MKV31F Power supply considerations

[aloblaw]

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!

[aloblaw]

Thank you Jingjing.

Can you please tell me about power supply sequencing? Should VDD or VDDA be powered up first (if they are on separate supplies of course). Is there a safe power-up time (e.g. 1ms? 100us?) for the VDDA supply to be powered up after VDD (or vice versa)?

Also, is there really no buffer zone for the VREFH to be SLIGHTLY higher than VDDA? What if my VREFH is a very stable voltage, and the VDDA has a very slightly noisy supply (even with capacitors placed close to the MCU there will still be some supply bounce). Those voltage ripples could cause VDDA to be lower than VREFH. Surely there is some tolerance here so that does not damage the part.

Thanks.

[kerryzhou · Accepted solution]

Hi aloblaw.

      The VDD should be powered up at first, actually, from the KV31 reference manual, we can get that  In some packages, VDDA is connected internally to VDD. If externally available, connect the VDDA pin to the same voltage potential as VDD. It is the MUST!

      So we don't recommend you use the separated power supply in the VDD and VDDA, just use the VDD,  VDDA, VREFH as the same power source.

    The following analog power supply is wildely recommended:

Wish it helps you!

Have a great day,
Jingjing

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[kerryzhou]

Hi aloblaw,

  Now answer your 3 questions:

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

  Actually, in the internal MCU circuit, the VDDA and VDD have the relationship, it determined the VDDA must be +/- 100mV within VDD.

We strongly recommend you keep the VDDA in the range which determined by the datasheet.

If the VDDA is out the range, it may cause the MCU internal circuit damaged.

2. As you know, the VDDA is the power for the analog circuit, If you use the clearner supply, it will keep the analog circuit have the stable powering, the data will be preciser.

  About the PSRR, we also don't have data until now, just as the datasheet recommended, you should keep the VDDA working in the area, then the ADC will working normally.

3.VREFH should in the range of 1.13V to VDDAv.

Just as the KV31 reference manual has said:"The ADC can be configured to accept one of two voltage reference pairs for VREFSH and VREFSL. Each pair contains a positive reference that must be between the minimum Ref Voltage High and VDDA, and a ground reference that must be at the same potential as VSSA."

You will find it is "MUST", normally we connect the VDDA and the VREFH toghther, if you are afraid the VREFH may higher than VDDA, I suggest use some external clamping circuit to keep the VREFH in the safe state.

Wish it helps you!

If you still have question, please contact with me!

Have a great day,

Jingjing

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