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TPS3760: I need help designing a circuit that can turn on a sub-circuit when the input supply goes above voltage threshold

Part Number: TPS3760
Other Parts Discussed in Thread: TPS37, TPS38,


We have a relay that we want to energize when our input voltage goes above a certain voltage, 20V probably.  that same input voltage is also the power for all circuitry of the design.  The input voltage goes to a few different voltages on start up to work on detecting PoE loads, and that voltage is always below 18V.  The voltage detection relies on measuring a resistance value at those various voltages.

Any impedance from the relay coil or other "downstream"parts must not interfere with that measurement.  So my idea was to have a voltage supervisor keep an output set low/high while the input is below 20V, and then once the input goes above 20V to a max of ~50V, then the supervisor flips that signal.  I would want to drive the gate of a FET with the idea that the FET is making sure the relay coil (and potentially other loads) are blocked and look like very high impedance when the input voltage is below 20V.

I want to make sure that of what happens to the operation of the TPS3760 (or the TPS37 or TPS38 as other choices) when the Supply Voltage and the Sense Voltage are identical and vary over time.  

Will this work as is, or is there something I should do to the sense line input? 

Perhaps using a fixed-reference version of the chip where it is a C4 option (or some other lower voltage) and I change my 7k to a divider to sneak off a lower voltage that would track the input voltage?

Can you recommend a particular version with a proper voltage reference?

So something like this, with maybe not needing R1 depending on TPSxx's abilities.

Thank you

  • We will respond shortly. Thank you.

  • Hi Jonathan,

    Thank you for your question. For the TPS3760, we do have the adjustable under voltage active low TPS3760A012DYYR and the adjustable over voltage active low TPS3760E012DYYR. For the adjustable variant, you will need to have a voltage divider to drop the 20V threshold to 800mV. If you are going to stick with the DN1509, Nmos, the under voltage TPS3760A012DYYR will do the trick. I can provide an example schematic with the proper values, I just have a few questions.

    • Is the rail going to be noisy? What is the pulse duration of transients you would want to filter out.
    • Needed reset delay for the system?


  • Thanks for the info.
    we are not at all married to the DN1509, it was just in there from a schematic I found online.  We could use a P-channel on the high side, N-chan on the low side, or any other combination. I'm happy to look at recommendations on any other parts that you think could do a better job than the TPS3760 series we started with, including FETs or high-side switcher all-in-one chips you might know of.

    We have our 20V classification voltage that is the max we should ever see during the initial up/down voltage levels.
    I am not sure the timing on this as it can be variable based on other system parameters, but our worst case scenario probably has ~750ms max before the voltage rail ever goes above 20V.

    The top dotted line is the 20V max voltage level.  This graph is one example of possible voltage input levels. As some point after this we will turn the "power on" and bring the voltage up to ~48V.  

    I am really looking for something like this over-simplified drawing:

    We have Vrail that does the up and down thing and some circuitry that looks like >2MOhm (infinite-ish) impedance for any Vrail <21V.
    Then once we have gone thru our start-up voltage dance and raise the Vrail over 21V** our black box will pass through the Vrail to Vo with minimal loss. 
    **(or some other voltage we can set using a voltage divider, zener, Power-on reset, supply monitor, etc.)

    Ideally we'll have some hysteresis where we'd be safe from any kind of droop on Vrail below that 20V, and this would be shorter blips less than 1ms.  In other words, Vout needs to always track Vrail any time after we've first crossed that 20V line even if the voltage drops down to 15V (or some other voltage we can set.)

    I hope this helps clarify better our scenario.

    If you provide an example schematic would you be able to provide guidance on any easy adjustments we can make for things like reset-delay times or easy transient filtering.


  • Hi Jonathan,

    Below is an example schematic to utilize the TPS3760 active low for your use case as well as the start up behavior of the device. 

    The reset time delay, tctr is controlled by the value of the capacitor attached to CTR pin, while the capacitor attached to the CTS controls the noise filter, tcts, of the sense pin, please refer to the data sheet for further details.

    Please let me know if you have additional questions. 


  • That's great, thank you so much!

    is that M1 FET in your schematic meant to be the DN1509?  (I am assuming yes as it's an N-chan depletion mode doing high-side switching)  If I were to want to use a P-channel I would just swap to the TPS37 overvoltage version.  I think using that essentially inverts the logic, right?

    I saw that there is an eval board available for the TPS3760.    Would that be something that would allow me to try different R and C values before doing my PCB?
    Is there another eval board that has a few different products that allows a side-by-side comparison of the functionality of a few different chips similar to this? (a long-shot, i know, but worth asking)


  • Hi Jonathan,

    Yes the M1 is suppose to represent the DN1509 FET. If you want to use a P-CH you can use the TPS3760E overvoltage monitoring vs the TPS3760A undervoltage monitoring. The logic does flip but you will need to pay attention to the asserting voltage and the de-asserting voltage. If you want to use and OV active LOW as a UV active HIGH, the RESET asserts at Vitp=Vit+Vhyst and de-assert at Vitn=Vit. Vit = Vthreshold.

    The TPS3760 will allow you to evaluate with other passive components but not other devices outside of the TPS3760 family. 

    Unfortunately, we do not have an EVM to evaluate multiple devices side by side.