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MKV31F Power supply considerations

Question asked by aloblaw on Mar 8, 2016
Latest reply on Mar 9, 2016 by Kerry Zhou

I am building a PCB using the MKV31F512VLH12 that uses some very sensitive analog sensors.

 

For starters, the overall board is powered by a 15V (nominal) battery. This battery powers a BLDC motor which generates some fairly ugly voltage ripple on the 15V supply line that cannot really be avoided.

I am using a 3.3V switching supply to power several peripherals as well as the power (VDD) pins of the MCU, which is powered by the 15V battery.

I am using a 12V LDO, which is powered by the 15V battery.

I need the analog sensors to have very low noise and a clean power supply, so I am using a precision 3.3V reference supply (LT1461CIS8-3.3), which is powered by the 12V LDO (the 12V LDO provides extra PSRR and filtering).

I want to use the 3.3V reference supply to power the VDDA and VREFH pins of the MCU and use the 3.3V switching supply to power the VDD pins of the MCU.

 

1. the datasheet suggests that VDDA must be +/- 100mV within VDD

What happens if my VDDA/VREFH is (for example) 150mV > than VDD? Will it damage the MCU? Will it give bad results?

What happens if my VDDA/VREFH is 0V when the VDD is at 3.3V (i.e. power sequencing is such that the 3.3V reference turns on after the 3.3V switching supply)? If this IS a problem, then what if it is only 0V for a very short time period (e.g. 1ms, 200us)?

 

2. Are there significant advantages in powering VDDA from a cleaner supply, or is there enough PSRR in the ADC of the MCU that I should not worry about it? (There are no numbers or figures about PSRR that I can find in any datasheets)

 

2b. The datasheet suggests that VREFH must be <= VDDA

If I power VDDA and VDD from the 3.3V switching supply, and use the 3.3V reference source for VREFH, is there any chance I could damage the MCU if the 3.3V reference source is slightly higher than the 3.3V switching supply (e.g. reference=3.3V, switching=3.2V = delta 100mV)?

 

 

Thank you in advance!

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