Post Go back to editing

LTC4041 CAPGD behaviour not as expected

Category: Hardware
Product Number: LTC4041

I am using the DC2642A-A evaluation board for the LTC4041 supercapacitor controller to prototype a fairly standard 5V power backup application.

I have found that the behaviour of the CAPGD output is not consistent with what one would expect based on reading of the datsheet and would appreciate it if somebody can confirm whether the observed reality is correct or whether I should be looking for some kind of problem.

The datasheet block diagram shows the CAPGD signal as being derived from a comparator that compares the supercapacitor voltage at CAPFB against a fixed reference. The CAPGD rising threshold is given as 92.5% of the supercap target charge voltage. The hysteresis is given as 2.5% of this, so one would expect CAPGD to go low when the supercap discharges to 90% of its target charge voltage.

What happens in reality is that, when the input power is removed to initiate a supercap backup, CAPGD goes low 1us after /PFO does. There is no change in the supercap voltage during this period. It then stays low until input power is restored and the supercap has recharged. At this point, the CAPGD rising threshold is as described by the datasheet, i.e. around 92.5% of the target charge voltage. It would appear that the CAPGD comparator output is being internally combined with /PFO in some way.

Unless I am missing something, this CAPGD falling behaviour is not described anywhere in the datasheet for the LTC4041 or the evaluation board. I had been hoping to use the falling edge of CAPGD to trigger shutdown of my load system, thereby allowing the system to ride out short power interruptions that discharge the supercaps by less than 10% or so without shutting down, but this will not be possible if the LTC4041 behaves as observed. It's not a dealbreaker (there are other ways to achieve this) but I would still be grateful if somebody could confirm that what I am seeing is the normal behaviour of CAPGD. If so, it would be nice to get it added to the datasheet.

Thanks for any help - kind regards,

Steve
Benden Technology

  • Hi Steve,

    On page 13 of datasheet, it is said that CAPGD pin is pulled low when the charger is disabled. What is not mentioned is when PFI falls below falling threshold, while charger is disabled and backup is enabled.

  • Hi Fil,

    Thank you for taking the time to reply.

    "On page 13 of datasheet, it is said that CAPGD pin is pulled low when the charger is disabled. What is not mentioned is when PFI falls below falling threshold, while charger is disabled and backup is enabled."

    Ah, OK. I had understood that statement to mean that CAPGD is invalid (and goes low) only when the charger is EXTERNALLY disabled by pulling /CHGEN low. You appear to be saying that the charger is also considered to be INTERNALLY disabled when PFI drops below its threshold, and that this has the same effect on CAPGD. Is that correct?

    If so, it confirms my observation that when PFI falls below its threshold, CAPGD is forced low regardless of the actual supercap voltage and that this behaviour is correct and not a fault. That's fine - we can live with that now that we know it is doing what it is meant to!

    Kind regards,

    Steve
    Benden Technology