PRODUCT DATASHEET
AAT1130
TM
SwitchReg
2.5MHz 500mA Step-Down DC/DC Converter
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:
Feedback Resistor Selection
Resistors R1 and R2 of Figure 4 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 minimum
suggested 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
sensitive to external noise and interference. Table 2
summarizes the resistor values for various output volt-
ages with R2 set to either 59kΩ for good noise immunity
or 221kΩ for reduced no load input current.
PTOTAL = IO2 · RDS(ON)H + IQ · VIN
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range. Given the
total losses, the maximum junction temperature can be
derived from the θJA for the SC70JW-10 package which
is 160°C/W.
V
VFB
1.5V
0.6V
R1 =
OUT -1 · R2 =
- 1 · 59kΩ = 88.5kΩ
TJ(MAX) = PTOTAL · ΘJA + TAMB
The AAT1130, combined with an external feedforward
capacitor (C3 in Figure 4), delivers enhanced transient
response for extreme pulsed load applications. The addi-
tion of the feedforward capacitor typically requires a
larger output capacitor C1 for stability.
Layout
The suggested PCB layout for the AAT1130 is shown in
Figures 1, 2, and 3. The following guidelines should be
used to help ensure a proper layout:
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ
R1 (kΩ)
1. The input capacitor (C1) should connect as closely as
possible to VCC/VP (pins 3 and 4) and PGND/GND
(pins 7-10 for adjustable output voltage and pins
6-10 for fixed output voltage).
2. C1 and L1 should be connected as closely as possi-
ble. The connection of L1 to the LX (pin 5) should be
as short as possible.
3. The feedback trace or FB (pin 1 for adjustable output
voltage) should be separate from any power trace
and connect as closely as possible to the load point.
Sensing along a high current load trace will degrade
DC load regulation. If external feedback resistors are
used, they should be placed as closely as possible to
the FB (pin 1 for adjustable output voltage) to mini-
mize the length of the high impedance feedback
trace.
4. The resistance of the trace from the load return to
the PGND/GND (pins 7-10 for adjustable output volt-
age and pins 6-10 for fixed output voltage) should
be kept to a minimum. This will help to minimize any
error in DC regulation due to differences in the
potential of the internal signal ground and the power
ground.
VOUT (V)
0.9
1
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
113
150
187
221
261
301
332
442
1.1
1.2
1.3
1.4
1.5
1.8
Table 2: Feedback Resistor Values.
Thermal Calculations
There are three types of losses associated with the
AAT1130 step-down converter: switching losses, conduc-
tion 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:
IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])
PTOTAL
=
VIN
+ (tsw · FS · IO + IQ) · VIN
w w w . a n a l o g i c t e c h . c o m
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1130.2008.08.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
