AAT1149 datasheet(13/20 Pages) ANALOGICTECH

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AAT1149

3MHz Fast Transient

400mA Step-Down Converter

1149.2006.11.1.0

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:

OUT · (VIN(MAX) - VOUT)

RMS(MAX)

L · FS · VIN(MAX)

=·

COUT =

3 · ΔILOAD

VDROOP · FS

Figure 1: AAT1149 Evaluation Board

Figure 2: Exploded View of Evaluation

Top Side.

Board Top Side.

Figure 3: AAT1149 Evaluation Board

Bottom Side.

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