APW7062B(2008) Ver la hoja de datos (PDF) - Anpec Electronics
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APW7062B Datasheet PDF : 18 Pages
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APW7062B
Application Information
Component Selection Guidelines
Output Capacitor Selection
The selection of COUT is determined by the required effec-
tive series resistance (ESR) and voltage rating rather than
the actual capacitance requirement. Therefore, select high
performance low ESR capacitors that are intended for
switching regulator applications. In some applications,
multiple capacitors have to be paralled to achieve the
desired ESR value. If tantalum capacitors are used, make
sure they are surge tested by the manufactures. If in doubt,
consult the capacitors manufacturer.
and vice versa. 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 maxi-
mum output current.
Once the inductance value has been chosen, select an
inductor that is capable of carrying the required peak cur-
rent without going into saturation. In some types 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.
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 , where
OUT
I is the load current. During power up, the input capaci-
OUT
tors have to handle large amount of surge current. If tanta-
lum 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 be-
tween 0.1µF to 1µF can be connected between the VCC
and the ground pin.
Inductor Selection
The inductance of the inductor is determined by the out-
put voltage requirement. The larger the inductance, the
lower the inductor’s current ripple. This will translate into
lower output ripple voltage. The ripple current and ripple
voltage can be approximated by:
IRIPPLE = VIN - VOUT
FSx L
x
VOUT
V
IN
∆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 tran-
sient response at the expense of higher ripple current
Compensation
The output LC filter introduces a double pole, which con-
tributes with –40dB/decade gain slope and 180 degrees
phase shift in the control loop. A compensation network
between the COMP pin and the ground should be added.
The simplest loop compensation network is shown in
Figure 5.
The output LC filter consists of the output inductor and
output capacitors. The transfer function of the LC filter is
given by:
GAINLC =
1+ s ×ESR × COUT
s2 ×L × COUT + s ×ESR + 1
The poles and zero of this transfer function are:
1
FLC = 2 × π × L × COUT
= FESR
1
2 × π × ESR × COUT
The FLC is the double poles of the LC filter, and FESR is the
zero introduced by the ESR of the output capacitor.
PHASE
L
Output
COUT
ESR
Figure 2. The Output LC Filter
Copyright © ANPEC Electronics Corp.
11
Rev. A.4 - Oct., 2008
www.anpec.com.tw
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