• State TI Thinks Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 478 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM27761: When first enabled output goes positive before going negative

Tom Lowdermilk97
Prodigy 50 points
Part Number: LM27761

I have an LM27761 being used to generate -1.7V. It's enabled by a +3.3V signal. 

When enabled I'm seeing the output rise to 380 mV within the span of about 400 us 

before going low.

Is this normal?

  • 0
    TI__Genius 14876 points
    Hello Tom,

    What is your Vin and your load when you enable the LM27761?
    Can you provide your circuit schematic as well as a waveform of Vout, when you observe this output rise before going low?

    Thank you very Much!

    Best Regards,

    Dorian
  • 0 in reply to Dorian Brillet de Cande
    Prodigy 50 points

    hello Dorian,

    Thank you for the quick reply. The only load is VS- input to two LMH5401IRMST. 

    Here is the regulator schematic:

     

    If I probe the Vout and C1+ pins I see the following:

    Here is the input +3.3V and output -1.7V

    And here is the -1.7V output and CPOUT 

    To me it looks like the +3.3V is leaking into the LDO output before the C1 capacitor begins 

    switching. What do you think?

    regards,

    Tom

  • 0 in reply to Tom Lowdermilk97
    TI__Genius 14876 points
    Hi Tom,

    According to the datasheet the EN pin brings the device into enable mode when it sees at its input a voltage greater than 1.2V.

    As you can see on your second scope shot, the system start operating as expected when your VIN is stable. I would recommend you to start controlling the EN pin when VIN is already stable.

    Does this make sense to you?
    Thank you very much!

    Best Regards,

    Dorian
  • 0 in reply to Dorian Brillet de Cande
    Prodigy 50 points

    hello Dorian,

    Thanks again for the quick support.

    So I'm still wondering why the -1.7 Volt output initially goes positive.

    Here is the schematic showing one of the amplifier loads. Since 

    +3.3V comes up before -1.7V could the amplifier be somehow driving 

    the -1.7V positive?

    regards,

    Tom

  • 0 in reply to Tom Lowdermilk97
    TI__Genius 14876 points
    Hi Tom,

    Looking at your schematic, as your amplifier is also supplied by this 3.3V/N1.7V power line. I would assume there is current flowing from 3.3V and N1.7V (which is 0V at t=0s). This current would then flow in the LDO, and would cause that output rise. After that your system enters normal mode the LDO can compensate for it.

    If you can start your system without load you would be able to see normal operation. Is this rise a major concern in your system?

    Thank you!

    Best Regards,

    Dorian
  • 0 in reply to Dorian Brillet de Cande
    Prodigy 50 points

    Dorian,

    Thanks again!

    I was just concerned that maybe the downstream ADC would have 

    problems because during this -1.7V positive time I see the amplifier 

    glitch its outputs.

    However, we keep the ADC in reset during power up so the -1.7V 

    output doesn't look nice but I'm pretty sure this isn't a problem.

    thanks again!

    Tom