ML4830

8

IC BIAS, UNDER-VOLTAGE LOCKOUT AND THERMAL

SHUTDOWN

The IC includes a shunt regulator which will limit the

a current limited source, typically derived from the ballast

– 0.7V, the IC draws less than 1.7mA of quiescent current

and the outputs are off. This allows the IC to start using a

“bleed resistor” from the rectified AC line.

VCCZ

t

V(ON)

V(OFF)

15mA

1.3mA

t

is started with a bleed resistor from the rectified AC line

and bootstrapped from the ballast transformer.

To help reduce ballast cost, the ML4830 includes a

temperature sensor which will inhibit ballast operation if

the IC’s junction temperature exceeds 120°C. In order to

use this sensor in lieu of an external sensor, care should be

taken when placing the IC to ensure that it is sensing

temperature at the physically appropriate point in the

ballast. The ML4830’s die temperature can be estimated

with the following equation:

TT

P

C W

JA

D

≅×

×

°

65

/

(7)

STARTING, RE-START, PREHEAT AND INTERRUPT

The lamp starting scenario implemented in the ML4830 is

designed to maximize lamp life and minimize ballast

heating during lamp out conditions.

The circuit in Figure 4 controls the lamp starting scenarios:

Filament preheat and Lamp Out interrupt. C(X) is charged

with a current of

I

or

R SET

and disch

ed

R SET

()

.

()

arg

4

0 625

through R(X). The voltage at C(X) is initialized to 0.7V

filament preheat time. During that time, the oscillator

ch

ing current I

is

R SET

in both VCO

es

CHG

arg

(

)

.

()

mod .

25

This will produce a high frequency (or low duty cycle) for

filament preheat, but will not produce sufficient voltage to

ignite the lamp.

After cathode heating, the inverter frequency drops to

If the voltage does not drop when the lamp is supposed to

have ignited, the lamp voltage feedback coming into Pin

and the inverter is inhibited until C(X) is discharged by

R(X) to the 1.2V threshold. Shutting off the inverter in this

manner prevents the inverter from generating excessive

heat when the lamp fails to strike or is out of socket.

Typically this time is set to be fairly long by choosing a

large value of R(X).

LFB OUT is ignored until C(X) reaches 6.8V threshold.

The lamps are therefore driven to full power and then

dimmed. The C(X) pin is clamped to about 7.5V.

A timing diagram of lamp ignition and restart sequences

provided by the circuit of Figure 4 is given in Figure 7.

12

10

R(X)

C(X)

6.8

+

–

1.2/3.4

HEAT

INHIBIT

.625

R(SET)

+

–

1.2/6.8

–

+

DIMMING

LOCKOUT

R(X)/C(X)

INT

Q

R

S

Figure 4. Lamp Preheat and Interrupt Timers

Mode

PWM

FM

[F(MAX) to F(MIN)]

Preheat

50%

2

Dimming

Lock-out

D(MAX)%

F(MIN)

Dimming

Control

0 to D(MAX)%

F(MIN) to F(MAX)

Figure 5. Lamp Starting Summary

A summary of the lamp starting scenarios are given in

figure 5 for both PWM and Frequency Modulation modes.

The PWM duty cycle is defined as:

Duty Cycle

t

t

ON

CLK

=