Datasheet

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SLUS395J - FEBRUARY 2000 - REVISED MARCH 2009
13
www.ti.com
APPLICATION INFORMATION
current loop (continued)
R
I
is the R
MOUT
resistor, previously calculated to be 3.9 kΩ. (refer to Figure 3). The gain of the current amplifier
is R
f
/R
I
, so multiplying R
I
by G
EA
gives the value of R
f
, in this case approximately 12 kΩ. Setting a zero at the
crossover frequency and a pole at half the switching frequency completes the current loop compensation.
C
Z
+
1
2 p R
f
f
C
C
P
+
1
2 p R
f
f
s
2
R
I
+
−
R
f
C
P
CAOUT
C
Z
Figure 3. Current Loop Compensation
The UCC3817 current amplifier has the input from the multiplier applied to the inverting input. This change in
architecture from previous Texas Instruments PFC controllers improves noise immunity in the current amplifier.
It also adds a phase inversion into the control loop. The UCC3817 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 UCC3817 current amplifier
configuration is shown in Figure 4.
+
−
+
−
R
SENSE
+−
MULT
Z
f
PWM
COMPARATOR
CA
Q
BOOST
L
BOOST
V
OUT
Figure 4. UCC3817 Current Amplifier Configuration