PRODUCT DATASHEET
AAT2153
TM
SwitchReg
2.5A Low Noise Step-Down Converter
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the evalu-
ation board input voltage pins. The inductance of these
wires, along with the low-ESR ceramic input capacitor,
can create a high Q network that may affect converter
performance. This problem often becomes apparent in
the form of excessive ringing in the output voltage dur-
ing load transients. Errors in the loop phase and gain
measurements can also result.
itance will reduce the crossover frequency with greater
phase margin.
Adjustable Output Resistor Selection
The output voltage on the AAT2153 is programmed with
external resistors R3 and R4. To limit the bias current
required for the external feedback resistor string while
maintaining good noise immunity, the minimum sug-
gested value for R4 is 59kΩ. Although a larger value will
further reduce quiescent current, it will also increase the
impedance of the feedback node, making it more sensi-
tive to external noise and interference. Table 1 summa-
rizes the resistor values for various output voltages with
R4 set to either 59kΩ for good noise immunity or 221kΩ
for reduced no load input current.
Since the inductance of a short PCB trace feeding the
input voltage is significantly lower than the power leads
from the bench power supply, most applications do not
exhibit this problem.
In applications where the input power source lead induc-
tance cannot be reduced to a level that does not affect
the converter performance, a high ESR tantalum or alu-
minum electrolytic should be placed in parallel with the
low ESR/ESL bypass ceramic capacitor. This dampens
the high Q network and stabilizes the system.
The external resistor R3, combined with an external
100pF feed forward capacitor (C8 in Figure 1), delivers
enhanced transient response for extreme pulsed load
applications and reduces ripple in light load conditions.
The addition of the feed forward capacitor typically
requires a larger output capacitor C3-C4 for stability. The
external resistors set the output voltage according to the
following equation:
Output Capacitor
The output capacitor limits the output ripple and pro-
vides holdup during large load transitions. A 10μF to
22μF X5R or X7R ceramic capacitor typically provides
sufficient bulk capacitance to stabilize the output during
large load transitions and has the ESR and ESL charac-
teristics necessary for low output ripple.
⎛
R3⎞
R4⎠
VOUT = 0.6V 1 +
⎝
or
⎛VOUT
⎣
⎞
⎦
⎡
⎤
- 1 R4
The output voltage droop due to a load transient is dom-
inated by the capacitance of the ceramic output capacitor.
During a step increase in load current, 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:
R3 =
⎝VREF
⎠
R4 = 59kΩ
R3 (kΩ)
R4 = 221kΩ
R3 (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
3.0
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
237
75
113
150
187
221
261
301
332
442
464
523
715
887
1000
3 · ΔILOAD
=
COUT
V
DROOP · FS
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equa-
tion 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 10μF. This is due to
its effect on the loop crossover frequency (bandwidth),
phase margin, and gain margin. Increased output capac-
267
Table 1: AAT2153 Resistor Values for Various
Output Voltages.
w w w . a n a l o g i c t e c h . c o m
2153.2008.09.1.0
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