L4984D datasheet(7/35 Pages) STMICROELECTRONICS

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L4984D

Block diagram

3MULT

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.

4CS

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

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.

5VFF

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 k

Ω (minimum) to 2 MΩ (maximum). The

voltage on this pin, a DC level equal to the peak voltage on pin MULT (3),

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.

6PFC_OK

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

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

following releases.

TIMER

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.

8GND

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).

Table 4. Pin functions (continued)

Name

Function

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