ML4836
FUNCTIONAL DESCRIPTION
(Continued)
After cathode heating, the inverter frequency drops to
F
START
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 C
X
charging current is shut off and the
inverter is inhibited until C
X
is discharged by R
X
to the
1.25V 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
.
LFB OUT is ignored by the oscillator until INTERRUPT is
above 1.20V The C
X
pin is clamped to about 7.5V.
Care should also be taken not to turn on the VCCZ clamp
so as not to dissipate excessive power in the IC. This will
cause the temp sensor to become active at a lower
ambient temperature.
A summary of the operating frequencies in the various
operating modes is shown below.
OPERATING MODE
Preheat
After
Preheat
Dimming
Control
OPERATING FREQUENCY
[F(MAX) to F(MIN)]
2
F(START)
F(MIN) to F(MAX)
7.5
6.75
R
X
/C
X
4.75
1.25
.7
0
HEAT
LEA_ENB OR
DIMMING LOCKOUT
INTERRUPT
INHIBIT
Figure10. Lamp Starting and Restart Timing
11
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
