Service manual

ViewSonic Corporation Confidential - Do Not Copy VA903b-1_ VA903m-1
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4. Circuit Description
A. AC-DC CONVERTER
The power supply with a high-integrated green-mode PWM controller provides several features to
enhance the performance of power flyback converters.
U801 is a PWM controller and provides many protection functions.
U802 is a photo couple to transfer the feedback signal from the second side which U803 detected both of
the output DC voltage on 5V and 24V.
Z802 detected the working voltage on U801 and Q802 would pull down the voltage on U801 pin5 to
shut down U801 if feedback loop was failed.
R828 would be a dynamic load which was active while backlight was turned off and system still
working on, this kept 24V output voltage under 32V to protect the panel. After system went to
power saving mode, the R828 would no loaded. This was detected by Z803 active the R828 to load or
not.
B，Scaling Controller
Overview
Realtek RTD2523B series products are all-in-one LCD monitor controllers support up to
SXGA/XGA(optional), and integrate Realtek high performance ADC, TMDS Rx(optional), scaling engine,
OSD engine, LVDS TX, RSDS TX and so on. Moreover, all products are pin compatible in low pin count
package to save cost and make the design easier
General
z Embedded dual DDC support DDC1, DDC2B, DDC/CI
z Zoom scaling up and down
z Embedded Pattern Generator
z No external memory required.
z Require only one crystal to generate all timing
z Embedded reset control output
z Embedded crystal output to MICROP
z 3 channels 8 bits PWM output, and selectable PWM clock frequency.
Pin Description
(I/O Legend: A = Analog, I = Input, O = Output, P = Power, G = Ground)
 ADC: 15 pins
Name
I/O
Pin No Description Note
ADC_GND AG 27 ADC ground
ADC_REFIO AP 28 ADC band-gap voltage de-coupling 1.20V
ADC_VDD AP 29 Analog power (3.3V)
BLUE+ AI 30 Analog input from BLUE channel
BLUE- AI 31
Analog input ground from BLUE channel
ADC_GND AG 32 ADC ground
SOG/ADC_TEST AIO 33 SOG in/ADC test pin
GREEN+ AI 34 Analog input from GREEN channel
GREEN- AI 35 Analog input ground from GREEN
channel
ADCB_VDD AP 36 Analog power (3.3V)
RED+ AI 37 Analog input from RED channel