AAT2513
Dual 600mA Step-Down
Converter with Synchronization
Table 1 displays the suggested inductor values for
the AAT2513.
This equation provides an estimate for the input
capacitor required for a single channel.
Manufacturer's specifications list both the inductor
DC current rating, which is a thermal limitation, and
the peak current rating, which is determined by the
inductor's saturation characteristics. The inductor
should not show any appreciable saturation under
all normal load conditions. Some inductors may
meet the peak and average current ratings yet
result in excessive losses due to a high DCR.
Always consider the losses associated with the
DCR and its effect on the total converter efficiency
when selecting an inductor.
The equation below solves for the input capacitor
size for both channels. It makes the worst case
assumption that both converters are operating at
50% duty cycle with in phase synchronization.
1
CIN =
⎛
⎝
VPP
IO1 IO2
⎞
⎠
- ESR ·4 ·FS
+
Because the AAT2513 channels will generally
operate at different duty cycles the actual ripple will
vary and be less than the ripple (VPP) used to solve
for the input capacitor in the above equation.
The 2.2uH CDRH2D11 series inductor selected
from Sumida has a 98mΩ DCR and a 1.27A DC
current rating. At full load the inductor DC loss is
35mW which corresponds to a 3.2% loss in effi-
ciency for a 600mA, 1.8V output.
Always examine the ceramic capacitor DC voltage
coefficient characteristics when selecting the prop-
er value. For example, the capacitance of a 10µF
6.3V X5R ceramic capacitor with 5V DC applied is
actually about 6µF.
Input Capacitor
A key feature of the AAT2513 is that the funda-
mental switching frequency ripple at the input can
be reduced by operating the two converters 180°
out of phase. This reduces the input ripple by
roughly half, reducing the required input capaci-
tance. An X5R ceramic input capacitor as small as
1µF is often sufficient. To estimate the required
input capacitor size, determine the acceptable
input ripple level (VPP) and solve for C. The calcu-
lated value varies with input voltage and is a maxi-
mum when VIN is double the output voltage.
The maximum input capacitor RMS current is:
VO1
VIN
⎛
⎝
VO1
VIN
⎞
⎠
VO2
VIN
⎛
· 1 -
VO2
VIN
⎞
⎠
⎛
⎞
⎛
⎞
IRMS = IO1
·
·
1
-
+ IO2
·
⎝
The input capacitor RMS ripple current varies with
the input and output voltage and will always be less
than or equal to half of the total DC load current of
both converters combined.
VO
VIN
⎛
⎝
VO
VIN
⎞
⎠
IO1(MAX) + IO2(MAX)
⋅
1 -
IRMS(MAX)
=
2
CIN =
⎛
⎝
VPP
IO
⎞
- ESR
⋅
FS
⎠
Configuration
Output Voltage
Inductor
Slope Compensation
0.6V adjustable
with external
resistive divider
0.6V-2.0V
2.5V
2.2µH
3.3µH
4.7µH
0.6A/µs
3.3V
Table 1: Inductor Values.
12
2513.2007.04.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
