ML4836CS Просмотр технического описания (PDF) - Micro Linear Corporation

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ML4836CS

Compact Fluorescent Electronic Dimming Ballast Controller

Micro Linear Corporation 

ML4836

FUNCTIONAL DESCRIPTION (Continued)

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 7.5mA. The operation of the oscillator is

hown in Figure 7.

Assuming that IDIS >>IRT:

tDIS(VCO) @ 600 ´ CT

(9)

IC BIAS, UNDER-VOLTAGE LOCKOUT AND

THERMAL SHUTDOWN

The IC includes a shunt clamp which will limit the voltage

at VCC to 14.5V (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. The ICC

start-up condition is shown in Figure 7. To help reduce

ballast cost, the ML4836 includes a temperature sensor

which will inhibit ballast operation if the IC’s junction

temperature exceeds 140°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

ML4836’s die temperature can be estimated with the

following equation:

TJ @ TA + (PD + 65° C / W)

(10)

STARTING, RE-START, PREHEAT AND INTERRUPT

The lamp starting scenario implemented in the ML4836

is designed to maximize lamp life and minimize ballast

heating during lamp out conditions.

The circuit in Figure 9 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 4.65V 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.

RX/CX

10

RX

0.625

RSET

+

CX

1.75/4.65

–

INTERRUPT

9

+

1.0/1.20

–

HEAT

LEA_ENB OR

DIMMING LOCKOUT

S Q INHIBIT

+

R

1.25/6.65

–

Figure 9. Lamp Preheat and Interrupt Timers

10

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