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LM2776: LM2776

Vivek V12
Prodigy 60 points
Part Number: LM2776

hi Team,

I am using LM2776 device to generate Negative 3.3V from +3.3V as Input supply. Both these voltages, +/- 3.3V, is applied as supply voltages of 3 OP-AMP and 1 Instrumentation Amplifier.

But I am getting Positive 0.5V approx at the output of Lm2776 instead of Negative 3.3V. When i am isolating the OP-AMPS and INA, output becomes -3.3V. In D/S max load current is mentioned as 200mA.

I have connected 1uF, with low ESR high stability, across C1 and C2. Also, input and output has 2.2uF in parallel to 0.1uF.

Please let me know of this behavior and any work around possible

Regards,

Vivek

  • 0
    TI__Guru*** 142880 points
    Hi Vivek,

    Can you post (or share with me via a conversation) your schematic and a waveform of Vin and Vout and EN at power up?

    Does this happen on all PCBs or just a few? Does it happen at every startup or just occasionally?

    Did you already try replacing each capacitor around the LM2776 with a capacitor of a known correct value, to rule out a manufacturer issue, wrong component, cracked cap, etc.?
  • 0 in reply to Chris Glaser
    TI__Guru*** 142880 points
    Hi Vivek,

    Did you resolve this issue already?
  • 0 in reply to Chris Glaser
    Prodigy 60 points

    hi Chris,

    Attached is the schematic of the circuit used on the board.

    Vin of LM2776 is constant 3.3V. Vout we are getting is +0.4V with all the capacitor values as per schematic. Also the placement of the parts are nearby to the IC as per datasheet.

    Answering your second question, yes we tried replacing the input, output and even switching capacitors to attain the output voltage but still no luck.

    Even we provided Input of IC through external power suppy without current limit, but still not able to get the desired voltage.

    Regards,

    Vivek

  • 0 in reply to Vivek V12
    TI__Guru*** 142880 points
    Hi Vivek,

    L2 and L3 needs to go outside of the Cin and Cout. Please replace them with shorts and retest.

    Can you share the part numbers of the caps?

    Can you post a waveform of Vin and Vout at power up?

    Does this happen on all PCBs or just a few? Does it happen at every startup or just occasionally?
  • 0 in reply to Chris Glaser
    Prodigy 60 points

    hi Chris,

    L2 and L3 we have shorted and checked but issue remains same. Even we removed L3 and connected external Power Supply but still same. 

    Vin is constant 3.3V with around 50mV ripple and output we are observing is constant +0.4V. I don't have the waveform handy.

    C69, C70 - 06036C225KAT2A

    C9, C10 - GRM155R71C104KA88

    C68 - GRM155C81C105KE11D

    This issue we are seeing in multiple boards we have assembled. And yes continuously the output voltage is +0.4V

    Regards,

    Vivek

    06036C225KAT2A
  • 0 in reply to Vivek V12
    TI__Guru*** 142880 points
    Hi Vivek,

    Thanks for sending. It all looks ok.

    Please send a waveform of the startup (Vin and Vout) when it works and when it doesn't.

    Also, if you are able to post the PCB layout, I can check this as well.
  • 0 in reply to Chris Glaser
    TI__Guru*** 142880 points
    Hi Vivek,

    Have you had a chance to get a waveform?