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TPS61071 soft start sequence strange behavior

Ferdinando Quattromani
Prodigy 60 points
Other Parts Discussed in Thread: TPS61071, TPS61070

Hello

I'm using TPS61071 component to step up a voltage from 3.3 Vdc to 5.0 Vdc with about 0.08 Amp of current .

I evaluate first measurements and tests , on the Texas evaluation board  TPS61070 EVM-062 - HPA062 Rev A .

Before proceed with tests , I have  changed compnents from TPS61070 to TPS61071 !

In the first measurements  the TPS61071 works fine ; subsequently I have added on the EVBOARD , in C4 place a 220 uF capacitor

and sometimes , I observed with oscilloscope that  the soft start time phase  , is not executed correctly .

To better explain , sometimes I see the correct soft start sequence like that on page 16 - Figure 24 of Texas data sheet TPS6107x  - SLVS510E –JULY 2006–REVISED MARCH 2015 , and sometimes I see that the oscillation phase starts immediately without precharge phase where the current is limited .

Where is the problem ? Is there a native problem with this component ?

I choose TPS61071 because I need this soft start feature !

  • 0
    TI__Mastermind 46905 points

    Hi

    Both TPS61070 and TPS61071 has the same soft start function. the difference is the PWM mode is enabled or not.

    the C4 is the input capacitor, large input capacitor should not have much impact on the normal operation of the boost converter. normally the larger input capacitor, the better for boost operation. could you capture the input voltage and output voltage waveform and share with me?

    during you test, the cable connecting the power supply and EVM must be short to avoid oscillation.

    thanks

    Jasper Li

  • 0 in reply to Jasper Li
    Prodigy 60 points
    Thank you for the replay !

    I think to understand where is the problem !

    Adding a big input capacitor (C4) , the discharge time of this capacitor
    is enoguh long when the load is at minimum value .

    So what's happening is this : when I interrupt main power supply and
    just after some milliseconds I connect main power supply again , the power
    input doesn't drop to about zero volt but instead it decreases slightly
    and then it returns to its original value ; this behavior , doesn't trigger
    correct TPS61071 soft start sequence .
    This behavior may be acceptable because TPS61071 doesn't see a true/complete
    sequence of power OFF and subsequently power ON !!!

    However I'm not agree with this behavior because during the short interruption
    of the main input voltage , the output voltage drops more quickly than input
    voltage ; I can observe that output voltage is almost pulled down to about
    1 Vdc , during the interruption of the main input voltage .

    I think that TPS61071 soft start sequence should be triggered when there is power ON
    event and always every time that there is a Voutput drop event (let me say when the output
    voltage drops at 0.5 * Vout)
  • 0 in reply to Ferdinando Quattromani
    TI__Mastermind 46905 points
    Could you share the input voltage and output voltage waveform, so i can understand what real happen.

    thanks
    Jasper Li
  • 0 in reply to Jasper Li
    Prodigy 60 points
    Sorry for my disappoint , but I don't know how I'm able to attach a bitmap to this reply ): !
  • 0 in reply to Jasper Li
    Prodigy 60 points

     As I have described there is not a full Vout discharge ; in the above sequence , Vout drops at about 2.2 Vdc

  • 0 in reply to Ferdinando Quattromani
    TI__Mastermind 46905 points
    i don't see problem from the figure
    the soft start of the boost device divide into pre-charge stage (this stage finish when VOUT close to VIN) and boost stage.please refer to datasheet about detail of SS principle. in your measurement, there is a voltage in the VOUT (approximate 3V)when you restart the device. so pre-charge stage time is very short, then enter boost stage.

    thanks
    Jasper
  • 0 in reply to Jasper Li
    Prodigy 60 points
    AS I said previously :

    "This behavior may be acceptable because TPS61071 doesn't see a true/complete
    sequence of power OFF and subsequently power ON !!!"

    I'm arrived to same your conclusion .
    To be honest , only now I'm discovering the full aspect of this TPS61071 soft start feature .

    When I choose this component , instead I would expect that soft start feature will limit current
    until reaching to nominal Vout (plus / minus a little threshold) while now I see that
    when I enable /disable TPS61071 (due to power save management)
    with very low battery energy residual , the Vinput voltage , drops under my min survive value that
    generate a microprocessor reset .
    THAT'S ALL !!

    Thank you for the support
  • 0 in reply to Ferdinando Quattromani
    TI__Mastermind 46905 points
    please refer to this application note about the how to limit the soft start current in TPS6107x series "Extending the Soft Start Time in the TPS6107x Family of Boost Converters". ( SLVA307A) it should help.

    the device do have soft start even when there is a pre-bias voltage. the current limit is limitedto half of its normal current limit value. but apparently, this current is still higher than output capability of the battery. other solution could be bulk capacitor in the battery output.

    thanks
    Jasper Li
  • 0 in reply to Jasper Li
    Prodigy 60 points
    Thank you very much Jasper !
    When you say

    " this current is still higher than output capability of the battery "

    you have completely hit my problem and the requirements to apply when
    develop an electronic system equipped with not rechargeable battery .

    I'm despondent because initially I was sure that this problem was
    resolved by the component TPS61071 and I don't understand why
    the TPS61071 soft start feature doesn't match this requirements !
    But anyway now I'm forced to step back and change my printed
    circuit board to correct this problem .
    Thank you and bye