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TPS61260 failure as a supercapacitor charger

Christian JAFFRE
Prodigy 70 points
Other Parts Discussed in Thread: TPS61260, TPS61260EVM-673

Hello,
A TPS61260EVM-673 is used in order to evaluate the TPS61260 as a supercapacitor charger.
Output voltage was set a 4.0V with R2=143K instead of 182K.
Output current limit was set at 30mA with R3=6.8K instead of 2.0K.
The supercapacitor is formed with two CAP-XX HW203 (0.55F/90mOhm) in parallel (with 47K balancing resistors to VOUT and GND).
Voltage source is a primary lithium element (SAFT LS14500 3.6V with 50mA continuous max current).
VOUT is used to power a radio board (pulsed load 1A/400ms, 50s period)

The first day of evaluation the board was tested with good results so behavior and voltage values was exactly the same as computed previously.
24 hours later we measured the output voltage without powering the board. VOUT was 1.2V, corresponding to the voltage discharge in the balancing resistors. After connecting the voltage source we saw  that the TPS61260 wasn't switching so the voltage remained constant.

After disconnecting the input source and discharging the output capacitor the voltage source was connected again and then VOUT rise and stabilize to 0.5V (Device Vref internal value is 500mV so TPS61260 behavior corresponding to VOUT directly connected to FB pin).
When I measure the middle node of the feedback resistor network R1/R2, the voltage is close to 0V but the ground resistor is not failed.

It seems that the TPS61260 is not anymore functional but i was not able to find in the datasheet a violating of any Absolute Maximum Rating.

The Supercap value is 1.1F so the device will support a extended period of time with VOUT>>VIN when the input source is disconnected but i suppose that a step-up regulator is designed to support this kind of situation.

Is anyone have an idea to explain the failure of the TPS61260?

Thanks and best regards

  • 0
    TI__Mastermind 46905 points
    please remove the super capacitor and disconnect the EVM with the load. then power the EVM with a 2V power supply to double check if the EVM is damaged.

    if the EVM is really damage, please help to measure the resistance between GND pin and other pins. i want to know which pin is damaged.
  • 0 in reply to Jasper Li
    Prodigy 70 points
    Hi Jasper,
    Please find below the measurements

    Load: disconnected
    Scap: disconnected
    Voltage source= 2V

    Voltages at different nodes after startup in respect to GND:
    VIN= 2V
    VOUT=0.832V
    VFB=103mV
    VRI=0V

    Resistance between nodes and GND
    RVIN=90K
    RVOUT=160K
    RFB=124K
    RRI=6.8K

    I bought another evaluation board and just changed R2 to 147K and R3 to 6.8K (no SuperCap mounted)

    TPS61260 nerver started on EVB #2 (from VIN=2V to 3.6V)
    VOUT was at 0.853V and no switching at all seen at the L node from VIN rising to
    VOUT rising to 0.853V.
    I tried to add a 10K charge resistance at VOUT without any positive result

    So i decided to replace the TPS61260 parts on the two EVB with new parts and everything is working fine now
    on both EVB

    Is there any startup issued knowed for TPS61260?

    Thanks and best regards

    Christian JAFFRE
  • 0 in reply to Christian JAFFRE
    TI__Mastermind 46905 points
    No. i don't see problem about the startup of the TPS61260.
    from you experiment on the second EVB, it seem the device is damaged during changing the feedback resistor. i met this kind of problem before, a bad iron could damage the IC during replacing its external components.
  • 0 in reply to Jasper Li
    Prodigy 70 points

    Maybe for the second EVB but never seen that in 25 years with so many feedback resistor changes...

    Seems to me that the TPS61260 is not very rugged, maybe a very limited ESD protection on some pins?

    And afterall it does not explain why the first EVB, fully functional after the resistor changed, has failed one day later

    Christian JAFFRE

  • 0 in reply to Christian JAFFRE
    TI__Mastermind 46905 points

    Then i don't know why the IC is damaged. the ESD protection of all the pin should meet the specification in the datasheet.
    did you check the EVB is good before replacing the feedback resistor?

    could you duplicate the supercap experiment again? if you can find the damage again, i'll try to reproduce the problem in the lab and find the root cause.