If the LAC781P is a custom IC (black epoxy blob or custom stamped), a schematic may not be publicly available. In this case, reverse engineering is required:
Usually found near the charging port (DC-IN jack) and battery connector.
"Come on, talk to me," Elias whispered. He traced the schematic’s path to the VIN circuit. According to the diagram, a pair of MOSFETs near the charging port should have been passing voltage, but they were as cold as ice. He checked the gate voltage—zero. The tiny PWM controller that was supposed to wake them up was silent.
Look for discolored ceramic capacitors that often crack and short to ground. No Power-On (No Trigger) If the laptop has 3.3V and 5V but won't turn on: Locate the PWR_SW# signal on the schematic. lac781p schematic top
, the top side is packed with critical components responsible for power regulation and system operation. A. Power Input and Charging Section
If you can of your board's top side, I can help you identify specific components.
Verify 19V is reaching the first Mosfet's drain. If the LAC781P is a custom IC (black
| Symptom | Possible Fault (per schematic top) | | :--- | :--- | | V_OUT = V_IN (no regulation) | Pass transistor shorted (collector-emitter). Replace IC. | | V_OUT = 0V but input present | Thermal shutdown active (check load current or heatsink). Or output shorted to ground. | | Ripple on output | Error amplifier decoupling poor. Add 0.1µF caps directly at pins. | | Output low but not zero | Current limit active. Reduce load or increase input voltage to reduce pass transistor drop. |
Before the power button is even pressed, the LAC781P generates "Always" voltages. Powers the Super I/O (KBC) and the BIOS chip. +5VALW: Powers secondary peripherals and USB standby ports.
The current on your DC power supply when you plug it in. Voltage readings on the 3.3V and 5V inductors . He traced the schematic’s path to the VIN circuit
Understand the power-on sequence (S5, S4, S3, and S0 states).
Often a BQ series IC (e.g., BQ24780S), managing battery charging and input voltage detection.
| Designator | Description | Example(s) / Significance | | :--- | :--- | :--- | | (e.g., PU1, PU2) | Power IC / Regulator / Controller. These are the brains of the power delivery system. | PU1 = BQ24725A Battery Charger IC PU2 = 8A 1L Standby PWM IC | | PQ# (e.g., PQ4, PQ5) | Power MOSFET. These act as high-speed switches to route power. | PQ4 , PQ5 = Input MOSFETs for B+ PQ12 = SMD marking "7934" | | PL# (e.g., PL8) | Power Inductor (Coil). Works with the PU# IC to filter and smooth the output voltage. | PL8 = Output inductor for a key power rail | | PR# (Implicit) | Power Resistor. Used for voltage sensing, current limiting, and pull-up/pull-down functions. | Often found near PU ICs. | | PC# (Implicit) | Power Capacitor. Used for filtering and stabilizing voltage rails. | Often found near PU ICs. | | EC | Embedded Controller. A specialized microcontroller managing keyboard, power sequencing, and other low-level tasks. | ENE Chip | | BIOS | Basic Input/Output System. The firmware responsible for booting the system. | A specific IC near the EC chip |
Typically managed by a microcontroller responsible for power sequencing and keyboard input. Understanding the LA-C781P Schematic