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LM60440: charging supercapacitor

Robert Hinchy
Prodigy 40 points
Part Number: LM60440
Other Parts Discussed in Thread: LM73605EVM-5V-400K, LM73605, LM5117

Dear Sir/Madam,

I am currently using an LM60440ARPKR to charge a 50 Farad 4.2 Volt supercapcaitor. It is succesfully charging this capacitor however it is taking about 50 seconds to do so. Is there anything I can do to speed up this charging time (i.e. increase in rush current).

Attached is a picture of my circuit.

  • The PWR_PROT line on left of the circuit is main supply and can be anything from 8 Volts to 32 Volts.
  • The output signal on the right after D39 TVS diode is fed to the supercapacitor. 

Also attached is an oscilloscope plot of the supercapacitor charging. 

  • Blue line shows when power prot is applied (15 Volts in this case).
  • Yellow line shows the voltage on the supercapacitor. Note ten seconds per  horizontal division. 1 Volt per vertical division. 
  • for 30 seconds the supercacitor charges from 0V to 1.48V in a low current PWM mode (apx 100 to 200 mA). 
  • After 30 seconds the supercaitors charges upto to 4.2V in a higer current linear maner (apx 1 amp)
  • Ideally I would like to charge the supercapacitor in a linear manner with a 4 amp current. 

I would really appreciate any help with this. Alternatively if you can suggent a better device to do this job I would really appricatie it. 

  • 0
    TI__Mastermind 22299 points

    Hi Robert,

    Are you able to scopeshot the L5 inductor current? Place the current loop between L5 and the output capacitance.

    If you cannot probe inductor current then the SW node voltage will be OK.

    I think the device is hitting current limit, and it is entering hiccup mode which results in an overall lower average current at the lower voltage.

    Many of our general DC/DC converters have hiccup mode current limiting.

    One solution is to use a device with customizable soft-start, a longer soft start should prevent sustained overcurrent and prevent hiccup mode at soft-start.

    LM73605 has this feature you can try the customizable soft start on LM73605EVM-5V-400K (or the other EVM).

    Hope this helps,

    -Orlando

  • 0
    TI__Mastermind 22299 points

    Hi Robert,

    Check out this application note for a CC/CV converter using a LM5117, this may be more useful than extended soft-start which only works at startup.

    https://www.ti.com/lit/an/snva829/snva829.pdf

    Hope this helps,

    -Orlando