Circuit Description - ViewSonic VE175-2 Service Manual

4. Circuit Description

A. DC-DC CONVERTER
The DC-DC converter converts the 12V input voltage to 5V for panel use, and 3.3V and 1.8V for controller use.
It consists of a PWM IC (LM2596), flywheel Diode (B340), buck choke (L108), and capacitor C173.
I102 (LM2596) is a PWM generator working at 150KHz.
Self protection features include a two stage frequency reducing current limit for the output switch and an over
temperature shutdown for complete protection under fault condition.
B. Scaling controller
The ADC is used to convert RGB analog signal to digital signal that scaling chip can acknowledge.
The HSYNC input receives a logic signal and provides the frequency reference for pixel clock generation.
The scaling IC converts the input signal ranging from VGA to SXGA into SXGA resolution that panel can
acknowledge. When power is first applied, the ADE3XXX is asynchronously reset. The uC typically programs the
ADE3XXX with a number of default values and sets up the ADE3XXX to identify activity on any of the input
pins. All preconfigured values and RAMs, such as line-lock PLL settings, OSD characters, LCD timing values
(output sequencer), scale kernels, gamma curves, sRGB color warp, APC dithering, output pin configuration
(OMUX), etc. can be preloaded into the ADE3XXX. The typical end state is that the ADE3XXX is initialized into
a low power mode, ready to turn active once the power button is pressed. When the monitor has been powered on,
the inputs can be monitored for active video sources. Based on the activity monitors, the uC chooses an input or
power down state.
Once an input source is selected, all available information on frequencies and line/pixel counts is measured for the
selected source and made available to the uC.
MTV312M64
The MTV312M micro-controller is an 8051 CPU core embedded device especially tailored for CRT/LCD Monitor
applications. It includes an 8051 CPU core, 1024-byte SRAM, 14 built-in PWM DACs, VESA DDC interface,
4-channel A/D converter, and a 64K-byte internal program Flash-ROM.
A "CMOS output pin" means it can sink and drive at least 4mA current. It is not recommended to use such pin as
input function.
An "open drain pin" means it can sink at least 4mA current but only drive 10~20uA to VDD. It can be used as
input or output function and needs an external pull up resistor.
A "8051 standard pin" is a pseudo open drain pin. It can sink at least 4mA current when output is at low level and
drive at least 4mA current for 160nS when output transits from low to high, then keep driving at 100uA to
maintain the pin at high level. It can be used as input or output function. It needs an external pull up resistor when
driving heavy load devices.
POWER CONFIGURATION
The MTV312M can work on either 5V or 3.3V power supply system.
In 5V power system, the VDD pin is connected to 5V power and the VDD3 needs an external capacitor, all output
pins can swing from 0~5V, input pins can accept 0~5V input range.
The ADC conversion range is 5V. However, X1 and X2 pins must be kept below 3.3V.
In 3.3V power system, the VDD and VDD3 are connected to 3.3V power, all output pins swing from 0~3.3V,
HSYNC, VSYNC and open drain pin can accept 0~5V input range, other pins must be kept below 3.3V. The ADC
conversion range is 3.3V.
C. INVERTER
In order to drive the CCFLs embedded in the panel module, there is a ROYER inverter to convert the input 12V up
to hundreds of AC voltage output.
The inverter is formed by symmetric circuitry, in order to drive the separate lamp modules.
The input stage consists of a PWM controller, buck choke, and switching MOSFET to convert DC input into AC
output.
The output stage consists of a tuning capacitor, transformer, and push-pull transistor pair to boost ac output up to
hundreds of voltage.
There is also one resister in series with the lamp for output current feedback.
A 6-pin connector is the only interface to control the inverter.
Pin 5/6 is 12V input, pin 1/2 is the returns, pin 3 is the control of output current, and pin 4 is the enable/disable
control.
ViewSonic Corporation
10
Confidential – Do Not Copy
VE175/b-2