ML4832CS datasheet(9/15 Pages) MICRO-LINEAR

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ML4832

In running mode, charging current decreases as the VPIN6

rises from 0V to VOH of the LAMP FB amplifier. The

highest frequency will be attained when ICHG is highest,

which is attained when LFB OUT is at 0V:

CHG

SET

()

(7)

Highest lamp power, and lowest output frequency are

attained when LFB OUT is at its maximum output voltage

(VOH).

In this condition, the minimum operating frequency of the

ballast is set per (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 (VLFB OUT). The discharge current is

set to 5.5mA. Assuming that IDIS >> IRT:

DIS VCO

() ≅×

600

(8)

IC BIAS, UNDER-VOLTAGE LOCKOUT AND THERMAL

SHUTDOWN

The IC includes a shunt regulator 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 0.48mA 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 ML4832 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 ML4832’s die temperature can be estimated

with the following equation:

C W

≅×

(9)

STARTING, RE-START, PREHEAT AND INTERRUPT

The lamp starting scenario implemented in the ML4832

is designed to maximize lamp life and minimize ballast

heating during lamp out conditions.

The circuit in Figure 7 controls the lamp starting scenarios:

Filament preheat and lamp out interrupt. CX is charged

with a current of IRSET/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 4.8V 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.

After cathode heating, the inverter frequency drops to

FMIN causing a high voltage to appear to ignite the lamp.

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

have ignited, the lamp voltage feedback coming into pin 9

rises to above VREF, the CX charging current is shut off and

the inverter is inhibited until CX is discharged by RX to the

1.2V threshold. Shutting off the inverter in this manner

prevents the inverter from generating excessive heat when

Figure 6. Typical VCC and ICC Waveforms when

the ML4832 is Started with a Bleed Resistor from

the Rectified AC Line and Bootstrapped from an

Auxiliary Winding.

Figure 7. Lamp Preheat and Interrupt Timers

FUNCTIONAL DESCRIPTION (Continued)

VCCZ

VON

VOFF

5.5mA

0.34mA

VCC

ICC

6.8

–

1.2/4.8

HEAT

INHIBIT

0.625

RSET

–

1.2/6.8

–

VREF

DIMMING

LOCKOUT

RX/CX

INT

ML4832CS Datasheet(PDF) 9 Page - Micro Linear Corporation

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