AAT1152
850kHz 1A Buck DC/DC Converter
For a 1A load and the ripple set to 30% at the max-
imum input voltage, the maximum peak-to-peak
ripple current is 300mA. The inductance value
required is 3.9μH.
Input Capacitor
The primary function of the input capacitor is to pro-
vide a low impedance loop for the edges of pulsed
current drawn by the AAT1152. A low ESR/ESL
ceramic capacitor is ideal for this function. To mini-
mize the stray inductance, the capacitor should be
placed as closely as possible to the IC. This keeps
the high frequency content of the input current local-
ized, minimizing radiated and conducted EMI while
facilitating optimum performance of the AAT1152.
Ceramic X5R or X7R capacitors are ideal for this
function. The size required will vary depending on
the load, output voltage, and input voltage source
impedance characteristics. A typical value is around
10μF. The input capacitor RMS current varies with
the input voltage and output voltage. The equation
for the RMS current in the input capacitor is:
VOUT
IO ⋅ k ⋅ FS
⎛
VOUT
⎞
L =
L =
⋅ 1 -
⎝
⎠
VIN
1.5
V
1.5V
4.2V
⎛
⎞
⎠
⋅ 1 -
⎝
1.0A ⋅ 0.3 ⋅ 830kHz
L = 3.9μH
The factor "k" is the fraction of full load selected for
the ripple current at the maximum input voltage.
The corresponding inductor RMS current is:
ΔI2
12
⎛
⎜
⎝
⎞
⎟
⎠
2
VO
VIN
⎛
VO ⎞
VIN ⎠
IRMS
=
IO
+
≈ IO =1A
IRMS = IO ⋅
⋅ 1 -
⎝
ΔI is the peak-to-peak ripple current which is fixed by
the inductor selection above. For a peak-to-peak cur-
rent of 30% of the full load current, the peak current
at full load will be 115% of the full load. The 4.1μH
inductor selected from the Sumida CDRH5D18
series has a 57mΩ DCR and a 1.95A DC current rat-
ing. At full load, the inductor DC loss is 57mW, which
amounts to a 3.8% loss in efficiency.
The input capacitor RMS ripple current reaches a
maximum when VIN is two times the output voltage
where it is approximately one half of the load cur-
rent. Losses associated with the input ceramic
capacitor are typically minimal and are not an issue.
Proper placement of the input capacitor can be seen
in the reference design layout in Figures 5 and 6.
Figure 5: AAT1152 Evaluation
Board Layout Top Layer.
Figure 6: AAT1152 Evaluation
Board Layout Bottom Layer.
12
1152.2006.09.1.7
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