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LTC4079 - not running in timer mode with cap on timer pin

Hello,

I have the LTC4079 up and running and am trying to bench test it as I integrated it into my product.  I want to use the part in timer mode and from what I understand from the datasheet this is achieved by loading a capacitor on the timer pin.  I have loaded a 0.1uF (100nF) so I should get a timer duration of about 5.5hrs.  I am a little confused by the data sheet and by what I'm seeing on the bench.  With the 0.1uF pin loaded I see the charge current run at my set current of 100mA for ~4.5 hours.  It then begins to taper down like it is CV mode waiting for the C/10 limit to hit before it shuts off completely.  

Questions:

1. With timer pin loaded with a capacitor like mine - 01.uF should the part charge my battery pack at a constant current until the timer expires and then completely shut off?

  • Also, what is the difference between run timer and safety timer in the state diagram on page 10?

  • Hello,

    The timer operation sounds correct - this sets the maximum charge duration regardless of charging mode. The charging mode is actually irrelevant to the timer; this is to say that the timer has no control over how long the battery receives the full programmed charging current. That's moreso a function of the battery itself.

    Once the battery reaches the target charging voltage, the voltage will start to taper. If you were seeing a timer expiration, the charging would just stop immediately.

    As for the "run timer", think of "run" as a verb. Wink It is running the safety timer (as opposed to pausing it).

    Regards,

    Zack

  • Hi zach, thank you so much for the reply.  That is helpful.  Do you know why I would be seeing my charge current correctly for 3 seconds then the chip stops charge and the charge current is 0 for 6 seconds.  This is repeating indefinitely. 

    I see that the temperature is sampled every three seconds but where would the 6 seconds off time be coming from?  I checked the enable pin and it is pulled up to the input voltage which is 8 volts.  The input to output voltage is 8v in and a max battery pack voltage of 7v at 100mA.  The ntc thermistor pin is exactly 50%of the ntcbias pin as well.  

    Do you know what would be shutting charging down for 6 seconds repeatedly?  

  • What state is the charger in when this happens? Near top-off? Any state? Does this happen on multiple units?

    What does the battery voltage look like when this happens? Watch on a scope for any wavering. And you are configured for TIMER termination still?

    Regards,

    Zack

  • I will investigate and get back to you.  How do you know what state it is in?  The led is pretty much always on and provides very little information.

    Also, if you have a cap loaded for timer mode is it correct in saying that when you go above the Vchg threshold of 1.172V it will switch to constant voltage mode until the timer expires?

  • Is it in constant current or constant voltage mode? That's to say, is it delivering the full programmed charge current or is it holding the battery at a constant voltage while the current tapers off?

    The LED stays on throughout the toggling?

    Yes, it will switch to constant voltage mode as you described. The switching to constant voltage mode will be the same regardless of whether you terminate using a timer or C/10 current.

    Regards,

    Zack

  • Ok.

    So I had a manufacturing problem.  One of the pins actually wasn't soldered well and after ultrasonically cleaning the board after a mod it cleared conductive material under the pad and that pin to pad connection was broken.  This was the mains source of my charging issue.

    So if I understand this correctly, regardless of charge mode, once the feedback voltage hits its threshold the charger will decrease current to keep the battery terminal voltage constant?  If so this makes sense, but isn't very clear in the datasheet.

    My other assumption is that if the charge IC does NOT reach the target charge voltage and the timer runs out the charging will be shut off assuming the voltage is above the Vrechag threshold.  If the voltage is below the Vrechg threshold the retry count will be incremented then the safety timer is cleared and another charge cycle is started over?  

    I am configured in constant current / timer mode.   

  • Ok, great, thanks for letting me know.

    Here's how CC/CV works, and I can elaborate:

    To be clear, the charger regulates current in CC mode and watches voltage. In CV mode, the charger regulates voltage and watches current (if C/10 termination is used). The charger does not control the current in CV mode. All it does is hold the battery voltage to the programmed termination voltage; the decrease in current is what the battery naturally will accept at that voltage - it decreases as the battery reaches capacity.

    Regarding retrying a charge cycle, your understanding is correct. It will restart the charge cycle until the retry count is 5 in an attempt to get the battery to the full charge voltage, but then it will give up for safety reasons.

    Regards,

    Zack

  • Zack thanks for the help your replies/support has been really outstanding!!

    I think one of my issues is that due to a lower battery charge current the cell voltage in my battery pack isn't rising as fast as I thought it would.  Slower charge current allows for a less aggressive thermal and voltage rise is another way of saying it.

    I think the state machine in the part is restarting as you stated above because the feedback voltage isn't getting over the 97% threshold.  This isn't a huge deal because I'm actually controlling this charger with an MCU that keeps track of charge in and out via a fuel gauge from you guys.  So in the end this operation is pretty ideal for me.  

    Thanks again!

  • Ok, great!

    By the way, if you have any load on your battery while it is charging, that might explain why it never reaches the 97% threshold. You could consider extending the charge timer.

    However, if this operation suits you then feel free to stick with it as it is.

    Let me know if anything else comes up!

    Regards,

    Zack