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TLV840: How to detect lithium battery correctly ?

Part Number: TLV840
Other Parts Discussed in Thread: TPS3780, TPS3779,

Hi Expert:

I make a long push button circuit with CD4013.

The circuit shows below 

I use the /Q of second CD4013 to enable the following regulator.

I use lithium battery voltage to control CD4013.

Our customer says there is some error sometimes.

After experiment when lithium battery below 3.5V, there is some error .

I wish when lithium battery voltage under 3.5V, I will turn off the /Q of CD4013. (product power will on turn on)

So I use TLV840NADL35DBVR to reset the /Q of CD4013.

After test, when lithium battery voltage is 3.5V,3.4V,  /Q of CD4013 is low so system is off.

 When lithium battery voltage is 4V, /Q of 4013 is high,so system is on.

But when lithium battery charging to 3.7V.

I measure the /Q of 4013.

The blue is lithium battery voltage.

The yellow is /Q of 4013.

It seems TLV840NADL35DBVR reset /Q for many times.

Maybe 3.5*(1.05)=3.675=>  Hysteresis on VIT– pin

TLV840NADL35DBVR will not judge correctly at 3.6~3.7.

In the voltage supervisor and reset IC tip trick document figure 1-3.

There is a demo circuit.

Because the lithium voltage is a slowly changing voltage.

There may be some problem on judging the voltage.

I do not know how to overcome this point.

If I use TPS3779 or TPS3780 will solve this problem?

Or there is better solution to solve this issue?

best regards

  • Hi,

    Can you plot the output of the TLV840?  The TLV840 has a typical of 5% hysteresis.  Based on the orderable variant, the 3.5V option should yield a hysteresis of 175mV.  Do you have a schematic that includes the TLV840?  


  • Hi Ben:

    I did not plot schematic of TLV840.

    I only connect Battery voltage to TLV840 VDD.

    And GND to pin3,

    /reset pin connect to the second /Q2 of CD4013.

    CD4013 also connect to Battery voltage.

    best regards

  • Hi,

    Can you provide a scope photo of waveforms VDD, /RESET, and /MR from the TLV840?


  • Hi Ben:

    The blue is lithium battery voltage.

    TLV840 VDD, CD4013 VDD both connect to lithium battery.

    The yellow is TLV840 /reset connect to /Q2 of CD4013


    When Lithium battery voltage is below 3.5V, TLV840 always reset the /Q2 of CD4013.

    The product will not turn on.


    When Lithium battery voltage is  4V, TLV840 always open.

    The CD4013 /Q2 will be high to drive the following Enable signal of regulator.

    3.When Lithium battery voltage is 3.7V, TLV840 sometimes reset the /Q2 of CD4013.

    So you will see yellow signal, /Q2 of CD4013 turn high but pull low by TLV840.

    I do not know why this way.

    best regards

  • Hi,

    Can you take another scope photo of the lithium battery voltage waveform?  This time can you zoom in?  I want to see if there is any ripple on the waveform.  Is there a capacitor on lithium battery voltage rail?


  • Hi Ben:

    This is the scope of zoom in.

    I think I will want to co-work with firmware engineer changing the power sequence to not dropping the VBAT.

    Maybe there is another power sequence problem.

    best regards

  • Hi,

    Just want to follow up with you if there are any other questions?  It seems that the lithium battery voltage is dipping and causing the TLV840 to output a reset event.  If there are no other questions, please close the thread by clicking on "resolved".  Thanks and good luck!