PRODUCT DATASHEET
AAT2113A
TM
SwitchReg
3MHz, FastTransient 1.5A Step-Down Converter in 2mm x 2mm Package
The AAT2113A adjustable version, combined with an
external feed forward capacitor (C2 in Figure 5), delivers
enhanced transient response for extreme pulsed load
applications. The suggested value for C2 is in the range
of 22pF to 100pF.
Output Capacitor
The output capacitor limits the output ripple and pre-
vents the output voltage droop during large load transi-
tions. A 10ꢀF to 22ꢀF X5R or X7R ceramic capacitor
typically provides sufficient bulk capacitance to stabilize
the output during large load transitions and has the ESR
and ESL characteristics necessary for low output ripple.
R2 = 59kΩ
R1 (kΩ)
R2 = 200kΩ
R1 (kΩ)
VOUT (V)
The output voltage droop due to a load transient is
dominated by the capacitance of the ceramic output
capacitor.
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
39.2
49.9
59
68.1
78.7
88.7
97.6
107
118
133
165
200
232
267
301
332
365
400
During a step increase in load current, the ceramic out-
put 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 voltage droop during the three switching cycles to
the output capacitance can be estimated by:
Table 2: Feedback Resistor Selection for
Adjustable Output Voltage Version.
3 · ΔILOAD
COUT
=
V
DROOP · FS
Thermal Calculations
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.
There are three types of losses associated with the
AAT2113A 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:
The internal voltage loop compensation also limits the
minimum output capacitor value to 10ꢀF. 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.
IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])
PTOTAL
=
Feedback Resistor Selection
VIN
Resistors R1 and R2 of Figure 5 program the output to
regulate at a voltage higher than 0.6V for the AAT2113A
adjustable version. To limit the bias current required for
the external feedback resistor string while maintaining
good noise immunity, the suggested value for R2 is
200kΩ. Table 1 summarizes the resistor values for vari-
ous output voltages with R2 set to either 59kΩ or 200kΩ.
Alternately, the feedback resistor may be calculated
using the following equation:
+ (tsw · FS · IO + IQ) · VIN
IQ is the step-down converter quiescent current. The
term tSW is used to estimate the full load step-down con-
verter switching losses. For the condition where the
step-down converter is in dropout at 100% duty cycle,
the total device dissipation reduces to:
V
VREF
1.8V
0.6V
PTOTAL= IO2 · RDS(ON)H + IQ · VIN
R1 =
OUT -1 · R2 =
- 1 · 200kΩ = 400kΩ
w w w . a n a l o g i c t e c h . c o m
12
2113A.2009.06.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
