AAT2511 Datasheet, PDF (12/20 Pages) Advanced Analogic Technologies – Dual 600mA, 1MHz Step-Down DC/DC Converter

AAT2511
Dual 600mA, 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.
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:
C_OUT
=
3 · ∆I_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
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 · V_IN(MAX)
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 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 1 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.
R1
=
⎛ ^VOUT
⎝ V_REF
-1^⎞⎠
·
R2
=
⎛ ^1.5V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
88.5kΩ
The adjustable version of the AAT2511 in combina-
tion with an external feedforward capacitor (C4 and
C5 of Figure 1) 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.
V_OUT (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
R2, R4 = 59kΩ
R1, R3 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
R2, R4 = 221kΩ
R1, R3 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
Table 2: Adjustable Resistor Values
For Use With 0.6V Version.
12
2511.2006.02.1.2