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LM2776 - Output not correct : -1.2V output not -5V

Alex Bussiere
Prodigy 20 points
Other Parts Discussed in Thread: LM2776, LM27762

I am having an issue with a prototype design with the LM2776. I have followed the suggested layout and component suggestions, but when I look at the output without a load, the output is -1.2V and not -5V (VIN is 5V) . When I apply 3.3V to VIN my output is around -1V. 

When I put a load on the output the voltage does decrease as it should with a load at around 150mA load. 

Any help or suggestions would be greatly appreciated.

  • 0
    TI__Intellectual 1120 points

    Hi Alex,

    What kind of power supply is providing VIN? Can you take a look at VIN waveform with oscilloscope at start-up situation?

    Regards,

    Juha

  • 0 in reply to Juha Karttunen
    Prodigy 20 points

    I think there was a problem with the length of my traces to the capacitors (they are about 4mm away from the leads. I soldered the components right the package leads and everything is working perfectly

    Thank you for the response,

    Alex

  • 0 in reply to Juha Karttunen
    Prodigy 30 points

    Hi Juha,

    I am having a problem just like Alex. However, the problem appears to be related to start-up timing. Could you help me understand what is causing this? Thanks in advance.

    -Jacob

    Schematic:

    PCB layout:

    In the two oscilloscope traces below, Ch1 = EN pin, Ch2 = +5V supply, Ch3 = -5V output

    When input power ramps up slowly it works fine:

    When input power is applied more quickly the converter does not work correctly:

    Here are traces of the C- terminal in the case of correct and incorrect operation. Channel is assignment is the same except for Ch1 which is now the C- terminal.

    Working:

    Not working:

  • 0 in reply to Jacob Quant
    TI__Intellectual 1120 points

    Hi Jacob,

    I took a look at your layout and there are a few things you could improve:
    -Trace to GND pin is quite long and narrow. You should connect input and output capacitors with short and wide traces to VIN,VOUT and GND pins.( Like you have done with fly capacitor)

    -Capacitors C110, C116 and C109 should be close to VIN pin.

    You can refer datasheet, page 15 for layout example.

    Regards,

    Juha

  • 0 in reply to Juha Karttunen
    Prodigy 10 points

    Hi Juha,

    Could you explain why the part does not work as I have shown it here? I found that soldering a 0.1uF capacitor directly across pins 1 and 2 (-5V output) makes it work correctly, but I did not see any explanation in the datasheet as to why this is. It says "Place C_OUT on the top layer (same layer as the LM2776) and as close to the VOUT and GND pins as possible." which is obviously good design practice (lower noise, etc.), but as you can see in my case, the part is able to work fine with the layout I shared as long as the supply voltage does not rise too fast. Why is the LM2776 so picky?

  • 0 in reply to Jacob Quant
    TI__Intellectual 1120 points

    Hi Jacob,

    The layout is important  because of  large switching currents and high switching frequency. A proper layout is  more than a good design practice. It is something you must have if you want to have stable operation of the LM27762.

    Regards,

    Juha