3/23

L6668

Table 4. Pin Description

Pin

Number

Pin Name

Function

1

HV

High-voltage start-up. The pin is to be connected directly to the rectified mains voltage. A

0.8 mA internal current source charges the capacitor connected between pin Vcc and GND

to start up the IC. When the voltage on the Vcc pin reaches the start-up threshold the gen-

erator is shut down. Normally it is re-enabled when the voltage on the Vcc pin falls below

5V, except under latched shutdown conditions, when it is re-enabled as the Vcc voltage falls

0.5V below the start-up threshold.

2

HVS

High-voltage spacer. The pin is not connected internally to isolate the high-voltage pin and

comply with safety regulations (creepage distance) on the PCB.

3

GND

Chip ground. Current return for both the gate-drive current and the bias current of the IC. All

of the ground connections of the bias components should be tied to a track going to this pin

and kept separate from any pulsed current return.

4

OUT

Gate-drive output. The driver is capable of 0.8A min. source/sink peak current to drive

MOSFET’s. The voltage delivered to the gate is clamped at about 15V so as to prevent too

high values when the IC is supplied with a voltage close to or exceeding 20V.

5

Vcc

Supply Voltage of both the signal part of the IC and the gate driver. The internal high volt-

age generator charges an electrolytic capacitor connected between this pin and GND as

long as the voltage on the pin is below the start-up threshold of the IC, after that it is dis-

abled. Sometimes a small bypass capacitor (0.1µF typ.) to GND might be useful to get a

clean bias voltage for the signal part of the IC.

6

N.C.

Connect the pin to GND.

7

DIS

Latched device shutdown. Internally the pin connects a comparator that, when the voltage

on the pin exceeds 2.2V, shuts the IC down and brings its consumption to a value barely

higher than before start-up. The information is latched and it is necessary to recycle the

input power to restart the IC: the latch is removed as the voltage on the Vcc pin goes below

the UVLO threshold. Connect the pin to GND if the function is not used.

8

VREF

Voltage reference. An internal generator furnishes an accurate voltage reference (5V±4%,

all inclusive) that can be used to supply up to 5 mA to an external circuit. A small film

capacitor (0.1µF typ.), connected between this pin and GND is recommended to ensure the

stability of the generator and to prevent noise from affecting the reference.

9

SKIPADJ

Burst-mode control threshold. A voltage is applied to this pin, derived from the reference

voltage VREF via a resistor divider. When the control voltage at pin COMP falls 50 mV

below the voltage on this pin the IC is shutdown and the consumption is reduced. The chip

is re-enabled as the voltage on pin COMP exceeds the voltage on the pin. The high-voltage

start-up generator is not invoked. The function is disabled during the soft-start ramp. The

pin must always be biased between 0.8 and 2.5V. A voltage between 0.8 and 1.4V disables

the function, if the pin is pulled below 0.8V the IC is shut down.

10

COMP

Control input for PWM regulation. The pin is to be driven by the phototransistor (emitter-

grounded) of an optocoupler 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, 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 50 mV lower than that on

pin SKIPADJ shuts down the IC and reduces its current consumption.

11

SS

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

the pin and GND generating a voltage ramp across it. This ramp clamps the voltage at pin

COMP during start-up, thus the duty cycle of the power switch starts from zero. During the

ramp all functions monitoring the voltage at pin COMP are disabled. The SS capacitor is

quickly discharged as the chip goes into UVLO.

12

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

gate drive output (previously asserted high by the clock signal generated by the oscillator)

is driven low to turn off the power MOSFET. The pin is equipped with 200 ns. min. blanking

time for improved noise immunity. A second comparison level located at 1.5V shuts the

device down and brings its consumption almost to a “before start-up” level.