APW7068 View Datasheet(PDF) - Anpec Electronics
Part Name
Description
MFG CO.
APW7068
Anpec Electronics
'APW7068' PDF : 26 Pages View PDF
APW7068
Application Information (Cont.)
PWM Compensation (Cont.)
and ripple voltage can be approximated by:
5.Set the second pole FP2 at the half of the switching
frequency and also set the second zero F at the
Z2
output LC filter double pole F . The compensation
LC
gain should not exceed the error amplifier open loop
gain, check the compensation gain at F with the
P2
capabilities of the error amplifier.
F = 0.5 X F
P2
S
FZ2 = FLC
IRIPPLE
=
VIN1 − VOUT1 × VOUT1
FS × L
VIN1
∆VOUT1 = 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.
Combine the two equations will get the following
component calculations:
R3 =
R1
FS − 1
2 × FLC
C3 =
1
π × R3 × FS
FZ1 FZ2 FP1 FP2
20log
(R2/R1)
Compensation
20log
Gain
(VIN/Δ V OSC)
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.
FLC
FESR
PWM & Filter
Gain
Frequency(Hz)
Converter
Gain
Figure 10. Converter Gain and Frequency
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
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
Copyright © ANPEC Electronics Corp.
17
Rev. A.2 - Jun., 2006
www.anpec.com.tw
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