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LM48511: Switiching frequency not at 1MHz in Continuous Mode

Franz Schellhase
Expert 3605 points
Part Number: LM48511

Hi team,

Can you comment on my customers inquiry?

I can't get the integrated boost regulator of LM48511 to regulate at the fixed switching frequency of 1MHz (according to the data sheet). In continuous mode I see a maximum of 740kHz. Is this considered normal scattering or did I build-in an error?

In discontinuous mode the frequency is between 135kHz and 740kHz depending on the load.

My requirements:
Vin = 4.1V
Cin =100uF
Feedback voltage dividers are 24k and 4k7 Ohm (-> Vout approx. 7.5V)
Cout = 100uF
Compensation network = 270pF + 2k4 Ohm (or 270pF + 3k6 Ohm in another attempt)
L = 4u7 H

My real problem is that in discontinuous mode (especially when idle) I still see too much EMI on the power supply line from the device (conducted EMI with 135kHz, 270kHz and 408kHz). I have to stay below 150kHz and the first harmonics are still higher. I have traced these back to the switching regulator. Therefore I would like to check the correct function of the switching regulator part first.

Thank you,

Franz

  • 0
    TI__Guru* 84915 points

    Hi, Franz,

    Our team will take a look at this thread and will answer as soon as possible.

    Best regards,
    Luis Fernando Rodríguez S.

  • 0
    TI__Guru* 99135 points

    Hi Franz,

    Have you tried to disconnect the boost from the amplifier? I mean disconnect V1 and PV1 from the boost? Not sure if this is possible on your board but this would help to identify if the amplifier load is causing the mismatch.

    I'll check this with the EVM. Can you share your schematic and layout so we can take a look?

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators

  • 0 in reply to Ivan Salazar
    Prodigy 30 points

    Hi Ivan,

    I'm the customer how asked the question. I'm not sure if I understand you the right way. In my design and also in the the eval board V1 and PV1 are hard wired to the net with the feedback network and boost diode. Do you mean to cut the net at the pcb?

    I also checked the eval board these days it's also not at 1MHz but a little bit more than at my design (around 825kHz).

    Best regards

    Michael 

  • 0 in reply to Ivan Salazar
    Prodigy 30 points

    I can only share the related part of the schematic.

  • 0 in reply to Michael Eichhorn
    TI__Guru* 99135 points

    Hi Michael,

    Unfortunately there is not much information we can provide for this device. This is a legacy device and the available information is basically what we have online.

    Since this is related to the boost and the frequency variation depends on the load, what I could suggest is to experiment with the boost inductor and capacitor (L1 and C2 from the data sheet schematics).
    It is most likely that at lower impedance loads, the current demand is higher and thus causing drift on the boost. You can try changing inductor value or saturation current and/or boost capacitor voltage rating.

    Additionally, I wouldn't short FB_GND pins together, not sure if this could cause variations on device performance.
    Your output filter has relatively low DC impedance so this can also affect the boost performance.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators