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Fabricante

ML4831

Electronic Ballast Controller

Micro Linear Corporation 

ML4831

The oscillator’s minimum frequency is set when ICH = 0

where:

FOSC

≅

1

0.51× RTCT

(5)

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 Pin 6 (lamp feedback amplifier

output)

In preheat condition, charging current is fixed at

ICHG(PREHEAT)

=

2.5

R(SET)

(6)

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 VPIN6 is at 0V:

ICHG(0)

=

5

R(SET)

(7)

Highest lamp power, and lowest output frequency are

attained when VPIN6 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 (VPIN6). The discharge current is set

to 5mA. Assuming that IDIS >> IRT:

tDIS(VCO) ≅ 490 × CT

(8)

IC BIAS, UNDER-VOLTAGE LOCKOUT AND THERMAL

SHUTDOWN

The IC includes a shunt regulator which will limit the

voltage at VCC to 13.5 (VCCZ). The IC should be fed with

a current limited source, typically derived from the ballast

transformer auxiliary winding. When VCC is below

VCCZ – 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.

To help reduce ballast cost, the ML4831 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 ML4831’s die temperature can be estimated

with the following equation:

TJ ≅ TA × PD × 65°C / W

(9)

VCC VCCZ

V(ON)

V(OFF)

ICC

t

15mA

1.3mA

t

Figure 6. Typical VCC and ICC Waveforms when

the ML4831 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 ML4831

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. C(X) is charged

with a current of IR(SET)/4 and discharged through R(X).

The voltage at C(X) is initialized to 0.7V (VBE) at power

up. The time for C(X) to rise to 3.4V is the filament preheat

time. During that time, the oscillator charging current

(ICHG) is 2.5/R(SET). 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 C(X) charging current is shut off 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).

8

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