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Workstation 5 Field Service Guide
Workstation 5 System Board Technical Descriptions
System Board Power Distribution
Workstation 5 Backlight Inverter Board - XBRE38
Functional Description
The Workstation 5 Backlight Inverter Board is mounted to the rear of the LCD
Plate. It drives the pair of CCFL tubes in the 15” LCD Panel. The board
contains a pair of configuration jumpers used to define the LCD panel type.
Technical Description (XBRE38 - Sheet 1)
The backlight inverter board is based on U1, a Dallas/Maxim DS3992Z-18P
Two Channel, Push-Pull CCFL Controller. It utilizes a push-pull drive scheme
to convert a DC voltage into the high voltage AC waveform required to drive
the backlight tubes. The device supports one lamp per channel with fully
independent lamp control and minimal external components.
Each channel drives dual n-channel MOSFETs Q1 and Q2. The MOSFETs are
connected between the primary windings of step-up transformers T1 and T2.
Each transformer includes a primary center tap connected to V12 (from the
workstations VCC12 supply). Circuit operation is outlined below.
Input Supply Monitoring
U1 internally monitors both the inverter power V12, (VCC12), and its own
VCC supply V5, (VCC5) to ensure proper operation. Additional fault
monitoring includes open-lamp, lamp over current, failure to strike, and
over voltage.
Lamp Strike
When both the V5 and V12 supplies are at acceptable levels, the controller
attempts to strike (start) the lamps. During lamp strike, U1 boosts the
normal lamp operating frequency by 33% to increase the voltage - then
slowly ramps up the MOSFET gate duty cycle until the lamp strikes. When
current starts following through R15 and R16 to indicate the lamps have
struck, U1 returns the operating frequency to normal and enters the Run
Lamp stage.
Run Lamp
In the Run Lamp stage, the DS3992 adjusts the MOSFET gate duty cycle
to optimize the lamp current and voltage.
To monitor lamp current, R15 and R16 are connected in series between the
CCFL tube and ground. The voltage across each resistor is fed back to the
LCM (lamp current monitor) inputs and compared to an internal reference
to determine the duty cycle of Q1 and Q2.
