ML4833CS datasheet(9/13 Pages) MICRO-LINEAR

9 / 13 page

ML4833

This assumes that tCHG >> tDIS.

When LFB OUT is high, ICH = 0 and the minimum

frequency occurs. The charging current varies according

to two control inputs to the oscillator:

1. The output of the preheat timer

2. The voltage at LFB OUT (lamp feedback amplifier

output)

In preheat condition, charging current is fixed at

CHG PREHEAT

SET

()

= 25

(6)

In running mode, charging current decreases as the

voltage rises from 0V to VOH at the LAMP FB amplifier.

The highest frequency will be attained when ICHG is

highest, which is attained when voltage at LFB OUT

is at 0V:

CHG

SET

()

(7)

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 5mA. Assuming that IDIS >>

IRT:

DIS VCO

() ≅×

600

(8)

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 0.48mA 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 ML4833 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 ML4833’s die temperature can be estimated

with the following equation:

C W

≅+

+ °

)

(9)

VCCZ

V(ON)

V(OFF)

5.5mA

0.34mA

VCC

ICC

Figure 7. Typical VCC and ICC Waveforms when

the ML4833 is Started with a Bleed Resistor from

the Rectified AC Line and Bootstrapped from an

Auxiliary Winding.

STARTING, RE-START, PREHEAT AND INTERRUPT

The lamp starting scenario implemented in the ML4833

is designed to maximize lamp life and minimize ballast

heating during lamp out conditions.

The circuit in Figure 8 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 4.8V 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 or cause significant glow current.

After cathode heating, the inverter frequency drops to FMIN

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 lamp voltage feedback coming into

pin 8 remains below 1.25V, 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 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.

ML4833CS Datasheet(PDF) 9 Page - Micro Linear Corporation

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