STSPIN32G4 Internal LDO Outputting 4.1V Instead of 3.3V - Configuration Issue

[Raj6135]

Hello,

I'm experiencing an issue with the internal LDO regulator on the STSPIN32G4 MCU. The LDO is outputting 4.1V instead of the expected 3.3V, and I've been unable to resolve it despite trying multiple configurations.

Problem Details:

• Target: 3.3V VDD from internal LDO
• Actual output: 4.1V when input voltage > 4V
• Behavior: When input voltage is 3.33V, output is 3.05V (which seems reasonable given dropout), but when input is >4V, output remains at ~4.1V instead of regulating to 3.3V

What I've Tried:

• All configuration options listed in Section 5.1.2 of the datasheet
• Replicated the three circuit configurations from Figure 9 ("3.3V LDO regulator supplied through VCC")
• Various register settings without success

Expected Behavior: The LDO should regulate the output to 3.3V whenever the input voltage is above 3.3V + dropout voltage, correct?

I've attached my schematic and screenshots showing the output voltages. Any insights into what might be causing this behavior or what configuration I might be missing would be greatly appreciated.

Thank you!

[TDK]

If the output of a 3.3V LDO is above 3.3V, it's either being backpowered or the device has been damaged. LDOs are tough--being backpowered is more likely in my opinion.

To diagnose further, try removing or replacing the part. Either one will give you more information.

[Raj6135]

Thanks for the quick response!

Backpowering was the first thing I suspected as well. However, I removed all external connections to the 3.3V rail from the system, and the voltage remained at 4.1V, which seems to rule that out.

I also noticed something else that might be relevant: when probing the UART signals, I'm seeing approximately 750mV voltage difference from ground (picture attached), though the UART communication itself appears to be functioning correctly. I'm not sure if this could be related to the LDO issue, but thought it might provide additional insight.

Given that the voltage persists even with the 3.3V rail isolated, do you think this could indicate a damaged LDO, or are there other diagnostic steps I should try?

Thanks again for your help!

[TDK]

The chip won't operate with 4.1 V. It looks like your tester ground may not be the same as the chip ground. Where/how are you measuring the UART? Where are you measuring the LDO output? Be precise. In particular, what is the ground of your scope connected to? If the board has a ground isolated from earth ground, that could be the problem. You will need a differential probe to measure it correctly, or sometimes you can measure both sides depending on the common mode behavior.

4.1 V - 3.3 V is about 750 mV. Maybe it's a measurement setup issue.

[Raj6135]

Thank you for the suggestion about the measurement setup!

I am using a single ground plane across the entire PCB. For this debug session, I hand-soldered the MCU, but unfortunately, the exposed pad (EPAD) underneath the chip is not properly soldered due to the difficulty of accessing it with hand soldering.

To verify the measurement setup, I tested the voltage with reference to pin 32 (GND pin) directly on the MCU. When measuring other power rails on the board, the voltages appear as expected. However, the STSPIN32G4's internal LDO output still shows 4.1V.

Could the lack of proper EPAD connection be contributing to this issue? The EPAD is supposed to be connected to ground, and I'm wondering if inadequate grounding through the EPAD could cause the LDO to behave abnormally or create a ground potential difference that affects the measurements.

I appreciate any insights on whether this could be the root cause or if there are other diagnostics I should pursue.

Thank you!

[Raj6135 · Accepted solution]

Here is one more update on this! I have changed the MCU after trying multiple things. Now it works fine. Thanks