UCC3817D UNITRODE, UCC3817D Datasheet - Page 10

• Manufacturer Part Number: UCC3817D
• Description: POWER FACTOR CNTRLR, 3817, SOIC16
• Manufacturer: UNITRODE

Specifications of UCC3817D

• Primary Input Voltage: 15V
• No. Of Outputs: 1
• Output Voltage: 5.5V
• Voltage Regulator Case Style: SOIC
• No. Of Pins: 16
• Operating Temperature Range: 0°C To +70°C
• Operating Temperature Max: 70°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

10
SLUS395J - FEBRUARY 2000 - REVISED MARCH 2009
multiplier (continued)
The I
pin of the UCC3817/18. This resistor (R
UCC3817/18 the maximum I
of its linear range. A smaller current level is functional, but noise can become an issue, especially at low input
line. Assuming a universal line operation of 85 V
voltage rating constraints of standard 1/4-W resistor, use a combination of lower value resistors connected in
series to give the required resistance and distribute the high voltage amongst the resistors. For this design
example two 383-kΩ resistors were used in series.
The current into the IAC pin is mirrored internally to the VFF pin where it is filtered to produce a voltage feed
forward signal proportional to line voltage. The VFF voltage is used to keep the power stage gain constant; and
to provid input power limiting. Please refer to Texas Instruments application note SLUA196 for detailed
explanation on how the VFF pin provides power limiting. The following equation can be used to size the VFF
resistor (R
resistance connected between the IAC pin and the rectified line voltage.
Because the VFF voltage is generated from line voltage it needs to be adequately filtered to reduce total
harmonic distortion caused by the 120 Hz rectified line voltage. Refer to Unitrode Power Supply Design
Seminar, SEM−700 Topic 7, [Optimizing the Design of a High Power Factor Preregulator.] A single pole filter
was adequate for this design. Assuming that an allocation of 1.5% total harmonic distortion from this input is
allowed, and that the second harmonic ripple is 66% of the input ac line voltage, the amount of attenuation
required by this filter is:
With a ripple frequency (f
at:
The following equation can be used to select the filter capacitor (C
filter.
The R
current. The maximum multiplier current, or I
R
1.5 %
f
C
I
66 %
P
MOUT(max)
VFF
VFF
IAC
+ 120 Hz
MOUT
signal is obtained through a high-value resistor connected between the rectified ac line and the IAC
+ 0. 022
+
+
VFF
resistor is sized to match the maximum current through the sense resistor to the maximum multiplier
V
2
) to provide power limiting where V
IN(min)
2 R
+
1.4 V
p
0.022 [ 2.6 Hz
I
IAC
IAC
R
1
VFF
0.9
R
@V
) of 120 Hz and an attenuation of 0.022 requires that the pole of the filter (f
[ 30 kW
IN(min)
IAC
f
P
current is about 500 µA. A higher current than this can drive the multiplier out
K
[ 2.2 mF
APPLICATION INFORMATION
V
VFF
V
IAC
VAOUT(max)
2
) is sized to give the maximum I
(min)
MOUT(max)
www.ti.com
IN(min)
RMS
to 265 V
is the minimum RMS input voltage and R
* 1V
, can be determined by the equation:
RMS
VFF
gives a R
) required to produce the desired low pass
IAC
IAC
value of 750 kΩ. Because of
current at high line. For the
IAC
P
) be placed
is the total