Datasheet
SLUS395J - FEBRUARY 2000 - REVISED MARCH 2009
11
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APPLICATION INFORMATION
multiplier (continued)
I
MOUT(max)
for this design is approximately 315 µA. The R
MOUT
resistor can then be determined by:
R
MOUT
+
V
RSENSE
I
MOUT(max)
In this example V
RSENSE
was selected to give a dynamic operating range of 1.25 V, which gives an R
MOUT
of
roughly 3.91 kΩ.
voltage loop
The second major source of harmonic distortion is the ripple on the output capacitor at the second harmonic
of the line frequency. This ripple is fed back through the error amplifier and appears as a 3rd harmonic ripple
at the input to the multiplier. The voltage loop must be compensated not just for stability but also to attenuate
the contribution of this ripple to the total harmonic distortion of the system. (refer to Figure 2).
R
IN
R
D
+
−
R
f
C
f
V
REF
V
OUT
C
Z
Figure 2. Voltage Amplifier Configuration
The gain of the voltage amplifier, G
VA
, can be determined by first calculating the amount of ripple present on
the output capacitor. The peak value of the second harmonic voltage is given by the equation:
V
OPK
+
P
IN
ǒ
2 p f
R
C
OUT
V
OUT
Ǔ
In this example V
OPK
is equal to 3.91 V. Assuming an allowable contribution of 0.75% (1.5% peak to peak) from
the voltage loop to the total harmonic distortion budget we set 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 UCC3817). 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 was chosen to be 1 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 500-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: