ML4836
FUNCTIONAL DESCRIPTION (Continued)
In this condition, the minimum operating frequency of the
ballast is set per equation 5 above.
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:
For the IC to be used effectively in dimming ballasts with
higher Q output networks a larger C value and lower R
T
T
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.
TJ TA (PD 65 C / W)
(10)
Assuming that I >>I :
DIS
RT
STARTING, RE-START, PREHEAT AND INTERRUPT
tDIS(VCO) 600 CT
(9)
The lamp starting scenario implemented in the ML4836
is designed to maximize lamp life and minimize ballast
heating during lamp out conditions.
IC BIAS, UNDER-VOLTAGE LOCKOUT AND
THERMAL SHUTDOWN
The IC includes a shunt clamp which will limit the voltage
at V to 14.5V (V ). The IC should be fed with
The circuit in Figure 9 controls the lamp starting scenarios:
Filament preheat and lamp out interrupt. C is charged
CC
CCZ
X
a current limited source, typically derived from the ballast
transformer auxiliary winding. WhenV is below
with a current of I
/4 and discharged through R . The
R(SET) X
voltage at C is initialized to 0.7V (V ) at power up. The
CC
X
BE
V
– 1.1V, the IC draws less than 0.55mA of quiescent
time for C to rise to 4.65V is the filament preheat time.
CCZ
X
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
During that time, the oscillator charging current (I
) is
CHG
2.5/R . This will produce a high frequency for filament
SET
preheat, but will not produce sufficient voltage to ignite
the lamp or cause significant glow current.
0.625
R
SET
R /C
X
X
+
–
10
9
HEAT
R
X
C
X
1.75/4.65
1.0/1.20
LEA_ENB OR
DIMMING LOCKOUT
+
–
INTERRUPT
S
Q
INHIBIT
+
–
R
1.25/6.65
Figure 9. Lamp Preheat and Interrupt Timers
10
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
