PRODUCT DATASHEET
AAT2784
SystemPowerTM
3-Channel Step-Down DC/DC Converter
affect the converter performance, a high ESR tantalum
or aluminum electrolytic should be placed in parallel with
the low ESR/ESL bypass ceramic capacitor. This damp-
ens the high Q network and stabilizes the system.
maintaining good noise immunity, the minimum sug-
gested value for R2 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 2 summa-
rizes the resistor values for various output voltages with
R2 set to either 59kΩ for good noise immunity or 221kΩ
for reduced no load input current.
Output Capacitor: Channels 1 and 2
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. The output volt-
age droop due to a load transient is dominated by the
capacitance of the ceramic output capacitor. During a
step increase in load current, the ceramic output capac-
itor 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
voltage droop during the three switching cycles to the
output capacitance can be estimated by:
VOUT
VIN
3.3V
0.6V
R1 =
- 1 · R2 =
- 1 · 59kΩ = 267k
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ
R1 (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
=
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 mini-
mum 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.
267
Table 2: AAT2784 Resistor Values for Various
Output Voltages.
Thermal Calculations
There are three types of losses associated with the
AAT2784 step-down converter: switching losses, con-
duction losses, and quiescent current losses. Conduction
losses are associated with the RDS(ON) characteristics of
the power output switching devices. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, assuming continuous con-
duction mode (CCM), a simplified form of the losses is
given by:
Output Capacitor: Channel 3
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.
IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])
PTOTAL
=
VIN
Adjustable Output Resistor Selection
+ (tsw · FS · IO + IQ) · VIN
The output voltage on the AAT2784 is programmed with
external resistors R1 and R2. To limit the bias current
required for the external feedback resistor string while
IQ is the step-down converter quiescent current. The
term tSW is used to estimate the full load step-down con-
w w w . a n a l o g i c t e c h . c o m
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2784.2007.11.1.1
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