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Voltage supervisor circuit using TPS3890-Q1: enter image description here

My Vin (5 V) is connected to Vmon. R1 and R2 are already defined. Intention is to calculate positive and negative threshold. So I think, this is a kind of reverse engineering.

R1 = 27 K,

R2 = 47 K

From the above read, I calculated as 5 V = Vitp ( 1 + 27/47)

Input Threshold Positive, Vitp = 3.176 V

Questions

1) I was not clear threshold calculation from the datasheet. Above calculation is correct or not?

2) What will be threshold negative value, -3.176 V or How to calculate it?

Please comment. Thanks.

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  • \$\begingroup\$ Where did R1 and R2 get defined? TPS389001 shown in the diagram is the adjustable version with Vitp = 1.157V, Vitp is not a value that you calculate. \$\endgroup\$ – EE_socal Mar 12 '18 at 19:39
  • \$\begingroup\$ R1 and R2 are the resistor divider, and these are going to define the thresholds. What do you mean by defining R1 and R2? \$\endgroup\$ – vt673 Mar 12 '18 at 19:59
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The specifications are pretty clear, the TPS389001 device ("adjustable" variant) will DEASSERT reset pin (pins go HIGH) if the voltage on pin "SENSE" reaches 1.157 V, and will assert RESET if the voltage will go below 1.150 V. This is how this kind of devices generally operate.

Therefore, if you have the 27k/47k voltage divider on the SENSE pin, the -RESET pin will go high if Vmon reaches 1.157*(27+47)/47 = 1.82 V (aka positive threshold), and will assert RESET pin LOW if the Vmon voltage drops below 1.81 V (aka negative threshold). Obviously both thresholds are positive relative to the common ground pin. This is a very sensitive voltage monitor!

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  • \$\begingroup\$ Just for academic purpose, Assume as Vmon connected to supply voltage (5 V). Simply applying VD principle, 5*(R2/(R2+R1)) = 5*(47/(47+27)) = 3.176. This means the circuit will be always above the positive threshold and RESET pin will keep HIGH. Make sense? \$\endgroup\$ – vt673 Mar 13 '18 at 5:22
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    \$\begingroup\$ @vt673, the VDD level has nothing to do with thresholds, it can be anything between 1.5 to 5.5 V, it is the power supply to internal electronics. The thresholds will remain the same regardless of VDD, see Section 8.3.4 of the device datasheet. \$\endgroup\$ – Ale..chenski Mar 13 '18 at 5:28

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