SO16N datasheet(10/51 Pages) STMICROELECTRONICS

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Pin settings

L6566BH

10/51

Doc ID 16610 Rev 2

ZCD

Transformer demagnetization sensing input for quasi-resonant operation and OVP

input. The pin is externally connected to the transformer’s auxiliary winding through

a resistor divider. A negative-going edge triggers MOSFET turn-on if QR mode is

selected.

A voltage exceeding 5 V shuts the IC down and brings its consumption to a lower

value (OVP). Latch OFF or auto-restart mode is selectable externally. This function

is strobed and digitally filtered to increase noise immunity.

MODE/SC

Operating mode selection. If the pin is connected to the VREF pin (7), quasi-

resonant operation is selected, the oscillator (pin 13, OSC) determines the

maximum allowed operating frequency.

Fixed-frequency operation is selected if the pin is not tied to VREF, in which case

the oscillator determines the actual operating frequency, the maximum allowed

duty cycle is set at 70% min. and the pin delivers a voltage ramp synchronized to

the oscillator when the gate-drive output is high; the voltage delivered is zero while

the gate-drive output is low. The pin is to be connected to pin CS (7) via a resistor

for slope compensation.

OSC

Oscillator pin. The pin is an accurate 1 V voltage source, and a resistor connected

from the pin to GND (pin 3) defines a current. This current is internally used to set

the oscillator frequency that defines the maximum allowed switching frequency of

the L6566BH, if working in QR mode, or the operating switching frequency if

working in FF mode.

Soft-start current source. At startup, a capacitor Css between this pin and GND

(pin 3) is charged with an internal current generator. During the ramp, the internal

reference clamp on the current sense pin (7, CS) rises linearly starting from zero to

its final value, therefore causing the duty cycle to increase progressively starting

from zero as well. During soft-start the adaptive UVLO function and all functions

monitoring the COMP pin are disabled. The soft-start capacitor is discharged

whenever the supply voltage of the IC falls below the UVLO threshold. The same

capacitor is used to delay IC shutdown (latch OFF or auto-restart mode selectable)

after detecting an overload condition (OLP).

VFF

Line voltage feedforward input. The information on the converter’s input voltage is

fed into the pin through a resistor divider and is used to change the setpoint of the

pulse-by-pulse current limitation (the higher the voltage, the lower the setpoint).

The linear dynamics of the pin ranges from 0 to 3 V. A voltage higher than 3 V

makes the IC stop switching. If feedforward is not desired, tie the pin to GND (pin 3)

directly if a latch-mode OVP is not required (see pin 11, ZCD) or through a 10 k

Ω

min. resistor if a latch-mode OVP is required. Bypass the pin with a capacitor to

GND (pin 3) to reduce noise pick-up.

AC_OK

Brownout protection input. A voltage below 0.45 V shuts down (not latched) the IC,

lowers its consumption and clears the latch set by latched protection (DIS > 4.5 V,

SS > 6.4 V, VFF > 6.4 V). IC operation is re-enabled as the voltage exceeds 0.45 V.

The comparator is provided with current hysteresis: an internal 15 µA current

generator is ON as long as the voltage on the pin is below 0.45 V and is OFF if this

value is exceeded. Bypass the pin with a capacitor to GND (pin 3) to reduce noise

pick-up. Tie to Vcc with a 220 to 680 kW resistor if the function is not used.

Table 1.

Pin functions (continued)

N°

Function

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