AAT1152 Datasheet, PDF (11/18 Pages) Advanced Analogic Technologies – 850kHz 1A Synchronous Buck DC/DC Converter

AAT1152
850kHz 1A Synchronous Buck DC/DC Converter
Power Good
The AAT 1152 features an integrated Power Good
(POK) comparator and open-drain output signal.
The POK pin goes low when the converter’s output
is 12% or more below its nominal regulation volt-
age or when the device is in shutdown. Connect a
pull-up resistor from POK to the converter’s input
or output. Typical resistor pull-up values range
from 100k to 10k.
Inductor
The output inductor is selected to limit the ripple
current to some predetermined value, typically 20-
40% of the full load current at the maximum input
voltage. Manufacturer's specifications list both the
inductor DC current rating, which is a thermal limi-
tation, and the peak current rating, which is deter-
mined by the saturation characteristics. The induc-
tor should not show any appreciable saturation
under all normal load conditions. During over load
and short circuit conditions, the average current in
the inductor can meet or exceed the I_LIMIT point of
the AAT1152 without effecting the converter per-
formance. Some inductors may have sufficient
peak and average current ratings yet result in
excessive losses due to a high DCR. Always con-
sider the losses associated with the DCR and its
effect on the total converter efficiency when select-
ing an inductor.
For a 1 Amp load and the ripple set to 30% at the
maximum input voltage, the maximum peak to
peak ripple current is 300 mA. The inductance
value required is 3.9µH.
L
=
V_OUT
I_O ⋅ k ⋅
F
⋅ ^1
-
V_OUT
V_IN 
L
=
1.0A
⋅
1.5V
0.3 ⋅
830kHz
⋅ ^1
-
1.5V 
4.2V 
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:
I_RMS =
^Io2
+
∆I²ï£¶
12
≈ I_o = 1.0A
∆I 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 57 mΩ DCR and a 1.95 Amp DC cur-
rent rating. At full load the inductor DC loss is 57mW
which amounts to a 3.8% loss in efficiency.
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 close as possible to the IC. This keeps
the high frequency content of the input current
localized, 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 typi-
cal value is around 10µF. The input capacitor RMS
current varies with the input voltage and the output
voltage. The equation for the maximum RMS cur-
rent in the input capacitor is:
I_RMS = I_O ⋅
V_O
V_IN
⋅ _1 -
V_O 
V_IN 
The input capacitor RMS ripple current reaches a
maximum when V_IN 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 not an issue. The
proper placement of the input capacitor can be seen
in the reference design layout in figures 5 and 6.
1152.2003.01.0.9
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