/tools/lcd-backlight
LCD backlight driver
A backlight is white LEDs in strings. Give it the shape and the rating of one LED — the only number the panel datasheet quotes — and it sizes the ballast resistor, or tells you a resistor will not do.
In series
per string
In parallel
strings
Driving it from
How that is wired
6S7P
+ anode rail
− cathode rail
6S7P · 42 LEDs
18.0 V
6 × 3 V
140 mA
7 × 20 mA
At its rated current the panel burns 2.52 W. Series count sets the volts, parallel count sets the milliamps — nothing else about the panel changes either number.
5.00 V is not enough. The string needs 18.0 V before it lights at all, so this wants a boost constant-current driver set to 140 mA — output around 18.4 V. At 85% it pulls about 606 mA from the 5.00 V rail, which is the number your regulator has to survive.
Backlight voltage18.0 V
Backlight current, rated140 mA
Headroom on this supplyshort by 13.0 V
Driver output18.4 V at 140 mA
Power in the LEDs2.52 W
Drawn from the supply606 mA at 5.00 V
Efficiency83% reaches the LEDs
Do not know the shape? Wind a bench supply up in current limit until the backlight just lights: the voltage where it does, divided by about 3, is your series count. Never drive the backlight pin straight from a GPIO — even one LED at 20 mA is at the pin’s limit, and a panel wants ten times that.
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