Datasheet
SLUS419C − AUGUST 1999 − REVISED NOVEMBER 2001
20
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TYPICAL APPLICATION
In this example V
OPK
is equal to 4 V. Assuming an allowable contribution of 0.75% (1.5% peak-to-peak) from
the voltage loop to the total harmonic distortion budget sets the gain equal to:
G
VA
+
ǒ
DV
VAOUT
Ǔ
(
0.015
)
2 V
OPK
Where ∆V
VAOUT
is the effective output voltage range of the error amplifier (5 V for the UCC38500). The network
needed to realize this filter is comprised of an input resistor, R
IN
, and feedback components C
F
, C
Z
, and R
F
. The
value of R
IN
is already determined because of its function as one-half of a resistor divider from V
OUT
feeding
back to the voltage amplifier for output voltage regulation. In this case the value is 1.12 MΩ. This high value was
chosen to reduce power dissipation in the resistor. In practice, the resistor value would be realized by the use
of two 560-kΩ resistors in series because of the voltage rating constraints of most standard 1/4 W resistors. The
value of C
F
is determined by the equation:
C
F
+
1
ǒ
2 p f
R
G
VA
R
IN
Ǔ
In this example C
F
equals 150 nF. Resistor R
F
and C
F
generate a pole in the voltage amplifier feedback to reduce
total harmonic distortion (THD). The location of the pole is found by setting the gain of the loop equation to one
and solving for the crossover frequency. The frequency, expressed in terms of input power, is calculated by the
equation:
f
VI
+
P
IN
Ǹ
2pDV
VAOUT
V
OUT
R
IN
C
BOOST
C
F
Ǹ
f
VI
for this converter is 10 Hz. A derivation of this equation can be found in the Unitrode Power Supply Design
Seminar SEM−1000, Topic 1, Power Factor Correction Circuit, TI Literature No. SLUP106.
Solving for R
F
becomes:
R
F
+
1
ǒ
2 p f
VI
C
F
Ǔ
Or R
F
equals approximately 118 kΩ.
Due to the low output impedance of the voltage amplifier, capacitor C
Z
is added to improve dc regulation. To
maintain good phase margin, the zero from C
Z
is set to 10% of f
VI
. For this design, C
Z
is a 2.2-µF capacitor. The
following equation is used to calculate C
Z.
C
Z
+
1
2p
ǒ
f
VI
10
Ǔ
R
F
(24)
(25)
(26)
(27)
(28)