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LTC4079 Ctimer

Hi,

In datasheet, CHRG status pin will change once the c/10 pass below its threshold regardless ctimer. However, we find our CHRG pin is abnormal in practical and simulation.

According to datasheet and flowchard, CHRG status pin will DEASSERT once the c/10 pass below its threshold. However, after ctimer is set, CHRG pin remain assert after c/10 below threshold. CHRG status pin will only DEASSERT after the ctimer termination.

The issue is observed in my simulation too. When the timer is set, the CHRG pin is not working even c/10 below threshold. It only happened when I remove ctimer feature.

In datasheet, CHRG status pin pulls low during a charging cycle until the charger output current falls below the C/10 threshold, regardless which termination scheme is
used.

Appreciate if you can advise how CHRG pin status behave with c/10 and ctimer.

Thank you. 

Mark

  • Hi Mark,

    I just tested on the demo board and I see the behavior as described by the datasheet. Even with the timer enabled, the /CHRG pin deasserts when the charge current drops below C/10.

    Can you send me your simulation file?

    Regards,

    Zack

  • Hi Zack,

    Thank for your feedback.

    I modify the simulation file "LTC4079_TA01A". Following is simulation with the timer. The CHRG pin deasserts only timer running out in the simulation. It is happened in practical as well

    4079 - Timer.asc

    Thank you.

    Mark

  • Hi Mark,

    I see the same operation in the simulation. However, I do not see it in reality. I would be willing to guess that the simulation is wrong there, but it is strange that you see it in practice.

    What size cap to you use in your circuit? Do you have a plot of this happening?

    Regards,

    Zack

  • Hi Zack,

    Thank for your feedback. It is strange. So you mean the simulation file is not correct? or my simulation circuit and setting is wrong?

    Could you advise your demo board set-up and config? so that, we may follow and try

    In practical, we apply the 220nF, which mean 12 hours. We set the programmed current as 185mA. In general, we shouldobserve CHRG deasserts when the charge below 1/10 as 18.5mA.

    However, CHRG remain ASSERT until the timer is running out around 12 hour.

    Please kindly advise.

    Thank you.

    Mark

  • Hi Mark,

    If the simulation behaves differently than how the part behaves, I generally disregard the simulation. The TIMER pin is absent and replaced by a text input in the simulation, so we already know it is a little different.

    My demo board is in the standard configuration except that I had a 1.5kOhm PROG resistor for a 200mA charge current. So, pretty similar to your charge current.

    This was configured for a 4.2V battery with the timer enabled using the default 68nF TIMER cap on the demo board.

    In this setup, the /CHRG pin was deasserted when my charge current went below 20mA.

    Regards,
    Zack

  • Hi Zack,

    Would the capacitor value cause the timer issue?

    I find the following statement in the datasheet:

    I am wondering charging will be terminated based on c/10, and timer pin must below 0.3V.

    Will the capacitor value and type affect the threshold 0.3V and termination as well?

    Thank you.

    Mark. 

  • Hi Mark,

    The capacitor type will not affect the 0.3V threshold itself. This is saying "if your battery finishes charging faster than the TIMER pin rises, then charging will terminate immediately".

    Isn't your issue that /CHRG is not deasserting at C/10? This seems like a totally different behavior.

    I recommend you try some different component values. This seems to work as described on the standard demo board. I've never heard of an issue with this behavior. If you can't get anything to work, we can discuss sending us a board for debugging. I also recommend you disregard the simulation's behavior in this aspect.

    Regards,

    Zack