AT1362A/B
Preliminary Product Information
Synchronous Buck Converter
With Power Good Detector & LDO
Application Information
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 AT1362A/B. 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 10µF 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
VIN
VO
VIN
IRMS = IOMAX
(1−
)
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
2 3
IRMS
=
VBG Capacitor
A VBG pin is provided to decouple the bandgap reference voltage. An external
capacitor connected form VBG to GND reduces noise present on the internal
reference voltage, which in turn significantly reduces output noise and also improves
PSRR. Larger capacitor values may be used to further improve PSRR, but result in a
longer time period (slower turn on) to settle output voltage when power is initially
applied.
7F, No.9, Park Avenue II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Tel: 886-3-563-0878
5/30/2006
Fax: 886-3-563-0879
WWW: http://www.aimtron.com.tw
Email: service@aimtron.com.tw
REV:1.0
8
AIMTRON [ AIMTRON TECHNOLOGY ]
