Hello Paul_B,
Thank you for your response.
From the datasheet, we had assumed that the LCD would draw 60mA from a 19.2 volt supply rail, since the specifications are not defined as Vf=19.2V @60mA (as per standard LED definitions).
Should the backlight specification be changed to state the following ?
- Backlight supply Current (ILED) - 45mA (min), 60mA (typ), 75mA (max)
- Backlight Supply Voltage @ ILED=60mA - 16.2V (min), 19.2V (typ), 20.4V (max)
Based on your comments, we have re-configured the LCD backlight power supply output to include a series resistor between the power supply output and the voltage to the LCD. Please note that the power supply has been successfully load tested up to an output current of 500mA (with purely resistive loads).
Is there any application note on how to determine the PSU output voltage and the series resistance between the PSU and LCD ?
Could you please comment on these test results ?
1. The PSU output was set to 21.88 Volts with a series 20 ohm resistor. The resulting series current of 175mA is too high.
2. The PSU voltage was reduced to 20.78 Volts and the series resistor was increased to 56 ohms. The resulting current (66mA) is within spec, but the LCD voltage (17.4V) is at the lower end of spec.
3. Increase the PSU output to raise the LCD voltage. PSU voltage set to 22.74 Volts, but LCD voltage only increased slightly, and series current (95mA) is now too high.
PSU Output | Series R | VLCD | Vres | ILCD | Comment
-----------------------------------------------------------------------------------------
1. 21.88 V | 20.2 ohm | 18.30 V | 3.58 V | 176 mA | current is too high
2. 20.78 V | 56.5 ohm | 17.03 V | 3.75 V | 66 mA | current is within range, voltage at lower end of spec
3. 22.74 V | 56.5 ohm | 17.41 V | 5.34 V | 95 mA | increased PSU voltage does not proportionally increase VLCD, current now above spec