Datasheet

   
   
SLUS419C − AUGUST 1999 − REVISED NOVEMBER 2001
18
www.ti.com
TYPICAL APPLICATION
R
MOUT
+
P
LIMIT
2
Ǹ
R
SENSE
V
IN (min)
I
MOUT(max)
current loop
The UCC38500 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 UCC38500 takes advantage of this phase inversion
to implement leading-edge duty cycle modulation. Please refer to Figure 10 for the typical configuration of the
current amplifier.
The following equation defines the gain of the power stage, where V
P
is the voltage swing of the oscillator ramp,
4 V for the UCC38500.
G
ID
(s) +
V
BOOST
R
SENSE
s L
BOOST
V
P
In order to have a good dynamic response the crossover frequency of the current loop was set to 10% of the
switching frequency. This can be achieved by setting the gain of the current amplifier (G
CA
) to the inverse of
the current loop power stage gain at the crossover frequency. This design requires that the current amplifier
have a gain of 2.581 at 10 kHz.
G
CA
+
1
G
ID
(s)
+ 2.581
R
I
is the R
MOUT
resistor, previously calculated to be 3.57 kΩ (refer to Figure 10). 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 9.09 kΩ. Setting a zero at
the crossover frequency and a pole at half the switching frequency to roll off the high-frequency gain completes
the current loop compensation.
C
Z
+
1
2p R
F
f
C
C
P
+
1
2p R
F
ǒ
f
s
2
Ǔ
(18)
(19)
(20)
(21)
(22)