ST1S12G18R Output Voltage Issue - Feedback Not Functioning Properly

[jackob]

Hello,

I'm encountering an issue with the ST1S12G18R DC/DC regulator. I've attached the reference schematic from the datasheet and my PCB layout, created in Altium. As you can see, my design matches exactly with the reference schematic provided by ST.

According to the datasheet, with approximately 5V input and the recommended reference schematic resistor values, I should obtain 1.2V at the output (56k ohm resistor used for both feedback resistors). However, I'm consistently measuring around 4V instead of the expected 1.2V.

Because I specifically need a 3.3V output in my design, I adjusted the feedback resistors to 150 k ohm and 33.2 ohm. Still, the output voltage remains stable at approximately 4.3V when the input voltage is in the range of 4.5V to 6V. Only when the input voltage drops below 4V does the output voltage start decreasing accordingly. This behaviour indicates to me that the feedback loop may not be actively controlling the output voltage as intended.

To further investigate, I reduced the feedback resistor from 150k to 100k, yet the output remained unchanged at 4.3V. Changing the feedback resistor values does not seem to affect the output voltage.

Additionally, I performed another test by leaving the output open-circuited (no load). Under these conditions, the output directly reflects the input voltage, further suggesting the feedback control loop is inactive.

I have five identical PCBs, and all show this same behaviour. Thus, it's unlikely to be related to a faulty component or assembly error specific to one board.

Could you please advise me on how I can properly achieve a stable 3.3V output from this regulator? Any recommendations or insights into resolving this feedback issue would be greatly appreciated.

Thanks in advance for your assistance.

[Peter BENSCH]

You got a little confused and used the schematics for the adjustable version, but use the fixed voltage version for 1.8V, which uses an internal voltage divider. By the way: you also mentioned a 33.2 ohms resistor in the text, but drew a 33.2k in the schematics.

Simply replace R20 with a zero ohms jumper and remove R24, then your desired output voltage of 1.8V should be measurable at the output.

Regards
/Peter

[jackob]

Hi Peter,

Thank you for your comment and clarification. I understand your point regarding the fixed version and appreciate you catching the typo—I indeed used a 33.2kΩ resistor in my design, not 33.2Ω.

Just to clarify, I’m not aiming for 1.8V output. My target output voltage is 3.3V. According to the converter’s design procedure, a feedback network with 150kΩ and 33.2kΩ resistors should produce approximately 3.3V, but I’m not achieving this output voltage in practice.

Any insights into what might be causing this discrepancy would be greatly appreciated.

Best regards,

[Chris21]

The datasheet is unclear, but it looks like ST1S12G18R is fixed 1.8 V variant and you need the ADJ variant, which is possibly Part ST1S12XX, Order code: ST1S12GR.

[jackob]

Thank you for the response, Chris. I agree with you. I ordered  ST1S12GR. 

Have a good day

Regards

Jacob