How Power IC LDO Circuits Work: Mobile Repair Guide for Dead Phones
Learn how to diagnose dead phones by checking LDO voltages like VDD_MX and VIO 1.8V after the buck circuit. Step-by-step troubleshooting for technicians.
What is an LDO Circuit?
The LDO (Low Dropout Regulator) circuit within a mobile phone’s power IC is responsible for providing stable, low-amperage voltages to smaller components. While the buck circuit supplies high current to the CPU, the LDO circuit powers parts like SIM cards, cameras, and memory modules, which require precise voltages to function.
- SIM Card Voltage: 1.8V
- Memory Card Voltage: 2.8V
- Camera Voltages: 1.8V, 2.8V (AVDD/DVDD)
- Peripheral Sensors: Various low-voltage lines
If any LDO voltage is missing or shorted, the phone will not turn on—even if the buck circuit is working fine.
The 6 Critical LDO Voltages to Check
For troubleshooting a dead phone, focus on these essential “default on” voltages generated by power ICs like the PM8953. Checking these can quickly identify faults.
- VDD_MX (0.6V - 1.2V): Essential for CPU operation.
- VIO 1.8V: Powers CPU, eMMC, and RAM. Shorts here are common failures.
- eMMC VCC (2.8V / 2.9V): Main supply for memory ICs, required for boot.
- VDD_XO (1.8V): Supports CPU PLL section for clocking.
- VDD_PX2 (0.6V - 1.2V): Another CPU line; essential for operation.
- USB VREG (3.0V / 3.3V): Not critical for boot but shorts can damage the phone.
How to Troubleshoot an LDO Circuit
Follow these troubleshooting steps if you suspect LDO-related issues.
- Check for Shorts: Use a multimeter in diode mode to check capacitors and LDO lines before powering the phone. A shorted line is a common cause of fluctuating buck voltages.
- Measure Critical Voltages: Power the phone and check the six essential LDO outputs listed above.
- Analyze Results:
- Missing Voltage: The power IC might be faulty.
- Shorted Line: Find and remove the short, often due to faulty capacitors or attached components like eMMC or CPU.