AAT1121
1.5MHz, 250mA Step-Down Converter
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.
This dampens the high Q network and stabilizes
the system.
The maximum output capacitor RMS ripple current
is given by:
1
VOUT · (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.
Output Capacitor
The output capacitor limits the output ripple and
provides holdup during large load transitions. A
4.7µF to 10µF X5R or X7R ceramic capacitor typi-
cally provides sufficient bulk capacitance to stabi-
lize the output during large load transitions and has
the ESR and ESL characteristics necessary for low
output ripple. For enhanced transient response
and low temperature operation application, a 10µF
(X5R, X7R) ceramic capacitor is recommended to
stabilize extreme pulsed load conditions.
Adjustable Output Resistor Selection
Resistors R1 and R2 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 resis-
tor string while maintaining good noise immunity, the
suggested value for R2 is 59kΩ. Decreased resistor
values are necessary to maintain noise immunity on
the FB pin, resulting in increased quiescent current.
Table 2 summarizes the resistor values for various
output voltages.
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:
V
V
3.3V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
- 1 ·
R1 =
OUT -1
·
R2 =
59kΩ = 267kΩ
⎠
⎠
REF
With enhanced transient response for extreme
pulsed load application, an external feed-forward
capacitor, (C3 in Figure 1), can be added.
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ
R1 (kΩ)
3
·
VDROOP FS
ΔILOAD
COUT
=
·
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
124
137
187
267
75
113
150
187
221
261
301
332
442
464
523
715
1000
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.
Table 2: Adjustable Resistor Values For
Step-Down Converter.
12
1121.2007.03.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
