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TPS65133: Can't start up when the load is heavy

Vio Liu
Expert 3045 points
Part Number: TPS65133
Other Parts Discussed in Thread: TPS65131, TPS65130

Hi team,

 

This is Vio from Guangzhou, China. My customer is using our TPS65133 to supply some operation amplifiers. The input voltage is Li battery, ranging from 3.3V to 4.2V.

They used our solution before and it works well. But now they decide to change the Operation amplifiers with high power consumption. The total power consumption would be about 50mA(Vreg). In this case the Vreg can't start up.

I can see in forum that there're many threads about this issue. I have read them carefully and tried every method, such as adding big buck capacitor as high as 1000uF, changing higher current inductor, removing the output capacitors, and so on. None of them work.

In our case, the Vreg output is -0.6V. Current for the -0.6V is 113mA.

Also i have tried on our EVM. I add a 50 Ohm resistor to the negative output, and then power on. The output volage is -4V not -5V. This test seems similar with customers' test on their board.

Can you please give some guide to solve this issue? 

Thanks!

 

Best regards,

Vio Liu

 

  • 0
    TI__Guru 65200 points

    Hello Vio,

    During startup the load current on the part is limited. If it is heavy (which is the case with 50 ohm load), the part will limit the current until the heavy load is removed.

    Due to the fact that you are not removing the 50 ohm, the part is working with a lower current limit and not able to achieve the 5V. As soon as you add some additional load, VNEG will drop further.

    How is the above mentioned Vreg generated? Is this done with an LDO connected to VNEG of TPS65133? Is it possible to delay the load current until the device has started?

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    TI__Expert 3045 points
    Hi Brigitte,

    Thanks for your help.
    It seems the only way to solve this issue to avoid the load during start up, right?
    Do we have some test result about the threshold, with which load can our device VREG start up and with which load can't start up? Thanks!

    Best regards,
    Vio Liu
  • 0 in reply to Vio Liu
    TI__Guru 65200 points

    Hello Vio Liu,

    I checked out the resistive load that can be connected to the part during startup. I measured it at the lowest input voltage because the input voltage influences the current capability heavily on boost and buck-boost converters.

    Vpos: 33 Ohm (150mA at 5V)

    Vneg: 83 Ohm (60mA at -5V)

    This was measured at room temperature on a typical unit. So for production these values need to be degraded a lot to make sure that all units are able to start, independent of the ambient temperature as well as the current limit of the converters.

    I would not recommend to have less than double the resistance value attached to the device. This means:

    Vpos: 66 Ohm (75mA at 5V)

    Vneg: 166 Ohm (30mA at -5V)

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    TI__Expert 3045 points
    Hi Brigitte,

    Thanks very much for your timely test and guidance.
    Our customer is using TPS65131 in their products. Can this issue happen on TPS6513x?

    Thanks!
    Best regards,
    Vio Liu
  • 0 in reply to Brigitte
    TI__Expert 3045 points
    Hi Brigitte,

    Can you help varify if our TPS65131 has such kind of issue?

    One more question, our customer accept our solution, and they will add soft start circuit after VREG output. But they need our max start up time spec. Is 0.3s enough for TPS65133(load on VREG is disabled) to start up?

    Thanks!
    Best regards,
    Vio Liu
  • 0 in reply to Vio Liu
    TI__Guru 65200 points

    Hello Vio Liu,

    Any boost converter and buck-boost converter has a maximum resistive load it can start into. The value is dependent on the maximum allowed startup current. The startup current is limited in these devices because every boost and buck-boost converter will go into current limit at startup because the output voltage is higher or inverted to the input voltage. So yes, if the output current is too high, TPS65130/1 might not be able to start. The maximum start current of TPS65131 is higher than for TPS65133, so this will happen at a higher output current.

    As long as there is no load connected, 0.3s is plenty of time for TPS65133 to start. Thank you for convincing the customer to use our device.

    Best regards,
    Brigitte