S12ZVL with external 5V supply

[laurentlamesch]

Hello,

when using the S12ZVL, is it possible to use an external 5V supply instead of the internal one?

In my specific case, the external supply delivers a voltage between 4.85 V and 5.15 V (worst case). It already supplies power to an external circuit.

Would it be sufficient to connect VDDX and VDDA to the external 5V supply?

The S12ZVL internal voltage regulator should not supply current to the external circuit connected to the external 5V supply (for the case that the internal regulator output voltage is larger than the external one), as the external circuit draws too much current for the S12ZVL internal regulator. The external PNP is not an option because of additional costs.

Also, the internal LIN transceiver (physical layer) of the S12ZVL should be used, and therefore I assume that VSUP cannot be connected to the external 5V supply, but must be connected to VBAT instead?

One of the reasons for this question is that the S12ZVL ADC input voltage range should exactly match the output voltage range of the external circuit supplied by the external 5V. There are other solutions to this problem, but they have their drawbacks.

Thank you and with best regards,

Laurent

[RadekS]

Hi Laurent,

As you mentioned – it is a slightly complicated case due to the fact, that S12ZVL was not exactly designed for powering from an external source.

When we use external 5V power supply for powering VDDX/VDDA, we should also connect VBAT input pin to this voltage for avoiding transient effects at the internal voltage regulator.

You may use external PNP transistor for powering MCU and the external circuit. I suppose that PNP transistor should not be more expensive than LDO. However, I guess, that you already thought about that option and it is not possible due to some reason.

It is clear, that best solution for measuring ratiometric (proportional) output voltage is when ADC reference voltage equals to supply voltage of the measured voltage source. In that case, we can omit power supply voltage error during ADC measurement.

In that case, I would like to recommend use External circuit power supply as ADC reference voltage (VRL and/or VRH signals). This should eliminate power supply voltage error. However, we should somehow manage that External circuit power supply (VRH signal) will be in range VDDA/2~VDDA. The following constraints exist to obtain full-scale, full range results:

VSSA<=VRL<=VIN<=VRH<=VDDA

The ADC reference Voltage signal VRH_1 is mapped to VDDA; VRH_0 is mapped to PAD0; VRL_1 is mapped to VSSA and VRL_0 is mapped to PAD1.

So, this solution will consume 1 or 2 additional PAD pins.

As you mention, the conversion through absolute voltage area may be too complex and brings an additional source of issues.

When we use VDDA as the reference voltage, the VDDA error may be reduced by additional bandgap voltage measurement:

https://community.nxp.com/docs/DOC-332475

The external circuit power supply error should be compensated similar way by additional external circuit power supply voltage measurement.

So, in this case, we need at least three ADC measurement (cannot be captured exactly at the same time) and at least one additional PAD input for external circuit power supply voltage (probably through voltage divider with their own error).

I hope it helps you.

Have a great day,
Radek

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