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TLV8544: detection error increase when supply voltage drops

Harry Ma
Intellectual 1880 points
Part Number: TLV8544
Other Parts Discussed in Thread: TIDA-00489

Hi Team

My customer uses the 8544 for the design of the PIR sensor. They use CRxxxx coin cells to power the 8544 and the connective IC. When Connective IC does RF transmission, the high power increases the voltage drop on the internal resistance of the battery and reduces the VCC power supply of TLV8544 and Connective IC. At this time, the entire system is prone to detection error.

Do you ever see this phenomenon?  and how to solve this problem. 

  • 0
    Guru 140200 points

    Hi Marsj,

    the best remedy is to use extensive supply voltage filtering and decoupling as shown in the TIDA-00489:

    If this is not sufficient and the transmitter still draws too much supply current, spend the PIR circuit and the transmitter individual CRxxxx coin cells.

    Can you show a schematic of your circuit?

    Kai

  • 0 in reply to kai klaas69
    TI__Genius 11510 points

    Hi Harry, 

    You can also reference the following Application Report that uses the TLV8544 in a PIR sensor design. As Kai mentioned, if you also have a schematic, feel free to share and we can help to troubleshoot as well. 

    Thank you!
    Best Regards,
    Ashley

  • 0 in reply to Ashley Kang
    TI__Intellectual 1880 points

    Hi Ashley and kai

    thanks for your support. I am trying to get the schematic from customer side.

    and another question hope you can help: why will the detection error increase when coin cell voltage drop to low? 

  • 0 in reply to Harry Ma
    Guru 140200 points

    Hi Harry,

    Harry Ma said:
    why will the detection error increase when coin cell voltage drop to low?

    The answer is very simple: When a single supply is used by the help of a coin cell voltage, a pseudoground is necessary in the circuit to prevent the OPAmp output from going into saturation. The pseudoground is also used as reference signal for the comparators. And because this pseudoground is derived from the coin cell voltage by the help of a voltage divider, when the coin cell voltage drops the pseudoground is dropping as well. This can cause a massive change of operating points of circuit and can make the circuit work improperly finally.

    Another entry for the voltage drop is the PIR sensor itself, which has only a very limited ability to suppress supply voltage changes.

    Because of all this, extensive supply voltage decoupling and low pass filtering is necessary in this sort of circuit.

    The idea behind this filtering is that the transmitter is only switched-on for very brief periods and that the filtering can suppress the short voltage dips caused by the transmitter. So the time constant of the filtering must be way higher than the active time of transmitter.

    Another cause of trouble can be the cell coin itself. Coin cells are aging and become more and more high ohmic. Also when the coin cells discharges with time it becomes more and more high ohmic. Then it cannot source any longer the high transmitter current but needs a longer time period to recover from the high current demand. The only remedy then is to replace the coin cell by a fresh one.

    Kai