ML4835CS datasheet(11/17 Pages) MICRO-LINEAR

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ML4835

Highest lamp power, and lowest output frequency are

attained when voltage at LFB OUT is at its maximum

output voltage (VOH).

In this condition, the minimum operating frequency of the

ballast is set per equation 5 above.

For the IC to be used effectively in dimming ballasts with

higher Q output networks a larger CT value and lower RT

value can be used, to yield a smaller frequency excursion

over the control range (voltage at LFB OUT). The

discharge current is set to 5.5mA.

Assuming that IDIS >>IRT:

DIS VCO

() @´

600

(9)

IC BIAS, UNDER-VOLTAGE LOCKOUT AND

THERMAL SHUTDOWN

The IC includes a shunt clamp which will limit the

voltage at VCC to 15V (VCCZ). The IC should be fed with

a current limited source, typically derived from the

ballast transformer auxiliary winding. When VCC is below

VCCZ – 1.1V, the IC draws less than 0.55mA of quiescent

current and the outputs are off. This allows the IC to start

using a “bleed resistor” from the rectified AC line.

To help reduce ballast cost, the ML4835 includes a

temperature sensor which will inhibit ballast operation if

the IC’s junction temperature exceeds 130°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 ML4835’s die temperature can be estimated

with the following equation:

C W

+ °

(

)

(10)

STARTING, RE-START, PREHEAT AND INTERRUPT

The lamp starting scenario implemented in the ML4835

is designed to maximize lamp life and minimize ballast

heating during lamp out conditions.

The circuit in Figure 10 controls the lamp starting

scenarios: Filament preheat and lamp out interrupt. CX is

charged with a current of IR(SET)/4 and discharged through

RX. The voltage at CX is initialized to 0.7V (VBE) at power

up. The time for CX to rise to 4.75V is the filament preheat

time. During that time, the oscillator charging current

(ICHG) is 2.5/RSET. This will produce a high frequency for

filament preheat, but will not produce sufficient voltage

to ignite the lamp or cause significant glow current.

After cathode heating, the inverter frequency drops to

FSTART causing a high voltage to appear to ignite the

lamp. If lamp current is not detected when the lamp is

supposed to have ignited, the CX charging current is shut

off and the inverter is inhibited until CX is discharged by

RX to the 1.25V 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 RX.

LFB OUT is ignored by the oscillator until INTERRUPT is

above 1.25V The CX pin is clamped to about 7.5V.

Care should also be taken not to turn on the VCCZ clamp

so as not to dissipate excessive power in the IC. This will

cause the temp sensor to become active at a lower

ambient temperature.

A summary of the operating frequencies in the various

operating modes is shown below.

OPERATING MODE

OPERATING FREQUENCY

[F(MAX) to F(MIN)]

Preheat

After

Preheat

F(START)

Dimming

Control

F(MIN) to F(MAX)

Figure 10. Lamp Preheat and Interrupt Timers

FUNCTIONAL DESCRIPTION (Continued)

HEAT

INHIBIT

0.625

RSET

LEA_ENB OR

DIMMING LOCKOUT

INTERRUPT

RX/CX

1.25/4.75

–

1.0/1.25

–

1.25/6.75

–

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