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zener current cause R819 voltage become up to 0.7V, Q801 is triggered and OVP starts. For
12V
output OVP, ZD804 is a Zener Diode, when 14V output voltage becomes up to 16V, the
DC
zener current cause R819 voltage become up to 0.7V, Q801 is triggered and OVP starts. The
collector current of Q801 is used to make build-in diode light. FB Current of IC802 will be changed;
it can change the voltage from T801.
Q802, R827, R828 and ZD801 make up of dummy loading circuit. For start-up sequence, during
5V output take place high loading first, this dummy loading circuit operated to insure 14V not be
increased.
4. 2 Inverter circuit
4.2.1Low voltage to high voltage circuit
12V
supplies the power to IC501 through F501; the control signals that BRIGHTNESS and
DC
ON/OFF come from I/F board. ON/OFF signal connect to pin8 of IC501 and makes IC501 enabled.
BRIGHTNESS is connected to pin7 of IC501 to adjust the panel luminance. R524, R529, C505
make up of a delay-time circuit and R528, R523, R524 make up of a voltage divided circuit. C504
is used to filter the high frequency noise. The operation frequency is determined by R522 and
C529. For BURST MODE, its dimming frequency is determined by R527 and C506. C502 is used
for soft start and compensation, C502, C528 are used to filter noise.
The output drives, including NDR4, NDRV2, PDRV3, PDRV1 (pins1, 3, 15, 16 respectively),
generate a square pulses to drive MOSFET U501, U502. And U501, U502 works as full-bridge
topology, it is high efficient, zero voltage switch.
During start up, VSEN (pin9) detects the voltage at the transformer secondary. When VSEN
reaches 3.0V, the output voltage is regulated. If no current is detected for around 1.5 seconds,
IC501 will shut down.
The current flowing through CCFL is detected and regulated through sense resistor R509, R511.
The feedback voltage through R506, R507, and C508 connected to Pin11 (ISEN), and then
compared with a reference voltage (1.5V) via a current amplifier, resulting in PWM drive outputs to
full-bridge switches.
4.2.2Protection circuit
Over Voltage Protection: R501and R502 are connected in high voltage output connector, the
divided AC voltage is inverted DC voltage through D508, R505 and C507are used to rectify wave
& dump noise. Then the voltage signal reaches Pin9 VSEN of IC501, when the voltage changes,
build-in PWM of IC501 will adjust output voltage.
Open Lamp Protection: In normal operation, the resistors R510, R511, R512, R509 are sensed a
high level AC voltage, the AC signal IS1 invert DC voltage through D509, R515, C533, and the
high level DC voltage reaches the gate pin of Q502, similarly, the gate pin of Q503, Q504, Q505
has high level DC voltage. So the gate pin of Q501 has a low level voltage, and the IC501 is
normal operation. Once one of signal IS1, IS2, IS3, and IS4 is low, the voltages of Q501 gate pin
is high level, and make the voltage of ISEN low level, the IC501 will shut down.
4.3I/F Board Circuit
4.3.1Power Input
ViewSonic Corporation
Confidential - Do Not Copy
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