UCC3818D UNITRODE, UCC3818D Datasheet - Page 9

• Manufacturer Part Number: UCC3818D
• Description: POWER FACTOR CNTRLR, 3818, SOIC16
• Manufacturer: UNITRODE

Specifications of UCC3818D

• 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

softstart
multiplier
Conduction loss is calculated as the product of the R
and the square of RMS current:
where K is the temperature factor found in the manufacturer’s R
Calculating these losses and plotting against frequency gives a curve that enables the designer to determine
either which manufacturer’s device has the best performance at the desired switching frequency, or which
switching frequency has the least total loss for a particular power switch. For this design example an IRFP450
HEXFET from International Rectifier was chosen because of its low R
R
can be found in the Unitrode Power Supply Design Seminar SEM1200, Topic 6, Design Review: 140 W, [Multiple
Output High Density DC/DC Converter].
The softstart circuitry is used to prevent overshoot of the output voltage during start up. This is accomplished
by bringing up the voltage amplifier’s output (V
slowly. Please use the following equation to select a capacitor for the softstart pin.
In this example t
In an open-loop test circuit, shorting the softstart pin to ground does not ensure 0% duty cycle. This is due to
the current amplifiers input offset voltage, which could force the current amplifier output high or low depending
on the polarity of the offset voltage. However, in the typical application there is sufficient amount of inrush and
bias current to overcome the current amplifier’s offset voltage.
The output of the multiplier of the UCC3817 is a signal representing the desired input line current. It is an input
to the current amplifier, which programs the current loop to control the input current to give high power factor
operation. As such, the proper functioning of the multiplier is key to the success of the design. The inputs to the
multiplier are VAOUT, the voltage amplifier error signal, I
voltage, and an input voltage feedforward signal, V
as:
where K is a constant typically equal to 1
The
multiplier. Additionally, curves in figures 10, 11, and 12 provide typical multiplier characteristics over its entire
operating range.
DS(on)
C
I
P
MOUT
COND
SS
electrical characteristics
+
of 0.4 Ω and the maximum V
+ I
10 mA
+ R
IAC
DELAY
DS(on)
7.5 V
t
DELAY
V
K
is equal to 7.5 ms, which would yield a C
VAOUT
K
V
VFF
I 2
* 1
table covers all the required operating conditions for designing with the
APPLICATION INFORMATION
RMS
2
DSS
V
of 500 V made it an ideal choice. An excellent review of this procedure
.
VAOUT
www.ti.com
DS(on)
VFF
) slowly which allows for the PWM duty cycle to increase
. The output of the multiplier, I
of the switch (at the worst case junction temperature)
IAC
, a representation of the input rectified ac line
DS(on)
SS
DS(on)
of 10 nF.
SLUS395J - FEBRUARY 2000 - REVISED MARCH 2009
vs. junction temperature curves.
and its V
DSS
MOUT
rating. The IRFP450’s
, can be expressed
9