STM6710 Adjustable Threshold

[blovejoy]

Hi,

I am looking to use the STM6710DWB6F as a quad-channel voltage supervisor, specifically for 3.3V, 5V, 6V and 12V rails.  The datasheet specifies V1IN = 5V, V2IN = 3.3V, V3IN = 1.8V, and V4IN = adjustable.  It seems obvious use V1IN and V2IN directly, while feeding either 6V or 12V through a voltage divider to provide 1.8V into V3IN.  However, I am uncertain how to use V4IN to monitor the remaining voltage.  The datasheet states a typical threshold voltage of 0.62V at this pin and to use a resistor divider to modify this value.  However, will this affect V1IN-V3IN?  Any help in understanding V4IN and the effects of setting threshold voltages would be greatly appreciated.

Thanks,

Brendon

[Peter BENSCH · Accepted solution]

Welcome @blovejoy, to the community!

Section 4.2 of the data sheet describes how to calculate any necessary voltage dividers. Only the input current needs to be taken into account, which for adjustable pins, in your case V4IN, is max. 0.4µA (the maximum R2 was also specified in the data sheet, but unfortunately there was a font problem, so that the omega is displayed as a W, for which we apologise).

However, with your STM6710DWB6F you must be careful with V3IN, which already has an internal voltage divider. According to the data sheet, table 5, an input current of typically 25µA, max. 40µA can flow there (corresponding to a typical total resistance of 72kohms, min. 45kohms), so that an external voltage divider should be dimensioned accordingly.

• Either you match the external voltage divider of each device to the respective input resistance with a variable resistor (complex, expensive)
• Or you allow a cross-current of at least 1mA, which corresponds to a voltage divider of 4.2k and 1.8k for a voltage of 6V to be monitored, with a total power loss of 6mW.

However, V4IN does not have an internal voltage divider in the STM6710DWB6F, so you can expect a maximum input current of 0.2µA. For the external voltage divider and 12V, this results in R1=1.835Mohms and R2=100k according to equation 2 of the data sheet.

Connecting voltage dividers to one input does not affect the other inputs.

Hope that answers your questions?

Regards
/Peter

[blovejoy]

Thanks, Peter!  I believe you have answered all of my questions.  And thanks also for the note about V3IN.

Brendon