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I'm planning to use two 3.3V LDOs to supply power for two MCUs, one is STM32, one is ESP-12. Input power of these two LDOs come from one SMPS module 5V, 3W (HLK-PM01). The reason for two seperate LDOs: I'm using STM32's ADC and I think separate power supply with ESP will make ADC more accurate. Another reason is there is a distance between HLK-PM01 and ESP, trace length almost 25mm. STM and ESP communicate through UART and both using same I2C F-Ram Memory IC, STM reads ADC, calculates something and save to Memory IC. Then ESP reads data from Memory IC and post to server. All same ground. LDO is RT9013 Datasheet.

So is it possible and working? Or it is a overkill?

Thanks very much

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Yes, it will work, as long as both devices share the same ground I don't see any reason why there should arise a problem. But I don't see the any reason to do it this way either. It will be no problem to connect both MCUs to the same regulator. Just make sure to have good filtering for the analog supply of the STM. If you check the STM documents they explain very detailed what should be done to achieve clean input voltages for analog supply and reference: Put an RC-Filter in front of the input of the analog supply or even use a ferrite bead. This will attenuate switching noise from the MCUs and the SMPS.

● The VREF+ pin can be connected to the VDDA external power supply. If a separate, external reference voltage is applied on VREF+, a 100 nF and a 1 μF capacitors must be connected on this pin. In all cases, VREF+ must be kept between 2.4 V and VDDA.

● Additional precautions can be taken to filter analog noise:
- VDDA can be connected to VDD through a ferrite bead.
- The VREF+ pin can be connected to VDDA through a resistor (typ. 47 Ω).

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  • \$\begingroup\$ Thanks so much, actually my current deisgn: SMPS output ---> 10uF --> Ferrite Bead ---> 47uF ---> 10uF ---> LDO, each LDO is: 10uF ---> LDO ---> 10uF. STM power supply I followed datasheet, but I missed ferrite bead part. If I use one LDO for 2 MCU, trace length from LDO to ESP VDD is almost 45mm, is it ok? Do I need more ferrite bead to VDDA with my current design? \$\endgroup\$ – Ngô Hữu Nam Jul 23 at 7:00
  • \$\begingroup\$ Place one ferrite bead directly before the the VREF+ pin and put a 100nF capacitor from there to ground. The 10u is good, but an additional 100nF will help to filter higher frequency noise from your reference voltage. \$\endgroup\$ – jusaca Jul 23 at 15:33
  • \$\begingroup\$ Hi, thanks, with your suggestion, I searched and read some acticles, but I'm too bad to understand how to choose which one is suitable. Could you please suggest some FBs that I can use or anyone will work? My MCU is STM32F373, I use Sigma Delta ADC and SAR ADC to mearsure Watt-Hour of 50Hz 230 VAC \$\endgroup\$ – Ngô Hữu Nam Jul 30 at 5:32

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