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TPS61178: Minimum Current Limit

Nassim Mediouni
Prodigy 30 points
Part Number: TPS61178
Other Parts Discussed in Thread: TPS61378-Q1, TPS61379-Q1

Tool/software:

Hi,

I have a question about the TPS61178/TPS1781 reference.

I want to limit the current at 1.2A for my application, but the graph below just goes to 3A.

Is there a minimum value below which one should not fall?

Thanks in advance.

Best regards.

  • 0
    TI__Genius 13575 points

    Hi Nassim,

    The current limit should be higher than the PFM peak current, which is about 3A.

    if it is set below this, the PWM will be unstable.

    Also, small current will be quite imprecise and is not recommended.

    Could you tell me your input and output condition so I can recommend another device with such small current limitation?

    Best Regards,

    Fergus

  • 0 in reply to Fergus He
    Prodigy 30 points

    Hi Fergus,

    thank you for your reply.

    I use the TPS11781, there is no PFM mode.

    In Webench, the Ilimit can go to 0,4A (see picture below).

    My inputs are Vin_min = 4,5V, Vin_max = 5,5V, Vout = 6V and Iout = 200mA.

    I need the Sync pin on the device.

    Best regards.

    Nassim.

  • 0 in reply to Nassim Mediouni
    TI__Genius 13575 points

    Hi Nassim,

    TPS61378-Q1 and TPS61379-Q1 are both preferred.

    For the Webench, I think it is not reliable on the minimum current limitation.

    When use TPS611781, the main problem could be the variation of the current limitation.

    Best Regards,

    Fergus

  • 0 in reply to Fergus He
    Prodigy 30 points

    Hi Fergus,

    in the datasheet of the TPS61378-Q1 it's wrote : 

    With an output current of 200mA, 800mA of ripple is huge.

    Best regards,

    Nassim.

  • 0 in reply to Nassim Mediouni
    TI__Genius 13575 points

    Hi Nassim,

    The limitation is for just same reason with TPS61178: the small peak will cause unstable on PWM and PFM change.

    When work in Forced PWM mode(synchronized to external clock), the peak current minimum limitation(800mA) does not exist.

    This applies to TPS611781, when in FPWM mode the peak current can be small and all due to inductor and frequency.

    The difference is the TPS61378-Q1's current gets a much smaller range, and the variation will be smaller(+-0.8A refer to 4.8A typical).

    For TPS611178/1, its current limitation variation is about +-1.6A(refer to the 8A typical)

    Best Regards,

    Fergus

  • 0 in reply to Fergus He
    Prodigy 30 points

    Hi Fergus,

    800mA it's not the minimum peak current but the minimum ripple current (delta between IL_min and IL_max).

    Best regards,

    Nassim.

  • 0 in reply to Nassim Mediouni
    TI__Genius 13575 points

    Hi Nassim,

    You are right. The 800mA limitation is the ripple current.

    Please see the picture that describe the changes from PWM to PFM.

    When the load decreases and the device detects the peak current comes to a typical value, it stops further decrease the peak and start to extend its period.

    As a result, when the ripple is set to be too small (like 800mA for TPS61378-Q1), it will start to extend its period before the valley current comes to 0.

    This could cause some stability problem as this is not desired working condition.

    You can see this does not apply to FPWM mode as there is not period extend in FPWM mode, so, the limitation no longer exists.

    Best Regards,

    Fergus