AAT2505
Dual Channel, Step-Down
Converter/Linear Regulator
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:
To limit the bias current required for the external
feedback resistor string while maintaining good
noise immunity, the minimum suggested value for
R2 is 59kΩ. Although a larger value will further
reduce quiescent current, it will also increase the
impedance of the feedback node, making it more
sensitive to external noise and interference. Table 2
summarizes the resistor values for various output
voltages with R2 set to either 59kΩ for good noise
immunity or 221kΩ for reduced no load input current.
3
·
VDROOP FS
ΔILOAD
V
V
1.5V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
- 1 ·
OUT -1
·
R2 =
59kΩ = 88.5kΩ
COUT
=
R1 =
·
⎠
⎠
REF
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 adjustable version of the AAT2505, combined
with an external feedforward capacitor (C8 in
Figures 2 and 5), delivers enhanced transient
response for extreme pulsed load applications.
The addition of the feedforward capacitor typically
requires a larger output capacitor C1 for stability.
The internal voltage loop compensation 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.
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ
R1 (kΩ)
VOUT (V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
75
113
150
187
221
261
301
332
442
464
523
715
1000
The maximum output capacitor RMS ripple current
is given by:
1
V
OUT · (VIN(MAX) - VOUT
FS VIN(MAX)
)
IRMS(MAX)
=
·
L
·
·
2 · 3
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.
124
137
187
267
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.
Table 2: Adjustable Resistor Values For Use
With 0.6V Step-Down Converter.
2505.2006.06.1.1
17
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