ISL6730 Datasheet, PDF (10/19 Pages) Intersil Corporation – Reduce component size requirements

ISL6730A, ISL6730B, ISL6730C, ISL6730D
Functional Description
VCC Undervoltage Lockout (UVLO)
The ISL6730A, ISL6730B, ISL6730C, ISL6730D 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 ISL6730A, ISL6730B, ISL6730C,
ISL6730D 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 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----^B----^O----^R-----^M----^A----^X----------
V_RMSmin – 2V_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
=
-----^K----^B----^O-------
1 – K_BO
⋅
R_I
N
2
(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
=
1_--
2
⋅
--^R-----^C----^S----
R_SEN
⋅
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.
10
FN8258.1
August 8, 2013