AAT1149 Datasheet, PDF (13/20 Pages) Advanced Analogic Technologies – 3MHz Fast Transient 400mA Step-Down Converter

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:
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
Once the average inductor current increases to the
DC load level, the output voltage recovers. The
above equation establishes a limit on the minimum
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
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:
I = _RMS(MAX)
1
2·
·
3
V_OUT · (V_IN(MAX) - V_OUT)
L · F_S · V_IN(MAX)
Figure 1: AAT1149 Evaluation Board
Top Side.
Figure 2: Exploded View of Evaluation
Board Top Side.
Figure 3: AAT1149 Evaluation Board
Bottom Side.
1149.2006.11.1.0
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