UCC28500 Datasheet, PDF (24/31 Pages) Texas Instruments – BICMOS PFC/PWM COMBINATION CONTROLLER

UCC28500, UCC28501, UCC28502, UCC28503
UCC38500, UCC38501, UCC38502, UCC38503
SLUS419C − AUGUST 1999 − REVISED NOVEMBER 2001
TYPICAL APPLICATION
Figure 13 shows the circuitry for the voltage feedback loop. D13 is a TL431 shunt regulator that functions as
an operational amplifier, providing feedback control.
VBOOST
VREF
R13
V_C = V_ERR
VOUT
G^GC_C^O_O⁽₍^s_s⁾₎
Q5
H11AV1
6
1
5
R36
D14
R16 PGND2
2
4
3
U3
C14
C15
R35
D13
SGND
R31
R27
Figure 13. UCC38500 Feedback Loop
UDG−01091
Initially the designer must select the resistor values for the divider gain H(s). Equation (42) is used to determine
resistor size. Selecting R27 to be a standard value of 10-kΩ requires R31 to be approximately 38.3 kΩ.
R31
+
R27
ǒ_{VOUT *}
V_REF
V_REF^Ǔ
(42)
It is important to correctly bias the TL431 and the optoisolator for proper operation. Zener diode D14 and a
depletion mode J-FET, Q5, supply the bias voltage for the TL431. Resistors R16 and R13 provide the minimum
bias currents for the TL431 and the optoisolator respectively and can be calculated with the following equations.
Where I_OP(min) is the minimum optoisolation current, and V_VERR(max) is the maximum voltage seen at the VERR
pin of the UCC38500. VERR has an internal clamp that limits this pin to 4.5 V. V_F is the typical forward voltage
of the diode in the opto isolator, and I_TL431(min) is the minimum cathode current of the TL431. For the
components used in this design example R13 is calculated to be approximately 2.0 kΩ and R16 was calculated
to be approximately 680 Ω. The optoisolator is configured to have dc gain of approximately 20 dB and the
optoisolator −3 dB point is approximately 8 kHz. Figure 14 shows the frequency response of the optoisolator.
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