APW7064 Просмотр технического описания (PDF) - Anpec Electronics
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APW7064 Datasheet PDF : 19 Pages
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APW7064
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 1.2V. The
output voltage is determined by:
VOUT
= 1.2 × 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 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.
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
switching regulator applications. In some applications,
multiple capacitors have to be parallel 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 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
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 R , reverse transfer capacitance
DS(ON)
(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
following:
PUPPER = IOUT 2(1+ TC )(RDS(ON) )D + (0.5)( IOUT )(VIN )( tSW )FS
PLOWER = IOUT2(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
t is the switching interval
SW
D is the duty cycle
Note that both MOSFETs have conduction loss while the
upper MOSFET include an additional transition loss. The
switching internal, t , is the function of the reverse trans-
SW
fer capacitance CRSS. The (1+TC) term is to factor in
the temperature dependency of the RDS(ON) and can be
extracted from the “RDS(ON) vs Temperature” curve of the
power MOSFET.
Copyright © ANPEC Electronics Corp.
11
Rev. A.3 - Aug., 2009
www.anpec.com.tw
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