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TLV840: How to detect lithium battery correctly ?

future chang1
Prodigy 250 points
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

  • 0
    TI__Mastermind 39895 points

    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?  

    Ben

  • 0 in reply to Ben C
    Prodigy 250 points

    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

  • 0 in reply to future chang1
    TI__Mastermind 39895 points

    Hi,

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

    Ben

  • 0 in reply to Ben C
    Prodigy 250 points

    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

    1.

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

    The product will not turn on.

    2.

    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

  • 0 in reply to future chang1
    TI__Mastermind 39895 points

    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?

    Ben

  • +1 in reply to Ben C
    Prodigy 250 points

    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

  • 0 in reply to future chang1
    TI__Mastermind 39895 points

    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!

    Ben