AAT2510
Dual 400mA, 1MHz Step-Down DC-DC Converter
lize the output during large load transitions and has
the ESR and ESL characteristics necessary for low
output ripple.
Adjustable Output Resistor Selection
For applications requiring an adjustable output volt-
age, the 0.6V version can be programmed exter-
nally. Resistors R1 through R4 of Figure 2 program
the output to regulate at a voltage higher than 0.6V.
To limit the bias current required for the external
feedback resistor string, the minimum suggested
value for R2 and R4 is 59kΩ. Although a larger
value will reduce the 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 val-
ues for various output voltages with R2 and R4 set
to either 59kΩ for good noise immunity or 221kΩ
for reduced no load input current.
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. As the loop
responds, the inductor current increases to match
the load current demand. This typically takes two
to three switching cycles and can be estimated by:
3 · ∆ILOAD
=
COUT
V
DROOP · FS
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.
V
V
1.5V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
⎠
R1 =
OUT -1 · R2 =
- 1 · 59kΩ = 88.5kΩ
⎠
REF
The adjustable version of the AAT2510 in combina-
tion with an external feedforward capacitor (C4 and
C5 of Figure 2) delivers enhanced transient
response for extreme pulsed load applications. The
addition of the feedforward capacitor typically
requires a larger output capacitor (C1 and C2) for
stability.
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:
R2, R4 = 59kΩ R2, R4 = 221kΩ
VOUT (V)
R1, R3 (kΩ)
R1, R3 (kΩ)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
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
1
V
OUT · (VIN(MAX) - VOUT
)
IRMS(MAX)
=
·
L · F · VIN(MAX)
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
Table 2: Adjustable Resistor Values
For Use With 0.6V Version.
12
2510.2005.08.1.5
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
