ISL6731A Datasheet, PDF (11/20 Pages) Intersil Corporation – Power Factor Correction Controllers

ISL6731A, ISL6731B
Functional Description
VCC Undervoltage Lockout (UVLO)
The ISL6731A and ISL6731B start automatically once the
voltage at VCC exceeds the UVLO threshold.
Shutdown
When the VFB pin is below 0.2V, the controller is disabled and
the PWM output driver is tri-stated. When disabled, the IC power
will be reduced. During shutdown, the COMP pin is discharged to
GND and the controller is disabled. The Over-Temperature
Protection (OTP) is still alive to prevent the controller from
starting up in a high temperature ambient condition.
In the event that the FB pin is disconnected from the feedback
resistors, the FB pin is pulled to ground by an internal current
source I_FB. When the FB pin voltage drops below 0.2V, the gate
driver is disabled. The ISL6731A or ISL6731B enters shutdown
mode.
Soft-Start
The COMP pin is released once the soft-start operation begins. A
13µA current sources out to the RC network connected from the
COMP pin until the FB pin voltage reaches 90% of the reference
voltage.
Switching is inhibited when the COMP pin voltage is below 1V.
When the COMP pin reaches 1V, the current error amplifier and
the gate driver are activated and the converter starts switching.
During UVLO, brownout and shutdown, the COMP is pulled to the
ground.
Input Voltage Sensing
The VIN pin is needed to sense the rectified input voltage. The
sensed semi-sinusoidal waveform is needed to shape the
inductor current, which helps achieves unity power factor. At the
same time, the voltage on the VIN pin is used to generate the
negative capacitive element at the input. This will cancel the
input filter capacitor, C_F. Canceling the effect of C_F will increase
the displacement power factor and alleviate the zero crossing
distortion, which is related to the distortion power factor.
EMI CHOKE
V_LINE
C_F3
C_F2
L_m
D_F1
D_F2
R_IN2
VIN
R_IN1
BO
C_BO
FIGURE 10. INPUT VOLTAGE SENSING SCHEMATIC
The BO pin also utilizes the VIN resistor divider for voltage
sensing. Set the resistor divider ratio to satisfy the brownout
requirement.
First, calculate the resistor divider ratio, K_BO.
K_BO = _V-----_R---^V-_M---^B--_S--^O--_m--^R--_i--_n-^M---–-^A----₂^X----_V----_F--
(EQ. 1)
Where V_F is the forward voltage drop of the bridge rectifier and
the voltage drop of D_F1; D_F2.
Then, select the R_IN2 based on the highest reasonable resistance
value. Then select the R_IN1 based upon the desirable minimum
RMS value of the line voltage for the PFC operation.
R_IN1 = ₁-----^K_–----^B-_K---^O-_B----_O--  R_IN2
(EQ. 2)
Inductor Current Sensing
The current sensing of the converter has two purposes. One is to
force the inductor current to track the input semi-sinusoidal
waveform. The other purpose is for overcurrent protection. Refer to
Figure 11 for the current sensing scheme. The sensed current I_CS
is in proportion to the inductor current, I_L as described in
Equation 3:
I_CS = ¹²--  _R--^R---_S--^C--_E--^S--_N--  I_L
(EQ. 3)
where:
R_CS is the current sensing resistor with low value in the return
path to the bridge rectifier.
R_SEN is the current scaling resistor connected between ISEN to
the R_CS.
V_I
L
VOUT
Q1
COUT
CF1
RCS
RSEN
CURRENT
MIRROR
2:1
ICS
ISEN
ICS > 0.5 IOC
FIGURE 11. INDUCTOR CURRENT SENSING SCHEME
A high value R_CS renders more accurate current sensing. It is
recommended to use the R_CS to render 120mV peak voltage at
the maximum line voltage during full load condition.
R_C
S

-¹---²---⁰----^m-----^V------^---^V----^R----^M-----^S----^M-----^A----^X-----^---^--
2  P_Omax
(EQ. 4)
Where is the efficiency of the converter at the maximum line
input with full load.
Submit Document Feedback ₁₁
FN8582.1
February 13, 2015