Datasheet

R
I
+
R
f
C
P
CAOUT
C
Z
+
+
R
SENSE
+
MULT
Z
f
PWM
COMPARATOR
CA
Q
BOOST
L
BOOST
V
OUT
Start Up
I
C
+ C
DV
Dt
UCC2817-EP
UCC2818-EP
www.ti.com
........................................................................................................................................................................................... SLUS716 DECEMBER 2008
Figure 3. Current Loop Compensation
The UCC2817 current amplifier has the input from the multiplier applied to the inverting input. This change in
architecture from previous TI PFC controllers improves noise immunity in the current amplifier. It also adds a
phase inversion into the control loop. The UCC2817 takes advantage of this phase inversion to implement
leading-edge duty cycle modulation. Synchronizing a boost PFC controller to a downstream dc-to-dc controller
reduces the ripple current seen by the bulk capacitor between stages, reducing capacitor size and cost and
reducing EMI. This is explained in greater detail in a following section. The UCC2817 current amplifier
configuration is shown in Figure 4 .
Figure 4. UCC2818 Current Amplifier Configuration
The UCC2818 version of the device is intended to have VCC connected to a 12-V supply voltage. The UCC2817
has an internal shunt regulator enabling the device to be powered from bootstrap circuitry, as shown in the
typical application circuit of Figure 1 . The current drawn by the UCC2817 during undervoltage lockout, or start-up
current, is typically 150 µ A. Once V
CC
is above the UVLO threshold, the device is enabled and draws
4 mA typically. A resistor connected between the rectified ac line voltage and the V
CC
pin provides current to the
shunt regulator during power up. Once the circuit is operational, the bootstrap winding of the inductor provides
the V
CC
voltage. Sizing of the start-up resistor is determined by the start-up time requirement of the system
design.
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