AP62600 Regulation Limit Question

DC-DC Power Converters
1

Hi,

I used three AP62600 regulators as shown in the attached schematic to power an FPGA.
The input is supplied via USB Type-C PD, and I selected 15V.

AP62600_Schematic
AP62600_Schematic1412×1597 150 KB

However, I observed that the 1.8V VCCAUX rail increased up to 2.6V, which caused permanent damage to my FPGA.
I would like to understand the root cause of this behavior. Could you please help?

From reviewing my schematic, it seems that the only potential issue might be that the selected inductor value is slightly lower than optimal.

Observed Behavior

The attached waveform shows how the 1.8V output rises to 2.6V:

  • Yellow trace: Input voltage (starts at 5V, then switches to 15V after PD negotiation)

  • Pink trace: 1.8V output

At the moment the input transitions to 15V, while the other regulators operate normally, the 1.8V rail jumps to approximately 2.6V and remains at that level.

1.8V_OutOfRegulation
1.8V_OutOfRegulation1024×600 26 KB

Additional Testing

I performed further tests using a bench power supply:

  • When increasing the input voltage gradually, the output jump occurs at around 13V

  • When decreasing the input voltage, regulation recovers at approximately the similar threshold

Second PCB Observation

On a new PCB, I assembled only the 3.3V and 2.5V regulators:

  • In this case, the 1.8V rail initially remained stable across the full input range

  • However, the 3.3V rail started to track the input voltage at higher Vin levels

  • Regulation seemed to degrade above ~9V, and at 15V input, the output rose to approximately 6V

After replacing the inductors with 1µH parts, both outputs returned to normal operation.

Final Confirmation

On the original FPGA board:

  • I replaced only the inductor of the 1.8V regulator with a 1µH inductor

  • After this change, all voltage rails are now operating correctly

    Normal
    Normal1024×600 25.3 KB

Current Situation

Measuring individual rail currents is difficult, since the damaged FPGA causes the system to draw approximately 8W total.
Adding or removing load does not significantly affect the output voltages.

This makes me think the issue is mainly related to the relationship between input voltage and inductor value.

Questions

  1. Is there a formula or guideline that defines the conditions under which the AP62600 goes out of regulation?

  2. How can I calculate a safe minimum inductor value to ensure stable regulation across my input voltage range?

  3. With the current (updated) inductor values, can I be confident that this fault condition will not occur again?

  4. Are there any other possible issues in my schematic or design that I should consider?

Reliability Concern

After replacing the inductors, the AP62600 regulators appear to be functioning normally again.

However, is it possible that the ICs were partially damaged during the fault condition?
Since I will replace the FPGA on the PCB, would you recommend also replacing the AP62600 devices as a precaution?

Regards,
Mete