ViewSonic VA702-1 Service Manual page 22

2 Inverter circuit
2.1 Low voltage to high voltage circuit
12V supplies the power to IC501 through F501; the control signals that BRIGHTNESS and ON/OFF
DC
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.
2.2 Protection 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.
3 I/F Board Circuit
3.1 Power Input
+5V is from the power board and supply for U101(FS8860-18PJ)﹑U102(FS8860-33PJ)﹑
U105(MCU:W78E65P) and panel. +3.3V output is generated from +5V through C169 and C102
filtering, and U102 outputs. +3.3V is used for U104 (Scaler: TSU16AK). +1.8V output is generated
from +5V through C169, C105 and C102 filtering, and U101 outputs. +1.8V is also used for U104.
3.2 MCU (W78E65P)
VDD is +5V and its frequency of XTAL1 is 22.1184MHz. U105 #2 is defined as panel-enable. When
the I/O port is low, Q101 and Q102 are conducted. And then after C109 and C110 filtering, obtain the
voltage of VLCD, which will be connected to CN103. U105 #3 is defined as CCFL-enable. When the
I/O port is low, Q103 is pulled up and the backlights are on; When the I/O port is high, Q103 is
conducted and the backlights are off. U105 #4 is defined as DET-VGA, connected with CN102 #5.
U105 #14, #36, #37, #38, #39, #40, #41, #42, #43 are the communications with U104 (Scaler), which
are connected to #72, #31, #78, #77, #30, #70, #71, #69, #32 of Scaler. U105 #43 outputs reset signal to
U104 (Scaler).
U106 is EEPROM used for saving EDID data, which is connected by SCL and SDA pins with #16 and
#17 of MCU. Connect #12 to #26 of U105 for ISP.
3.3 Scaler (TSU16AK)
The frequency of XTAL2 is 14.318MHz. U104 #1, #102-#103, #106-#113, #118-#125, #128 output
LVDS digital data of 8 bit to panel control circuit through CN103. U104 #73 generates a PWM
waveform by regulating the duty to control the brightness of the backlights. U104 #30-#32, #69-#72,
#77-#78 are the communications with U105 (MCU) that are connected to #36-#43 of MCU. These
communications include HWRESET, CSZ/ALE, SCL/RDZ, SDA/WRZ, and AD0-AD3.
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VA702-1_VA702b-1