AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
Figure 2: AAT1147 Evaluation Board
Top Side.
Figure 3: Exploded View of Evaluation
Board Top Side Layout.
Figure 4: AAT1147 Evaluation Board
Bottom Side.
The maximum output capacitor RMS ripple current
is given by:
higher than 0.6V. To limit the bias current required for
the external feedback resistor string while maintain-
ing good noise immunity, the minimum suggested
value for R2 is 59kΩ. Although a larger value will fur-
ther 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
voltages with R2 set to either 59kΩ for good noise
immunity or 221kΩ for reduced no load input current.
1
VOUT · (VIN(MAX) - VOUT)
IRMS(MAX)
=
·
L · FS · VIN(MAX)
2 · 3
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.
V
V
1.5V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
- 1 ·
OUT -1
·
R2 =
59kΩ = 88.5kΩ
Output Resistor Selection
R1 =
⎠
⎠
REF
The output voltage of the AAT1147 0.6V version can
be externally programmed. Resistors R1 and R2 of
Figure 5 program the output to regulate at a voltage
12
1147.2006.05.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
