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Regulator LD39050PU33R for Nucleo Boards doesn't work.

Question asked by Vasileios Amoiridis on Apr 13, 2018

I have built a custom STM32F446ZE board and I used the ST890CDR current protector and the LD39050PU33R voltage regulator in exactly the same way they are implemented in the Nucleo Board from ST. The only difference is that I used a pull-down resistor in the ON pin of the ST890CDR because it is an active low pin and in the ST board is implemented with a pull-up resistor but it goes LOW from a signal in the programmer. I don't have a programmer in my board so I pulled it down from the start. My problem is this:


First of all, the regulator does not output a stable 3.31V as it does on the ST board. It outputs 3.18V and it oscillates at +/- 60mV. Secondly, when I raise the speed of the uController from 16MHz HSI at 90MHz HSE via PLL the output drops at 2.9V and works at this level. If I use the maximum speed at 180MHz the regulator shuts down itself, then powers up again, then again it powers down and after powers up and that's an endless loop. The datasheet states that the regulator has a thermal shutdown at 160 C but there is no way the chip gets so hot. So does anyone have an idea why could this be happening?


Btw, when I used an external 3.3V power supply directly to the uController the board worked perfectly.