TPS3700: Resistance supervision

Hi Michael,

This idea to use the TPS3700 is not possible.  Monitoring the voltage via R2 changing from 10.2Kohms to 21Kohms and keeping the other two resistors fixed can be possible but the TPS3700 does not have the reference voltage range on INA+ and INB- to monitor a wide change in R2.  

My suggestion, although I haven't fully thought about it, is to use two TPS3703 for your application.  Both of the inputs to the TPS3703 will be monitoring the voltage at R2, not across R2.  You can set one up where it will detect an OV fault and the other will detect an UV fault.

Ben   

Hi Ben,

thanks for this. I am not yet giving up on using a single TPS3700.

What do you think about the follwoing constellation:

R1 = Rus; Resistor under supervision (10.2k ≤ 15k ≤ 21k)
R2 = 909Ω
R3 = 1.0kΩ
OUTA is asserted when Rus > 21kΩ
OUTB is asserted when Rus < 10,2kΩ
Please check my calculations.

BR

MIchael

Hi Michael,

I am a bit confused with your new setup.  Is INA+ connection still above R2 and INB- connection above R3?  The threshold voltages for INA+ and INB- are 394.5mV and 400mV.  I don't believe your resistor selection gives you those values close to the threshold voltages.

One thing to note, the change in resistance is over 30%.  The threshold values for INA+ and INB- changes only 1.375% (394.5mV to 400mV).  I really don't think using the TPS3700 is the right approach.  You might want to reconsider the two-part solution, using two voltage supervisors, for your application.  

Ben

Hi Ben,

here is the schematic of the new setup:

The resistor inside the LED606 need to be supervised. Its nominal value is 15kΩ. If the resistance is outside the bounds of

Rmin = 10.2kΩ

Rmax = 21kΩ

the TPS3700 should assert either output.

Here are my calculations:

BR

Michael

Hi Michael,

Thank you for providing me a schematic.  I now understand your question.  According to my calculations, if you stay with the new setup where R1 is varying from 10.2Kohms to 21Kohms, you will not meet your design goals.  R1 needs to reach 22.3Kohms for OUTA to go low.  For OUTB, the signal starts high and as R1 approaches 10.2Kohms, OUTB will go low at 10.59Kohms.

Ben

Hi Ben,

so which resistor values should I use?

BR

Michael

Hi Michael,

If you use 806ohms for R2, then INA+ will be 394.5mV when R1 reaches 21.08Kohms. For INB-, INB- will be at 400mV when R1 decreases to 10.69K.  R1 will not go down to 10.2Kohms.  Will the resistor numbers suffice your application?

Ben

Hi Ben,

thanks for this. Have you simulated these values or should I simulate the design?

Do you recommend PSpice or TINA?

BR

Michael

Hi Michael,

I have not simulated these values.  The values are based from my calculations.  You can try Pspice.

Ben