AAT1120IES-0.6-T1 View Datasheet(PDF) - Skyworks Solutions
Part Name
Description
MFG CO.
'AAT1120IES-0.6-T1' PDF : 19 Pages View PDF
Output Capacitor
The output capacitor limits the output ripple and pro-
vides holdup during large load transitions. A 4.7μF to
10μF 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. For enhanced
transient response and low temperature operation appli-
cation, a 10μF (X5R, X7R) ceramic capacitor is recom-
mended to stabilize extreme pulsed load conditions.
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:
COUT
=
3 · ΔILOAD
VDROOP · 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 4.7μF. 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.
The maximum output capacitor RMS ripple current is
given by:
I = RMS(MAX)
1
2·
·
3
VOUT · (VIN(MAX) - VOUT)
L · FS · VIN(MAX)
DATA SHEET
AAT1120
500mA Step-Down Converter
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
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 resistor string
while maintaining good noise immunity, the suggested
value for R2 is 59k. Decreased resistor values are nec-
essary to maintain noise immunity on the FB pin, result-
ing in increased quiescent current. Table 2 summarizes
the resistor values for various output voltages.
R1
=
⎛ VOUT
⎝ VREF
-1⎞⎠
·
R2
=
⎛ 3.3V
⎝ 0.6V
-
1 ⎞⎠
·
59kΩ
=
267kΩ
With enhanced transient response for extreme pulsed
load application, an external feed-forward capacitor, (C3
in Figure 1), can be added.
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
R2 = 59k
R1 (k)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R2 = 221k
R1 (k)
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 2: Adjustable Resistor Values For
Step-Down Converter.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
12
201973B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013
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