APW8855 Datasheet, PDF (36/43 Pages) Anpec Electronics Coropration – High Current, High Frequency 7 Outputs Voltage Regulator and Power Management IC for High Performance Tablet and Ultra Notebook Applications

APW8855
16.Application Information
Output Voltage Selection
The duty cycle (D) of a buck converter is the function of the input voltage and output voltage. Once an output voltage is
fixed, it can be written as:
VOUT
D=
VIN
For PWM converter, the inductor value (L) determines the sum of the inductor ripple currents Δ I , and affects the load
P-P
transient response. Higher inductor value reduces the output capacitors¡¦ ripple current and induces lower output
ripple voltage. The ripple current can be approximated by:
ΔIP - P = ^{VIN − VOUT} × ^VOUT
FSw × L
VIN
Where F is the switching frequency of the regulator, although the inductor value and frequency are increased and the
SW
ripple current and voltage are reduced, a tradeoff exists between the inductor¡¦s ripple current and the regulator load
transient response time. A smaller inductor will give the regulator a faster load transient response at the expense of
higher ripple current. Increasing the switching frequency (F ) also reduces the ripple current and voltage, but it will
SW
increase the switching loss of the MOSFETs and the power dissipation of the converter. The maximum ripple current
occurs at the maximum input voltage. A good starting point is to choose the ripple current to be approximately 30% of
the maximum output current. Once the inductance value has been chosen, select an inductor that is capable of
carrying the required peak current without going into saturation. In some types of inductors, especially core that is
made of ferrite, the ripple current will increase abruptly when it saturates. This results in a larger output ripple voltage.
Output Capacitor Selection
Output voltage ripple and the transient voltage deviation are factors that have to be taken into consideration when
selecting output capacitors. Higher capacitor value and lower ESR reduce the output ripple and the load transient drop.
Therefore, selecting high performance low ESR capacitors is recommended for switching regulator applications. In
addition to high frequency noise related to MOSFET turn-on and turn-off, the output voltage ripple includes the capaci-
tance voltage drop ΔV_COUT and ESR voltage drop ΔV_ESR caused by the AC peak-to-peak sum of the inductor’s current. The
ripple voltage of output capacitors can be represented by:
ΔVCOUT =
ΔIP − P
8 × COUT × FSw
ΔVESR = ΔIP − P ×RESR
These two components constitute a large portion of the total output voltage ripple. In some applications, multiple
capacitors have to be paralleled to achieve the desired ESR value. If the output of the converter has to support another
load with high pulsating current, more capacitors are needed in order to reduce the equivalent ESR and suppress the
voltage ripple to a tolerable level. A small decoupling capacitor in parallel for bypassing the noise is also recommended,
and the voltage rating of the output capacitors are also must be considered. To support a load transient that is faster
than the switching frequency, more capacitors are needed for reducing the voltage excursion during load step change.
For getting same load transient response, another aspect of the capacitor selection is that the total AC current going
through the capacitors has to be less than the rated RMS current specified on the capacitors in order to prevent the
capacitor from overheating.
Copyright © ANPEC Electronics Corp.
36
Rev. A.1 - Dec., 2015
www.anpec.com.tw