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LM27762: Regulator not working properly (both positive and negative)

Vincenzo Costanza
Prodigy 170 points
Part Number: LM27762

Hi,

I'm using the LM27762 to generate +3V and - 3V from 5V (USB generated). I have used as guidelines the schematic and layout provided in the LM27762 EVM user guide and modified the values of resistors and a couple of capacitors (find attached the pic of the schematic and layout)

Schematic:

Components:

Top Layer - Signal:

Layer 2 - GND:

Layer 3 - PWR:

Bottom Layer - Signal:

PCB Picture:

I have also bought the EVM Module to compare the noise performances and also to understand better how the LM27762 works. However, on my PCB, the output voltages are not correct: I measure +2.8V and -2V instead of the +/-3V.

I measured with the scope the signals at Cp and at Cfly and at the outputs. There is definitely something wrong with the switching:

Cp (this signal only is AC coupled):

C1+: 

C1-:

Vout+:

Do you know what's going on?

Thanks for the help

  • 0
    Guru 316420 points

    The datasheet says:

    8.2.2.5 External Capacitor Selection

    The LM27762 requires 5 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive, and have very low ESR (≤ 15 mΩ typical). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended for use with the LM27762 due to their high ESR compared to ceramic capacitors.

    And why is Cfly so large?

  • 0 in reply to Clemens Ladisch
    Prodigy 170 points

    Thanks for the reply! I'll try to change the input and output capacitors with ceramic capacitors. For the Cfly in the datasheet says it's said that for higher current it is recommended a 10uF capacitor instead of the 2.2uF. 

  • 0 in reply to Vincenzo Costanza
    Guru 316420 points

    8.2.2.5.3 actually says:

    In typical high-current applications, 0.47-µF or 1-µF 10-V low-ESR ceramic capacitors are recommended for the flying capacitors.

  • 0 in reply to Clemens Ladisch
    Prodigy 170 points

    I changed the values of the capacitors according to your suggestion. Now I have

    Cin and Cp: 4.7 uF

    Cout: 2.2 uF

    and Cfly: 1uF

    the waveforms look way cleaner, but I fear these values do not work for me:

    Cin:

    Cp:

    Cfly+ : 

    Cfly-:

    and OUT:

  • 0 in reply to Vincenzo Costanza
    TI__Genius 14876 points

    Hello Vincenzo,

    What exactly are the last two scope shots you sent? Are those AC? 

    I would recommend you use wider traces for the output power rails, using long trace lengths and narrow widths will add parasitic inductance and resistance. This add in impedance will result in inferior performances. 

    Why not using same resistor bridge for out+ and out-? If your concern is losses, you can even shift your regulation for a regulation a bit higher than the one you have now.

    Thank you very much!

    Best Regards,

    Dorian

  • 0 in reply to Vincenzo Costanza
    Guru 316420 points

    If I interpret the waveforms correctly, then the regulator stops switching every 200 ms or so. I suspect that this is either the current limit or the thermal protection kicking in.

    What load is there on the output?

    Dorian Brillet de Cande said:

    Why not using same resistor bridge for out+ and out-?

    The positive and negative LDOs have slightly different reference voltages.

  • 0 in reply to Dorian Brillet de Cande
    Prodigy 170 points

    Hi Dorian,

    The last two scope images are the voltage outputs from the LM27762 (measured on the output capacitors). I'll try to directly add a power plane from the output. Do you think that could be problem?

    Yes as Clemens already pointed out the regulators reference are a little bit different (1.2V for the positive and -1.22V for the negative rail)

  • 0 in reply to Clemens Ladisch
    Prodigy 170 points

    Hi Clemens,

    right now I have no load connected to the circuit and I don't feel any heat coming off the regulator. My understanding is that the switching is not working properly and the output is not constant because of that. Plus if you notice, while the positive output is almost at 3V as intended, the negative voltage is way lover than the targeted -3V (it's around -1V)

    Thanks again for the help

  • 0 in reply to Clemens Ladisch
    Prodigy 170 points

    Hi Clemens,

    that was actually the issue. When resoldering there was a short circuit on the chip. Thanks a lot for the advice

    Cheers,

    Vince