AT1361
1MHz,1A Synchronous Buck Converter
Application Information
Short Circuit Protection Function
The AT1361 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 AT1361. 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:
VO
VO
IRMS = IOMAX
(1−
)
VIN
VIN
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:
1
∆VOUT ≅ ∆IL (ESR +
)
8 fCOUT
The output capacitor RMS ripple current is given by:
VOUT × (VIN −VOUT
L× f ×VIN
)
1
IRMS
=
2 3
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1/11/2008 REV:1.0
Fax: 886-3-563-0879
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10
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