PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
Sinceꢀ theꢀ inductanceꢀ ofꢀ aꢀ shortꢀ PCBꢀ traceꢀ feedingꢀ theꢀ
input voltage is significantly lower than the power leads
from the bench power supply, most applications do not
exhibit this problem.
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
R4ꢀsetꢀtoꢀeitherꢀ59kW for good noise immunity or 221kW
for reduced no load input current.
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
lowꢀ ESR/ESLꢀ bypassꢀ ceramicꢀ capacitor.ꢀ Thisꢀ dampensꢀ
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
followingꢀequation:
Output Capacitor
The output capacitor limits the output ripple and pro-
vides holdup during large load transitions. A 10µF to
22µFꢀ X5Rꢀ orꢀ X7Rꢀ ceramicꢀ capacitorꢀ typicallyꢀ providesꢀ
sufficient bulk capacitance to stabilize the output during
largeꢀloadꢀtransitionsꢀandꢀhasꢀtheꢀESRꢀandꢀESLꢀcharac-
teristics necessary for low output ripple.
R3
R4
VOUT = 0.6V 1 +
or
VOUT
R3 =
- 1 R4
VREF
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
capacitanceꢀcanꢀbeꢀestimatedꢀby:
R4 = 59kW
R3 (kW)
R4 = 221kW
R3 (kW)
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 · ∆ILOAD
=
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.
267
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-
itance will reduce the crossover frequency with greater
phase margin.
Table 1: AAT2158 Resistor Values for Various
Output Voltages.
The typical circuit shown in the AAT2158 evaluation
schematic is intended to be general purpose and suitable
for most applications. An additional example schematic
is shown in Figure 2 according to the design guidelines
for cases where transient load steps are more severe
and the restriction on output voltage deviation is more
stringent. To handle these cases some simple adjust-
ments can be made.
Adjustable Output Resistor Selection
The output voltage on the AAT2158 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-
gestedꢀvalueꢀforꢀR4ꢀisꢀ59kW. Although a larger value will
further reduce quiescent current, it will also increase the
w w w . a n a l o g i c t e c h . c o m
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2158.2008.10.1.5
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