AAT2500M
400mA Step-Down Converter and 300mA LDO
Figure 3: AAT2500M Evaluation Board Top Side.
Figure 4: AAT2500M Evaluation Board
Bottom Side.
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
capacitor. This dampens the high Q network and
stabilizes the system.
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 4.7ꢀF. 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.
Output Capacitor
The step-down converter output capacitor limits the
output ripple and provides holdup during large load
transitions. A 4.7ꢀF to 10ꢀF X5R or X7R ceramic
capacitor typically provides sufficient bulk capaci-
tance to stabilize the output during large load tran-
sitions and has the ESR and ESL characteristics
necessary for low output ripple.
The maximum output capacitor RMS ripple current
is given by:
1
V
OUT · (VIN(MAX) - VOUT
)
IRMS(MAX)
=
·
L · FOSC · VIN(MAX)
2 · 3
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:
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 Voltage Resistor
Selection
For applications requiring an adjustable output volt-
age (VO or VOUT), the 0.6V version can be externally
programmed. Resistors R1 and R2 of Figure 5 pro-
gram the output to regulate at a voltage higher than
0.6V. To limit the bias current required for the exter-
nal feedback resistor string while maintaining good
noise immunity, the minimum suggested value for R2
3 · ∆ILOAD
DROOP · FOSC
COUT
=
V
2500M.2007.06.1.0
15
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