APW7060 데이터 시트보기 (PDF) - Anpec Electronics
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APW7060 Datasheet PDF : 16 Pages
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APW7060
Application Information
Inductor Selection
The inductance of the inductor is determined by the
output voltage requirement. The larger the inductance,
the lower the inductor’s current ripple. This will trans-
late into lower output ripple voltage. The ripple current
and ripple voltage can be approximated by:
IRIPPLE =
VIN - VO×UTV OUT
Fs×L V IN
PUPPER
=
I2
out
(1+
TC)(RDS(ON))D
+
(0.5)(Iout)(VIN)(tsw)FS
PLOWER
=
I2
out
(1+
TC)(RDS(ON))(1-D)
where IOUT is the load current
TC is the temperature dependency of RDS(ON)
FS is the switching frequency
t is the switching interval
sw
D is the duty cycle
∆VOUT = IRIPPLE x ESR
where Fs is the switching frequency of the regulator.
There is a tradeoff exists 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 cur-
rent and vice versa. The maximum ripple current oc-
curs at the maximum input voltage. A good starting
point is to choose the ripple current to be approxi-
mately 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
type of inductors, especially core that is make of
ferrite, the ripple current will increase abruptly when it
saturates. This will result in a larger output ripple
voltage.
PWM Regulator 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.The
losses in the MOSFETs have two components: con-
duction loss and transition loss. For the upper and
lower MOSFET, the losses are approximately given
by the following :
Note that both MOSFETs have conduction losses while
the upper MOSFET include an additional transition
loss.The switching internal, tsw, is a function of the
reverse transfer capacitance CRSS. Figure 4 illustrates
the switching waveform of the MOSFET.
The (1+TC) term is to factor in the temperature depen-
dency of the RDS(ON) and can be extracted from the
“RDS(ON) vs Temperature” curve of the power MOSFET.
Linear Regulator Input/Output Capacitor Selec-
tion
The input capacitor is chosen based on its voltage
rating. Under load transient condition, the input ca-
pacitor will momentarily supply the required transient
current. A 1uF ceramic capacitor will be sufficient in
most applications.
The output capacitor for the linear regulator is chosen
to minimize any droop during load transient condition.
In addition, the capacitor is chosen based on its volt-
age rating.
Linear Regulator MOSFET Selection
In addition to choosing the pass MOSFET for its abil-
ity to sustain the load current requirement (see Maxi-
mum Output Voltage of Linear Controller), another cri-
teria is its efficiency of heat removal. The power dissi-
pated by the MOSFET is given by:
Pdiss = Iout * (VIN - VOUT2)
Copyright ANPEC Electronics Corp.
11
Rev. A.3 - Oct., 2003
www.anpec.com.tw
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