How the Backlight Circuit Generates 45Vā50V
MacBook batteries supply between 11.4V and 12.8V. However, the strings of white or Mini-LED backlights behind your screen require 38V to 52V DC to illuminate.
A specialized boost converter IC (like the Texas Instruments or Renesas driver on Apple boards) boosts the battery rail up to high voltage on the PPVOUT_S0_LCDBKLT line.
The Blown Backlight Fuse Syndrome
If water droplets enter the display connector or a worn flex cable shorts high voltage to ground, the 0402-size fast-acting backlight fuse (F8400 or F8500) blows instantly to protect the CPU.
When this happens, the LCD still forms images, but with zero illumination (verifiable via the Flashlight Test).
Test your screen using Mac Screen Test and stabilize software assertions with DisplayGuard.