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LM2776: Pulse frequency modulation of LM2776

Part Number: LM2776
Other Parts Discussed in Thread: TPS60403

Hello

I have an appplication, that is sensitive to switching frequencies at about 10 bis 20 kHz. I uses the LM2667, which as a fundamentel switching frequency of 2MHz. In my application, the load of the LM2667 is very light, so that the pulse frequency modulation is working. Consequently, the LM2667 switched only a few pulses with 2MHz, afterwards it waits for about 70us (13kHz). This 13kHz frequency disturbs the signal of my ciruit. I can set the LM2667 to a continuous switching mode, if I reduce the flying capacitor to just 10 or 20nF. Althogh, in the datasheet of the LM2667 is mentioned that the flying capacitor C1 should not be less then 200nF. What is the reason for this minimum value of the flying capacitor?

Thanks for your help.

Best regards

Basil

  • Hi Basil,

    Let me check internally and I will come back to you early next week.

    Regards,
    Davor

  • Hi Basil,

    I have some additional questions about your application:

    What is the VIN, VOUT, load current profile what are is the total CIN and COUT of your implementation of the LM2776? Those parameters will be essential in order to give you a proper answer on your query.
    Also if you could provide the temperature range of your application that would be useful as well.

    Are you worried about audible noise between 10kHz and 20kHz and that is why you don't want to be operating with that switching frequency or is it something different?

    Regards,
    Davor

  • Hi Davor

    Vin is +5V. Vout is -5V. The oparating temperature of my application will be room temperature (25°C).

    At the input, I have a LC-Pi-Filter with two 10uF capacitors and the 47uH inductor. And also a 100nF capacitor diretly at the input pin.

    At the output of the LM2776, there is a output capacitor of 2.2uF followed by a LC-Filter with 47uH and 2.2uF. Furthermore, at this -5V supply there are seven 100nF-decoupling capacitors. The output current is nominal approximately 2mA.

    I use this -5V supply for operational amplifiers in a low-noise analog signal processing chain (BW ca. 25kHz). I have built a prototype of my circuit and there, I see the switching frequency of the LM2776 in the analog output signal.

    I hope this information helps to unterstand my application.

    Thanks for your further answers.

    Best regards

    Basil

  • Hi Basil,

    Thanks for the detailed explanation.
    I'll come back to you early next week with an answer.

    Regards,
    Davor

  • Hi Basil,

    It seems there is no particular reason why the device should experience an issue when working with a smaller flying capacitance than 0.2uF.

    However, since your application needs to operate in PWM constantly, I would not recommend using the LM2776. Main reason is that the precise point when the converter will transition from PWM to PFM is not easy to predict and it depends on the DC-bias, Temp. characteristics of the flying capacitor and also from the operating current of the switches which is drawn from the output.

    Basically there might be some corner cases (very rare situations) where the LM2776 might operate in PFM, even though for your tests it seems to work PWM constantly.

    One alternative could be using our TPS60403, which is a charge pump inverter with a fixed frequency operation at 250kHz. It is not however p2p to the LM2776 unfortunately.

    Regards,
    Davor

  • Hi Davor

    Thanks for the detailed explanation and the suggested alternative. I think, we will firstly work with a smaller flying capacitor, because we already have some assemblied prototype PCBs. Although, I think we will switch to the TPS60403 in the future.

    Regards,

    Basil