ANP015 Datasheet, PDF (12/15 Pages) List of Unclassifed Manufacturers – Application Note AP2011 High Efficiency Synchronous PWM Controller

Application Note
AP2011 High Efficiency Synchronous PWM Controller
ANP015
Preliminary
z Inductor Selection
The inductor plays a central role in the proper operation of the buck circuit. To find the inductor
value it is necessary to consider the inductor ripple current. Choose an inductor to maintain
continuous-mode operation down to 10 percent (Io(min)) of the rated output load:
∆IL = 2 x 10% x Io = 2 x 0.1 x 5 = 1A
The inductor “LB” value for this system is connected to be:
LB ≧
(Vin - Vds(sat) – Vo) x Dmin
=
∆IL x fs
(25 – 0.1 – 12) x 0.484 _{= 21µH}
1 x (180 x 10^3)
So we choose buck inductor value to be 33uH for this case, and if the core loss is a problem, increasing
the inductance of L will be helpful.
z Output Capacitor Selection
A. The output capacitor is required to filter the output and provide regulator loop stability. When selecting
an output capacitor, the important capacitor parameters are; the 100KHz Equivalent Series Resistance
(ESR), the RMS ripples current rating, voltage rating, and capacitance value. For the output capacitor,
the ESR value is the most important parameter. The ESR can be calculated from the following formula.
VI _ESR
=


2
×
RIPPLE _
 _{LOAD (min)}
An aluminum electrolytic capacitor's ESR value is related to the capacitance and its voltage rating. In
most case, higher voltage electrolytic capacitors have lower ESR values. Most of the time, capacitors
with much higher voltage ratings may be needed to provide the low ESR values required for low output
ripple voltage. If the selected capacitor's ESR is extremely low, resulting in an oscillation at the output.
It is recommended to replace this low ESR capacitor by using two general standard capacitors in
parallel.
B. The capacitor voltage rating should be at least 1.5 times greater than the output voltage, and often
much higher voltage ratings are needed to satisfy the low ESR requirements needed for low output
ripple voltage.
Anachip Corp.
www.anachip.com.tw
12/15
Rev. A.0 Oct, 29, 2004