APW8713
Application Information
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, selecting an in-
ductor that is capable of carrying the required peak cur-
rent 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 re-
sults in a larger output ripple voltage. Besides, the induc-
tor needs to have low DCR to reduce the loss of efficiency.
Output Inductor Selection
The output voltage is adjustable from 0.8V to 12V with a
resistor-divider connected with FB, GND, and converter?¦s
output. Using 1% or better resistors for the resistor-di-
vider is recommended. The output voltage is determined
by:
RTOP
VOUT = 0.8 ´ (1+
)
RGND
Output Capacitor Selection
Where 0.8 is the reference voltage, RTOP is the resistor
connected from converter¡¦s output to FB, and RGND is the
resistor connected from FB to GND. Suggested RGND is in
the range from 1k to 20kW. To prevent stray pickup, locate
resistors RTOP and RGND close to APW8713.
Output voltage ripple and the transient voltage deviation
are factors that have to be taken into consideration when
selecting an output capacitor. 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 turnoff, the output voltage ripple
includes the capacitance voltage drop DVCOUT and ESR
voltage drop DVESR caused by the AC peak-to-peak
inductor’s current. These two voltages can be represented
by:
Output Inductor 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 =
V
IN
IRIPPLE
DCOUT
=
8 ´ COUT ´ FSW
The inductor value (L) determines the inductor ripple
current, IRIPPLE, and affects the load transient response.
Higher inductor value reduces the inductor?¦s ripple cur-
rent and induces lower output ripple voltage. The ripple
current and ripple voltage can be approximated by:
DVESR = IRIPPLE ´ 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 (1mF) in parallel for bypassing the
noise is also recommended, and the voltage rating of the
output capacitors are also must be considered.
V
IN - VOUT VOUT
IRIPPLE
=
´
FSW ´ L
V
IN
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.
A smaller inductor will give the regulator a faster load
transient 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 occurs
at the maximum input voltage.
To support a load transient that is faster than the switch-
ing frequency, more capacitors are needed for reducing
the voltage excursion during load step change. Another
aspect of the capacitor selection is that the total AC cur-
rent going through the capacitors has to be less than the
rated RMS current specified on the capacitors in order to
prevent the capacitor from over-heating.
Copyright ã ANPEC Electronics Corp.
17
www.anpec.com.tw
Rev. A.3 - Sep., 2013
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
