APW7073A View Datasheet(PDF) - Anpec Electronics
Part Name
Description
MFG CO.
'APW7073A' PDF : 20 Pages View PDF
APW7073A
Application Information
Output Voltage Selection
The output voltage can be programmed with a resistive
divider. Use 1% or better resistors for the resistive divider
is recommended. The FB pin is the inverter input of the
error amplifier, and the reference voltage is 0.6V. The
output voltage is determined by:
VOUT
=
0.6 × 1+
ROUT
RGND
Where ROUT is the resistor connected from VOUT to FB, and
RGND is the resistor connected from FB to GND.
Output Inductor Selection
The inductor value determines the inductor ripple current
and affects the load transient response. Higher inductor
value reduces the inductor’s ripple current and induces
lower output ripple voltage. The ripple current and ripple
voltage can be approximated by:
IRIPPLE
=
VIN − VOUT
FS × L
×
VOUT
VIN
∆VOUT = IRIPPLE × ESR
where Fs is the switching frequency of the regulator.
Although increase of the inductor value and frequency
reduces the ripple current and voltage, a tradeoff will
exist 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 (FS) also reduces the
ripple current and voltage, but it will increase the
switching loss of the MOSFET 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 will result in a larger
output ripple voltage.
Output Capacitor Selection
Higher capacitor value and lower ESR reduce the output
ripple and the load transient drop. Therefore, selecting high
performance low ESR capacitors is intended for switch-
ing regulator applications. In some applications, mul-
tiple capacitors have to be parallelled to achieve the
desired ESR value. A small decoupling capacitor in
parallel for bypassing the noise is also recommended,
and the voltage rating of the output capacitors also must
be considered. If tantalum capacitors are used, make
sure they are surge tested by the manufactures. If in doubt,
consult the capacitors manufacturer.
Input Capacitor Selection
The input capacitor is chosen based on the voltage rating
and the RMS current rating. For reliable operation, select
the capacitor voltage rating to be at least 1.3 times higher
than the maximum input voltage. The maximum RMS
current rating requirement is approximately I /2,
OUT
where IOUT is the load current. During power up, the input
capacitors have to handle large amount of surge current.
If tantalum capacitors are used, make sure they are surge
tested by the manufactures. If in doubt, consult the
capacitors manufacturer. For high frequency decoupling,
a ceramic capacitor 1µF can be connected between the
drain of upper MOSFET and the source of lower MOSFET.
MOSFET Selection
The selection of the N-channel power MOSFETs are
determined by the RDS(ON), reverse transfer capacitance
(CRSS) and maximum output current requirement. There
are two components of loss in the MOSFETs: conduction
loss and transition loss. For the upper and lower
MOSFET, the losses are approximately given by the fol-
lowing equations:
P = I 2 ( 1+ TC)(R )D + (0.5)( I )(V )( t )F
UPPER OUT
DS(ON)
OUT IN SW S
PLOWER = IOUT 2 (1+ TC)(RDS(ON))(1-D)
Where I is the load current
OUT
TC is the temperature dependency of RDS(ON)
FS is the switching frequency
tSW is the switching interval
D is the duty cycle
Copyright © ANPEC Electronics Corp.
12
Rev. A.5 - Nov., 2012
www.anpec.com.tw
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