Symptom · Diagnostic

No backlight MacBook:
U7701 and LP8550 checks

April 28, 2026 Level: Technician MacBook Pro / Air

Backlight supply chain: boost topology overview

MacBook backlight systems use a two-stage converter: U7701 (boost controller) steps battery voltage to approximately 24V nominal, which then feeds LP8550 (WLED driver IC) that regulates current to the LED string. When backlight is absent with display visible, the fault lies in this chain.

The boost stage uses PPBUS_G3H (typically 10.5–12.6V from battery) as input. U7701 outputs roughly 24V into a boost inductor network, feeding the LP8550 input rail. Loss at either stage kills backlight instantly.

The LP8550 requires minimum 16V input to enable; below this threshold, no LED current flows. U7701 boost failure is the most common root cause in no-backlight complaints.

Testing U7701:
boost controller checks

On most MacBook models (13", 15", 16" Pro/Air post-2015), U7701 is a Richtek RT8209A or similar boost controller in an 8-pin USON package, located near the LCD connector.

Step 1: Verify power rails

  • Probe U7701 pin 1 (VIN): expect 10.5–12.6V when PPBUS_G3H is present. If 0V, backlight fault originates upstream (battery, PMU).
  • Probe pin 2 (GND): confirm zero volts reference. Floating ground indicates power plane disconnection.
  • Probe pin 7 (VOUT/boost output): expect 23–25V when system is awake and display active. Dead 0V output confirms boost failure.
Do not probe VOUT with a DMM in circuit while powered. Use a high-impedance probe or disconnect LP8550 input to read stable voltage.

Step 2: Inductor resonance test

Inspect the boost inductor (typically L7201, 10 µH ±20%, 1.5A rated) adjacent to U7701. Open windings produce zero output.

  • In-circuit: Measure DC resistance at pin 7 to pin 1: typically 1–3 Ω. Above 10 Ω suggests inductor failure or open FET.
  • Out-of-circuit: Inductor resistance should be 0.05–0.2 Ω. Measure with resistance mode; if open (∞ Ω), replace.

Step 3: Bootstrap/feedback network

U7701 pin 5 (FB) sets output voltage via a resistor divider. Expected voltage at FB pin: 0.6–0.8V (internal reference).

  • Locate divider resistors (typically 120 kΩ top, 47 kΩ bottom, labeling R7501/R7502). Measure top resistor to ground: if open, VOUT drifts high.
  • Measure bottom resistor: open causes FB to float, disabling output.
  • Scope pin 5 during operation: should show stable DC, not ripple.

Step 4: Enable/shutdown line

Pin 4 (SHDN or EN) gates the boost. Active-low enable requires 3.3V logic high from EC (Embedded Controller).

  • Probe pin 4 at rest: should be 3.3V high. If 0V, trace back to EC output Q7151 or equivalent gate driver.
  • Toggling display on/off should pulse this rail. Static 0V = backlight deliberately held off (software kill) or EC fault.

Testing LP8550:
WLED driver verification

LP8550 is a TI/National Semiconductor WLED driver (typically 12-pin VFBGA), located on the display cable or logic board. It requires valid boost voltage to function.

Supply and control inputs

  • Pin VIN (typically pin 1): 16–28V from U7701 boost output. Measure first; if below 16V, U7701 is at fault, not LP8550.
  • Pin GND (pins 2, 8): 0V reference. Check continuity; open ground = no LED drive.
  • Pin EN/SHDN (typically pin 3): 1.2V logic from EC. Pulled low disables driver. If stuck low, check EC output or trace for short.
  • Pin LED-/CATN (LED cathode pins 10–12): These connect to LED string. Measure DC current: should be 20–30 mA nominal in "on" state. 0 mA = open LED string or shorted sense resistor.

Current sense resistor check

LED current flows through sense resistor R_LED (usually 10 Ω, 0.1W, next to LP8550). Measure across it:

  • Backlight ON: Expect 0.2–0.3V drop (20–30 mA × 10 Ω). No voltage = open string or bad solder.
  • Backlight OFF: 0V. If 0.3V persists, IC leaking current (replace LP8550).

Frequency and oscillation

Scope the LED cathode or boost output pin: should see 500 kHz–2 MHz square wave when backlight is active. No switching = regulator not operating, likely disabled by EC or failed internal circuit.

If U7701 VOUT is 24V DC stable and LP8550 shows 500 kHz oscillation on LED cathode but no visible light, suspect open LED string or backlight panel failure, not electronics.

Diagnostic decision tree

U7701 VIN U7701 VOUT LP8550 VIN Probable Cause Action
10.5–12.6V 23–25V 23–25V LP8550 fault or LED short Replace LP8550; check LED string continuity
10.5–12.6V 0–5V N/A U7701 failure or enable stuck low Check pin 4 (SHDN); test boost inductor; measure FB divider
0V N/A N/A Power loss upstream (battery, PMU) Verify PPBUS_G3H at board edge; test battery voltage
10.5–12.6V 28–32V (high) N/A FB divider open; runaway boost Inspect R7501/R7502; remove LP8550 load to prevent secondary failure

Quick isolation procedure

  1. Power on; measure U7701 pin 1 (VIN). If 0V, entire backlight subsystem is de-powered: skip to power delivery audit.
  2. If pin 1 is 10.5–12.6V, measure pin 7 (VOUT). If 0V, U7701 is the fault. If 23–25V, go to step 3.
  3. Measure LP8550 pin 1 (VIN). Should mirror U7701 pin 7. If present, probe LED cathode with scope for 500 kHz activity. If no switching, replace LP8550. If switching present, LED string open or display is bad.