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LM25011: Is LM25011 Good for charging Supercapacitor?

Alex Poon14
Prodigy 50 points
Part Number: LM25011
Other Parts Discussed in Thread: LM22673

Hi,

I am looking for a buck converter with adjustable current limit for charging 4pcs of super capacitor connected in series. 

My requirements:

Vin: 9-36V

Vout: 9V

Charging Current: 1A

Is LM25011 Good for the application of supercapacitor charging?

What will happen on Vout if Vin is lower than the Vout we set? For example, if Vin=8V, will Vout equal to 8V minus some dropout, i.e. around 7V? Will the charging current also be affected in this case?

Is LM22673 also good for my application?

Thanks.

AlexP

  • 0
    TI__Mastermind 25590 points

    Alex,

    LM25011 would be better than LM22673 because LM22673 folds back the frequency in short circuit conditions. Charging a supercap from 0V is essentially a short circuit condition so you'd run into this feature accidentally.

    As described in this thread, you may run into issues with a pre-biased output. To prevent issues you can add a diode in series with the inductor to allow the pre-charge FET to pull SW to GND regardless of output voltage.

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Mastermind 25590 points

    Alex,

    VOUT > VIN will cause current to flow through the body diode of the high-side FET to VIN. This may cause the device to switch and quickly deplete the charge on VIN which may cause some oscillations. We recommend disallowing this condition by keeping VIN > VOUT or adding the diode in series with L and adding a high-value bleed resistor on SW or VIN to prevent the leakage through the diode from charging up SW and VIN.

    -Sam

  • 0 in reply to Samuel Jaffe
    Prodigy 50 points

    Hi Sam,

    Thanks for your reply.

    I agree that LM25011 is more suitable in my application because it can switch to constant current mode when the current limit is triggered.

    I also agree adding a schottky diode and bleed resistor can avoid reverse current when input voltage is lower than output voltage.

    And actually I would like to know what will be the "actual output voltage" if input voltage is smaller than the "target output voltage" we set.

    For example.

    Target output voltage: 9V

    Input voltage: 6 - 9V only

    Actual output voltage: will it be 0V or nearly equal to input voltage?

    Thanks.

    Alex Poon

  • 0 in reply to Alex Poon14
    TI__Mastermind 25590 points

    Alex,

    The EVM users guide has data at 1AOUT at 5VOUT at 7.5VIN but I'm sure it can regulate at lower VIN. Let me check with the team. I should have an answer by early next week.

  • 0 in reply to Samuel Jaffe
    TI__Mastermind 25590 points

    Alex,

    VOUT_MAX_AT_DROPOUT = VIN * Max_duty_cycle - IOUT*DCR_L - IOUT*RDS(ON)*Max_duty_cycle - Diode_Forward_Voltage_at_IOUT*(1-Max_duty_cycle)

    where

    • Max_duty_cycle = TON/(TON+TOFF_MIN)
    • TOFF_MIN = 208ns max in the datasheet.
    • TON = Set by you. See datasheet RT
    • DCR_L = inductor DCR
    • RDS(ON) = 0.6 ohms max (see datasheet)

    Also keep in mind the device may go unstable at dropout. This is a constant on-time which relies on the output ripple for stability. Dropout conditions reduce that ripple. You may want to add ripple injection.

    -Sam