L6591 datasheet(5/32 Pages) STMICROELECTRONICS

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L6591

Pin settings

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Pin N.

Name

Function

ISEN

Current sense (PWM comparator) input. The voltage on this pin is internally

compared with an internal reference derived from the voltage on pin COMP

and when they are equal the high-side gate drive output (previously

asserted high by the clock signal generated by the oscillator) is driven low to

turn off the upper power MOSFET; the lower MOSFET is turned on after a

delay programmed by the timing capacitor at pin OSC (#5). The pin is

equipped with 200 ns blanking time for improved noise immunity. A second

comparator referenced at 0.8 V turns off the upper MOSFET if the voltage

at the pin exceeds the threshold, overriding the PWM comparator (pulse-by-

pulse OCP). A third comparison level located at 1.5 V shuts the device

down and brings its consumption almost to a “before start-up” level (hiccup-

mode OCP) to prevent uncontrolled current rise. A logic circuit improves

sensitivity to temporary disturbances.

4SS

Soft-start. An internal 20 µA generator charges an external capacitor

connected between the pin and GND (#11) generating a voltage ramp.

During the ramp, the internal reference for pulse-by-pulse OCP (see pin #3,

ISEN) rises linearly starting from zero to its final value, thus causing the

duty cycle of the upper MOSFET to rise starting from zero as well, and all

the functions monitoring pin COMP (#7) are disabled. The same capacitor

is used to delay IC’s shutdown (latch-off or auto-restart mode selectable)

after detecting an overcurrent condition. The SS capacitor is quickly

discharged as the chip goes into UVLO.

5OSC

Oscillator pin. A resistor to VREF (#6) and a capacitor from the pin to GND

(#11) define the oscillator frequency. The maximum duty cycle is limited

below 50 % by an internal T-flip-flop. As a result, the switching frequency will

be half that of the oscillator. The capacitor value defines the dead-time

separating the conduction state of either MOSFET. This capacitor should

not be lower than 220 pF.

6VREF

Voltage reference. An internal generator furnishes an accurate voltage

reference (5 V±4 %, all inclusive) that can be used to supply up to 5 mA to

an external circuit. A small film capacitor (0.1 µF typ.), connected between

this pin and GND (#11) is recommended to ensure the stability of the

generator and to prevent noise from affecting the reference.

7COMP

Control input for PWM regulation. The pin is to be driven by the

phototransistor (emitter-grounded) of an octocoupler to modulate the

voltage by modulating the current sunk from (sourced by) the pin (0.4 mA

typ.). It is recommended to place a small filter capacitor between the pin

and GND (#11), as close to the IC as possible to reduce switching noise

pick up, to set a pole in the output-to-control transfer function. A voltage

lower than 1.75 V shuts down the IC and reduces its current consumption.

The chip restarts as the voltage exceeds 1.8 V. This function realizes burst-

mode operation at light load.

8PFC_STOP

Open-drain ON/OFF control of PFC controller. This pin is intended for

temporarily stopping the PFC controller at light load in systems comprising

a PFC pre-regulator, during burst-mode operation (see pin COMP, #7). The

pin, normally open, goes low if the voltage on COMP is lower than 1.75 V

and opens when the voltage on pin COMP exceeds 1.8V. Whenever the IC

is shut down (SS > 5 V, DIS>4.5, ISEN >1.5 V) the pin is low as well,

provided the supply voltage of the IC is above the restart threshold (typ. 5

V). It is open during UVLO. Leave the pin open if not used

Table 1.

Pin functions (continued)

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