www.irf.com

21

Lamp Protection & Automatic Restart Circuitry Operation

from oscillator

section

+V

BUS

from lower

lamp cathode

R

CS

R3

R4

R5

R2

C1

R1

Q2

R

OC

C2

V

CC

OVER-

TEMP

DETECT

2

4.0V

5.1V

9.5V

3.0V

5.1V

1

1.0uA

7

CPH

DT

VDC

Q

T

RQ

13

SD

14

0.2V

CS

Q

S

RQ

QD

R

Q

CLK

Q

S

R

Q

UNDER-

VOLTAGE

DETECT

5.1V

8

50uA

7.6V

7.6V

7.6V

OC

3V

1V

FAULT

COUNTER

QUICK

RESTART

LOGIC

C

OC

2V

1meg

C4

from upper

lamp cathode

R7

R6

C3

Figure 12: Lamp Protection & Automatic Restart circuitry block diagram with external component connection

Sensing the AC Line Voltage

The first of these protection pins senses the voltage on the

AC line by means of an external resistor divider (R1, R2 and

capacitor C1) and an internal comparator with hysterisis.

When power is first supplied to the IC at system startup,

three conditions are required before oscillation is initiated:

1.) the voltage on the VCC pin must exceed the rising

undervoltage lockout threshold (11.5V), 2.) the voltage at

the VDC pin must exceed 5.1V, and 3.) the voltage on the

SD pin must be below approximately 4.85V. If a low ac line

condition occurs during normal operation, or if power to the

ballast is shut off, the ac line will collapse prior to the VCC of

the chip (assuming theVCC is derived from a charge pump

off of the output of the half-bridge). In this case, the voltage

on the VDC pin will shut the oscillator off, thereby protecting

the power transistors from potentially hazardous hard switch-

ing. Approximately 2V of hysterisis has been designed into

the internal comparator sensing the VDC pin, in order to

account for variations in the ac line voltage under varying

load conditions. When the ac line recovers, the chip re-

starts from the beginning of the control sequence, as shown

in timing diagram (See Figure 13).