AT1366B Datasheet, PDF (9/14 Pages) Aimtron Technology – 1MHz,800mA Synchronous Buck Converter

Application Information
AT1366B
Preliminary Product Information
1MHz,800mA Synchronous Buck Converter
Short Circuit Protection Function
The AT1366B has short circuit protection(see the Electrical Characteristics frequency
item). When the output is shorted to ground, the frequency of the oscillator is reduced
to about 200KHz. This frequency foldback ensures that the inductor current has more
time to decay. It will prevent the inductor current and internal P-MOSFET current
runaway to damage the device. The frequency will return to the normal values when
the short circuit condition is removed and the feedback voltage reaches 0.6V.
Capacitor Selection
In continuous mode, the source current of the top MOSFET is square wave of duty
cycle. The Primary function of the input capacitor is to provide a low impedance loop
for the edges of pulsed current drawn by the AT1366B. A load step at the output can
induce ringing at the input VIN. This ringing can couple to the output and be mistaken
as loop instability. The oscillation can be improved by add the capacitance of the input
capacitor. A typical value is 4.7~22µF ceramic (X5R or X7R), POSCAP or Aluminum
Polymer. These capacitors will provide good high frequency bypassing and their low
ESR will reduce resistive losses for higher efficiency. The input capacitor RMS
current varies with the input voltage and the output voltage. The equation for the
maximum RMS current in the input capacitor is:
I _RMS = I_OMAX
V_{O (1 −} V_{O )}
V_IN
V_IN
The output capacitor depends on the suitable ripple voltage. Low ripple voltage
corresponds to lower effective series resistance (ESR). The output ripple voltage is
determined by:
∆V_OUT
≅
∆I
L
(ESR
+
8
1
fC_OUT
)
The output capacitor RMS ripple current is given by:
7F, No.9, PARK AVENUE II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
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1/18/2006 REV:1.2
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