Service manual

15
ViewSonic Corporation Confidential - Do Not Copy VA503b-2_VA503m-2
2. 2 Inverter Circuit
2.2.1 Low voltage to high voltage circuit
14VDC provides the power for IC501; the control signals Brightness and ON/OFF come from I/F board.
ON/OFF signal connect to pin10 of IC501 and makes IC501 enable. Brightness signal connect to pin4 of IC501
and regulates the panel brightness, R526, R529, C505 make up a network of delaying time circuit and R523, R524
make up a divided voltage network, C504 is used to dump noise. The operation frequency is determined by the
external Resistor R522 and capacitor C529 connected to pin13 of IC501. BURST MODE regulated dimming
frequency is determined by the external resistor R527 and capacitor C506 connected to pin11 of IC501. C508 is
used for soft start and compensation, C507, C505 are used for dump noise.
The output drives, include DRV1, DRV2 (pins1,15 respectively) output square pulses to drive MOSFET U501,
U502, and each of U501, U502 , is consist of a N channel MOSFET. U501,OR U502 work as Push-Pull- topology, it
is high efficient, PWM switching.
During start up, VSEN (pin6) senses the voltage at the transformer secondary. When VSEN reaches 3.0V, the
output voltage is regulated. If no current is sensed approximately 2seconds IC501 shut off.
The current flowing through CCFL is sensed and regulated through sense resistor R509, R534. The feedback
voltage connected to Pin5 (ISEN), then compared with a reference voltage (1.5V) via a current amplifier, resulting in
PWM drive outputs to PUSH-PULL switches.
2.2.2 Protection circuit
Over Voltage Protection and over-current protection are monitored by the voltage on VSEN(Pin 6) During
normal operation , if a CCFL is damaged or removed ,the voltage at VSEN (Pin6) increases .Once the voltage at
VSEN exceeds 2.0V (OVPT Setting) the driver output duty cycle is regulated and the shutdown delay timer is
activated.OVPT set the overall protection threshold voltage that is lower than 3V (VSEN threshold). Once the
voltage at TIMER pin reached about 3v ,the IC will shut down and latch .R501, R503,C525,C527 are connected in
high voltage output connector, the divided AC voltage is inverted DC voltage through D503,
D504,D507,D508 ,R530 and C516are used to rectify wave & dump noise. Then the voltage signal reaches Pin6
VSEN of IC501, when the voltage changes, build-in PWM of IC501 will adjust output voltage.
Open Lamp Protection: In normal operation, R509 are sensed a high level DC voltage,If a CCFL is removed
or damaged during normal ,the voltage at SSTCMP(Pin12) rises rapidly .when the voltage at SSTCMP reaches a
threshold of approximately 2.5V,a current source charges the capacitor(C511)connected to TIMER(Pin3).Once the
voltage level at the TIMER pin reaches a threshold of approximately 3v,The drive outputs shut down and latch.