APW7065C View Datasheet(PDF) - Anpec Electronics
Part Name
Description
MFG CO.
'APW7065C' PDF : 19 Pages View PDF
APW7065C
Application Information
Output Voltage Selection
The output voltage can be programmed with a resistive
divider. Using 1% or better resistors for the resistive di-
vider is recommended. The FB pin is the inverter input of
the error amplifier, and the reference voltage is 0.8V. The
output voltage is determined by:
V OUT
=
0.8
× 1 +
R OUT
R GND
where ROUT is the resistor connected from VOUT to FB and
R is the resistor connected from FB to GND.
GND
Output Inductor Selection
The inductor value determines the inductor ripple cur-
rent and affects the load transient response. Higher in-
ductor 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 F is the switching frequency of the regulator.
S
Although increase of the inductor value reduces the ripple
current and voltage, a tradeoff will exist between the
inductor’s ripple current and the regulator load tran-
sient response time.
A smaller inductor will give the regulator a faster load
transient response at the expense of higher ripple current.
The maximum ripple current occurs at the maximum in-
put voltage. A good starting point is to choose the ripple
current to be approximately 30% of the maximum out-
put current. Once the inductance value has been chosen,
select an inductor that is capable of carrying the re-
quired 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 out-
put ripple and the load transient drop. Therefore, select-
ing high performance low ESR capacitors is intended
for switching regulator applications. In some applications,
multiple capacitors have to be parallel to achieve the de-
sired 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 have been done surge test by the manufactures.
If in doubt, consult the capacitors manufacturer.
Input Capacitor Selection
The input capacitor is chosen based on the voltage rat-
ing 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 IOUT/2,
where I is the load current. During power up, the input
OUT
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 ca-
pacitors 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 de-
termined by the R , which reverses transfer capaci-
DS(ON)
tance (C ) and maximizes output current requirement.
RSS
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 following:
PUPPER = IOUT 2(1+ TC)(RDS(ON))D + (0.5)( IOUT)(VIN)( tSW)FS
P
= I 2(1+ TC)(R )(1-D)
LOWER OUT
DS(ON)
Where IOUT is the load current
TC is the temperature dependency of RDS(ON)
FS is the switching frequency
Copyright © ANPEC Electronics Corp.
12
Rev. A.2 - Mar., 2008
www.anpec.com.tw
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