens the high Q network and stabilizes

the system.

Output Capacitor

The output capacitor limits the output ripple and

provides holdup during large load transitions. A

4.7µF to 10µF X5R or X7R ceramic capacitor typi-

cally provides sufficient bulk capacitance to stabi-

lize the output during large load transitions and has

the ESR and ESL characteristics necessary for low

output ripple.

The output voltage droop due to a load transient is

dominated by the capacitance of the ceramic out-

put capacitor. During a step increase in load cur-

rent, the ceramic output capacitor alone supplies

the load current until the loop responds. Within two

or three switching cycles, the loop responds and

the inductor current increases to match the load

current demand. The relationship of the output volt-

age droop during the three switching cycles to the

output capacitance can be estimated by:

Once the average inductor current increases to the

DC load level, the output voltage recovers. The

above equation establishes a limit on the minimum

value for the output capacitor with respect to load

transients.

The internal voltage loop compensation also limits

the minimum output capacitor value to 4.7µF. This

is due to its effect on the loop crossover frequency

(bandwidth), phase margin, and gain margin.

Increased output capacitance will reduce the

crossover frequency with greater phase margin.

The maximum output capacitor RMS ripple current

is given by:

Dissipation due to the RMS current in the ceramic

output capacitor ESR is typically minimal, resulting in

less than a few degrees rise in hot-spot temperature.

Adjustable Output Resistor Selection

For applications requiring an adjustable output volt-

age, the 0.6V version can be externally pro-

grammed. Resistors R1 and R2 of Figure 5 program

the output to regulate at a voltage higher than 0.6V.

To limit the bias current required for the external

feedback resistor string while maintaining good

noise immunity, the minimum suggested value for

VOUT · (VIN(MAX) - VOUT)

RMS(MAX)

L · F · VIN(MAX)

=·

COUT = 3 · ΔILOAD

VDROOP · FS

AAT2500

1MHz Step-Down Converter/LDO Regulator

2500.2006.05.1.16

Figure 3: AAT2500 Evaluation Board Top Side.

Figure 4: AAT2500 Evaluation Board

Bottom Side.

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