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LM27762: Strange behavior from LM27762

majd ghadab
Prodigy 60 points
Part Number: LM27762

hello everyone,

i was trying to make some testing on LM27762 before i use it in other projects.

so i made a pcb with Altium and designed the schematic based on the values from Texas Instruments site using WEBENCH so the voltage divider were 499K and 1.58M because i wanted to have +5 and - 5 from 5.5 Power..

anyway i made the circuit and tested it and i got from the positive output the same voltage from as input and from the negative output always - 3.8 as Minimum

the value of negative output is more positive like - 3 when i reduce the input voltage and the output of positive output is always the same as input.

now i replaced the chip because i thought it might be damaged or so, but i got the same results.

can someone helps me. Thanks in Advance.

  • 0
    TI__Mastermind 36681 points

    Hello Majd,

    Can you please share the schematic diagram and layout so I can check it? If you are not comfortable sharing it here, then I can send an email.

    Thank you.

    Best regards,

    Excel

  • 0
    TI__Expert 3645 points

    Hello,

    can you send me a schematic and can you give more info about your application Vin, Vout and the load for the positive and negative rails?

  • 0 in reply to Alfonso Furio
    Prodigy 60 points

    Hello thanks for your reply,

    these are the photos, and as a notice i used 1.5M resistor because i don't have right now 1.58. and this is only a test and when i see that this IC can work with my projekt i planned to change the components to 0805 or and put the caps on the bottom layer.

    I forgot to say that the Enable + and - is working correctly but the problem that described above is still exist.

  • 0 in reply to Alfonso Furio
    Prodigy 60 points

    I still didn't use any load on the output.. because i still see a problem with the output.. negative voltage is not correct and positive, negative voltage is not regulated

  • 0 in reply to majd ghadab
    Prodigy 60 points

    hello

    until now i didn't discover the solution. I checked all tracks an connectivity under microscope but it seems all is correct.. No short and all pins are connected with each other, i bought the 2 Chips from Mouser, could it be that both are damaged?

    because even the positive rail has always exactly the same input voltage.

  • 0 in reply to majd ghadab
    Guru 316440 points

    The capacitors must be ceramic. Electrolytic capacitors have too much ESR.

    Please read section 10 of the datasheet; all those guidelines need to be followed.

  • 0 in reply to Clemens Ladisch
    Prodigy 60 points

    okay thanks for reply. i would re design the circuit and order all Ceramic Capacitors. i also would buy new LM27762 chips to test with the new board. 

    but i need jetzt sometime.. i would post the results in the future. 

    anyway i didn't take these rules into consideration because that was a quick test and i didn't thought at this too low oscillating frequency that this going to cause this Problem, because i sometime make circuits that works upto 100 MHz on breadboard. Thanks