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1. Input supply voltage
2. Input 2Vdc from microprocessor *
* This 2Vdc becomes a 372 Hz 4Vp-p pulse after the lamp turns on.
Ballast Block Operation
As you can conclude from the turn-on sequence in table 9, once the microprocessor
provides the turn-on signal to the ballast board, the function of the ballast circuit is to:
1. Generate 20kVac to ignite the lamp.
2. Inform the microprocessor that the lamp is on, to prevent it from re-triggering the
ballast.
3. Internally monitor the lamp current and regulate the voltage applied to the lamp
for consistent brightness.
4. Shut off voltage to the lamp if it does not ignite (and inform the microprocessor).
1. Generate 20KV to ignite the lamp:
Refer to figure 18. The operation of the ballast circuit can be divided into 3 parts: Start
voltages, Control Circuits, and HV Output.
• Start Voltages to the Ballast board - When the TV is powered on, the power supply
delivers +335Vdc to the ballast board connector B1/pin 1. Use either B1/pin 3 or its
nearby test point CN005 for ballast ground. The ballast board ground is floating and
is not referenced to either hot or cold ground.
The +335V is filtered by L001 and C001, before passing through R106, to the DC-to-
DC converter stage, consisting of IC001, T001, and D003. The converter stage is a
regulated 15Vdc, for the ballast control circuit (MC001/2) operation. No PWM or
drive signals are output from this control circuit until it receives a ballast-on
command.
Ballast-ON - This command comes from the microprocessor (DG board) as a High
(B2/pin 1 = 2.16Vdc, with respect to cold chassis ground or B2/pin 2). This High
lasts for approximately 3 seconds and then switches to 4Vp-p pulses after the lamp
turns on. The High or 4Vp-p pulses keep all three sub-circuits within the ballast
control modules MC001 & MC002 turned on.
• Three Control Circuits - The first sub-circuit controls the switching regulator, Q002.
This PWM circuit adjusts the output voltage in the range of 20 to 300Vdc (after
filtering by L004). It gradually increases the lamp brightness to insure long lamp life
after the initial start voltage. The base frequency of this PWM circuit is 82.6 kHz.
The second sub-circuit generates a variable frequency square wave signal starting
at a high frequency, settling down to 372 Hz when the lamp is warmed. This second
circuit drives the four MOSFET drivers located on the heat sink and ultimately
becomes the frequency applied to the lamp. With two circuits, to adjust the voltage
and frequency, the lamp is offered a wide range of parameters to insure reliable
starting and long life.
Ballast On Requirements
Voltage
+335Vdc
2Vdc / 4Vp-p *
Table 11
24
Location
Ground
B1/pin 1
CN005 or B1/pin 3
B2/pin 1
Chassis (cold gnd)
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