Unlike simple Zener diodes, the LAC781P uses a . This generates a stable 1.25V or 2.5V reference voltage that drifts very little with temperature. From the schematic top view, this block connects to the inverting input of the error amplifier.
Pin capacitor for +3V_EC , Reset line ( WRST# ), and Crystal Oscillator clock output. The Charger IC Chip Designation: Typically an ISL88739 or BQ24735 .
: The gate voltage for the isolation MOSFETs must be driven approximately 6V higher than VIN (around 25V ) to fully turn on the N-channel gates via the charging IC’s ACDRV pump. 2. The Step-Down Section (3.3V & 5V Always-On)
: Accepts raw power from the DC Jack and distributes it across the board as B+ . lac781p schematic top
Below is the typical for using the LAC781P as a +12V regulated power supply from an unregulated DC source (e.g., transformer + rectifier).
Comprehensive Repair Guide: Compal LA-C781P Motherboard Architecture and Troubleshooting
Always-on power blocks powering the Super I/O chip and the power button mechanism. Unlike simple Zener diodes, the LAC781P uses a
Corrupted SPI BIOS/EC firmware array, or missing RAM power rail.
The hardware components issue confirmation pulses back to the processing unit, which drops the system reset flag ( SYS_RST# ) to initiate active calculations. 4. Troubleshooting Common Faults on the LA-C781P
| 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. | Pin capacitor for +3V_EC , Reset line (
A full-bridge rectifier (usually four 1N4007 or similar) converts AC to pulsating DC. A large electrolytic capacitor (typically 100–220 µF at 400–450V) provides a smooth high-voltage DC bus (~310V DC for 230V AC).
The should note that a heatsink is required for loads >500mA.
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When troubleshooting a "No Power" issue, technicians check these specific points on the LA-C781P: Power Rail Description Test Point / Component Main adapter voltage Check DC-in Mosfets and CLR (Current Limit Resistor). +3VLP / LDO Low drop-out always-on voltage Available at the RTC cell and battery connectors. +3VALW / +5VALW Always-on system power Generated by the RT8243 (PU2) step-down IC. ACDRV Gate control for entry Mosfets Should be ~25V if the bq25A IC detects a valid adapter. 3. Troubleshooting "No Power" (Dead Board)
When using the schematic to diagnose "No Power" issues, focus on these top-layer sections: Check for 19V at the drain of the first N-channel MOSFET.