APW7098
Application Information (Cont.)
Output Inductor Selection (Cont.)
caused by the AC peak-to-peak sum of the inductor’s
current. The ripple voltage of output capacitors can be
represented by:
VOUT
D =
V
IN
DIP - P
DVCOUT =
For two-phase converter, the inductor value (L) determines
the sum of the two inductor ripple currents, DIP-P, and af-
fects the load transient reponse. Higher inductor value
reduces the output capacitors’ ripple current and induces
lower output ripple voltage. The ripple current can be
approxminated by:
8´ COUT ´ FSW
DVESR = DIP - 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 capaci-
tors 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.
VIN - 2VOUT VOUT
DIP - P =
´
FSW ´ L
VIN
Where FSW is the switching frequency of the regulator.
Although the inductor value and frequency are increased
and the ripple current and voltage are reduced, a tradeoff
exists between the inductor’s ripple current and the regu-
lator load transient response time.
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, the output
capacitance of two-phase converter only needs around
half of output capacitance of single-phase converter.
A smaller inductor will give the regulator a faster load tran-
sient response at the expense of higher ripple current.
Increasing the switching frequency (FSW) also reduces
the ripple current and voltage, but it will increase the
switching loss of the MOSFETs and the power dissipa-
tion of the converter. The maximum ripple current oc-
curs 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 out-
put ripple voltage.
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 ca-
pacitors in order to prevent the capacitor from over-
heating.
Input Capacitor Selection
Use small ceramic capacitors for high frequency
decoupling and bulk capacitors to supply the surge cur-
rent needed each time high-side MOSFET turns on. Place
the small ceramic capacitors physically close to the
MOSFETs and between the drain of high-side MOSFET
and the source of low-side MOSFET.
Output Capacitor Selection
Output voltage ripple and the transient voltage de-
viation are factors that have to be taken into con-
sideration 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 fre-
quency noise related to MOSFET turn-on and turn-off,
the output voltage ripple includes the capacitance
voltage drop DVCOUT and ESR voltage drop DVESR
The important parameters for the bulk input capacitor are
the voltage rating and the RMS current rating. For reliable
operation, select the bulk capacitor with voltage and cur-
rent ratings above the maximum input voltage and larg-
est RMS current required by the circuit. The capacitor volt-
age rating should be at least 1.25 times greater than the
maximum input voltage and a voltage rating of 1.5 times
is a conservative guideline. For two-phase converter, the
Copyright ã ANPEC Electronics Corp.
23
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Rev. A.6 - Oct., 2009
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