Table 4. Pin functions (continued)
Main input to the multiplier. This pin is connected to the rectified mains
voltage via a resistor divider and provides the sinusoidal reference to the
current loop. The voltage on this pin is used also to derive the information on
the RMS mains voltage. At startup this pin is used also to perform soft-start.
This pin can also be used as a remote ON-OFF control input by means of the
internal brownout comparator. In this case the IC performs the soft-start
function when the pin is released.
Input to the PWM comparator. The current flowing in the MOSFET is sensed
through a resistor; the resulting voltage is applied to this pin and compared to
an internal sinusoidal-shaped reference, generated by the multiplier, to
determine the turn-off instant of the external Power MOSFET. The pin is
4 CS equipped with about 220 ns digital leading-edge blanking for improved noise
immunity. A second comparison level set at 1.7 V detects abnormal currents
(e.g. due to boost inductor saturation) and, on this occurrence, activates a
safety procedure that temporarily stops the converter and limits the stress of
the power components.
Second input to the multiplier for 1/V2 function. A capacitor and a parallel
resistor must be connected from the pin to GND. They complete the internal
peak-holding circuit that derives the information on the RMS mains voltage.
The resistor should range from 100 kΩ (minimum) to 2 MΩ (maximum). The
voltage on this pin, a DC level equal to the peak voltage on pin MULT (3),
5 VFF compensates the control loop gain dependence on the mains voltage. This
pin is also internally connected to a comparator in order to provide brownout
(AC mains undervoltage) protection. A voltage below 0.8 V shuts down (not
latched) the IC and brings its consumption to a considerably lower level. The
IC restarts as the voltage at the pin goes above 0.88 V. Never connect the pin
directly to GND.
PFC pre-regulator output voltage monitoring/disable function. This pin
senses the output voltage of the PFC pre-regulator through a resistor divider
and is used for protection purposes. If the voltage on the pin exceeds 2.5 V,
the IC stops switching and restarts as the voltage falls below 2.4 V. However,
if at the same time the voltage on the INV pin falls below 1.66 V, a feedback
failure is assumed. In this case the device is latched off. Normal operation
6 PFC_OK can be resumed only by cycling VCC. If the voltage on this pin is brought
below 0.23 V, the IC is shut down. To restart the IC the voltage on the pin
must go above 0.27 V. This pin can also be used as a burst-mode control
input to synchronize the burst-mode of the IC to the one of a D2D converter
controller. Do not use this pin as remote ON/OFF control input because the
soft-start function is performed only at the startup by PFC_OK but not on the
LM-FOT modulator setting. A capacitor connected between this pin and
ground is charged by an accurate internal generator during the off-time of the
external Power MOSFET (i.e. while pin GD is low), therefore generating a
voltage ramp. As the voltage ramp equals the voltage on the MULT pin, the
off-time of the Power MOSFET is terminated, the GD pin is driven high and
the ramp is reset at zero.
Ground. Current return for both the signal part of the IC and the gate driver.
Keep the PCB trace that goes from this pin to the “cold” end of the sense
resistor separate from the trace that collects the grounding of the bias
components (output voltage sensing divider, multiplier bias divider and LM-
FOT modulator setting).
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