• State
  • Replies 9 replies
  • Subscribers 62 subscribers
  • Views 99 views
  • Users 0 members are here
Support feedback
Options
Options
Related

BQ25173: Super capacitor charging issue

SELVAGANAPATHI K
Prodigy 135 points

Part Number: BQ25173

Hi TI team, 

I'm using BQ25173DSGR charger in my design to charge 0.47F super capacitor to the maximum voltage of 1.4V with the configured current limit of 30mA. From the datasheet it is understood that super cap will be charged by a constant configured current of 30mA, when charging the SCAP from 0V. image.png

But I couldn't see this kind of behaviour. It looks somewhat strange. 
image.png
With my understanding the theoritical charging time of supercap from 0V to 1.4V would be 23 seconds, but we are observing the sudden change of super capacitor voltage from 0 to 1.4V in 341us.
Note: C197 is removed


Can you explain the charging method we are observing?

With C197, the super capacitor charging voltage togles, below is the snip
image.png
image.png
 
Can you suggest a fix for the above issues?

 

  • 0
    TI__Guru* 77966 points

    Hello

    The Schematic looks good do not see problems.

    Do you have a current meter in the output between BQ25173 and super capacitor?  I have seen problems with the current shunts in the meters before.  Current can be monitored using ISET pin, voltage is proportional to output current.  

    Can you repost the scope captures, resolution is low.

  • 0
    Prodigy 135 points

    Hi john,

    Thanks for your reply.

    We don't have current meter in the output between BQ25173 and supercapacitor and also, we didn't provide any option to measure the super capacitor charging current.
    Can you please explain how the output current is proportional to ISET voltage other than below?
    ...

    Scope captures:
    Super capacitor's voltage during charging
    ...

    For our understanding, can you pls explain how charging will be done for completely drained supercapacitor and how much time it will theoretically takes for complete charge to 1.4V.

  • 0 in reply to SELVAGANAPATHI K
    Prodigy 135 points

    Any other relation to output current with voltage on ISET PIN other than below

    Scope capture:
    Super capacitor's voltage during charging:

  • 0 in reply to SELVAGANAPATHI K
    TI__Guru* 77966 points

    The ISET pin is a sample of output current at a ratio of 1 to 300.  The ISET current develops a voltage across the ISET resistor, at 1V the unit is in current regulation. 

  • 0 in reply to Bill Johns
    TI__Guru* 77966 points

    Have you tried FB configuration that does not use the PG Pin?

  • 0 in reply to Bill Johns
    Prodigy 135 points

    Hi John,

    Yes, we have tried that too but same behavior.

    Can you please suggest what is the expected charging time of the super capacitor in my case and typical test waveform that you have for our reference?

  • 0 in reply to SELVAGANAPATHI K
    TI__Guru* 77966 points

    Hello

    "Can you please suggest what is the expected charging time of the super capacitor in my case and typical test waveform that you have for our reference?"  -- The expected charge time I calculated is about 22 sec.  

    I have tested a similar configuration using the BQ25173 EVM with 0.5F capacitor configuration.  The charge current was 34mA with charge time was about 19.5sec.

    The lab test did not show any charge problem for this configuration.

  • 0 in reply to Bill Johns
    Prodigy 135 points

    Hello John.

    Can you share any charging waveform captured for charging 0.5F in your lab. 

    First of all, the regulated output maximum voltage itself differs. 

    Designed Vout range:
    1.45 V to 1.46 V

    Actual maximum Vout observed:
    1.38V

    Any specific reason for this, are we missing out anything?

  • 0 in reply to SELVAGANAPATHI K
    TI__Guru* 77966 points

    Hi

    "Designed Vout range: 1.45 V to 1.46 V"

    "Actual maximum Vout observed: 1.38V"

    With the 8.2k and 11.5k resistor combination I am calculating Vout value of 1.3704V which is very close to your measurement of 1.38V.

    UPDATE -- In the below test I measured output voltage with a DVM and it is running 1.3698V

     

    SELVAGANAPATHI K said:
    Can you share any charging waveform captured for charging 0.5F in your lab. 

    See below scope capture of a start up in your configuration.