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AM3354: TL5209 function on Beaglebone Black board

FRZE
Intellectual 1890 points
Part Number: AM3354
Other Parts Discussed in Thread: TL5209

Looking at the schematics of the Beagle Bone Black board there is the TPS65217C and a TL5209 to create the required voltages. 

Why do we have this additional LDO (TL5209) to supply the flash on the BBB together with the TPS65217C. Could the TPS65217C not supply the flash as well? Why do we have this separation between VDD_3V3A and VDD_3V3B? Drive strength of the TPS65217C or something to do with an enable function?

Best regards,

Frank

  • 0
    TI__Guru**** 393215 points
    This is additional 500mA 3.3V power supply. TPS65217C LDO4 can provide max. 400mA.
  • 0 in reply to Biser Gatchev-XID
    Prodigy 40 points
    The point is that the typical consumption current of the used flash is about 80mA. From point of the maximum current both solution should be possible. But the more expansive one is chosen. Why? Maybe the timing of the VCC(flash)-starting makes it necessary? Best regards Andrej
  • 0 in reply to Biser Gatchev-XID
    Prodigy 40 points
    From point of the maximum current both solutions should be possible. The typical consumption flash current is about 80mA. My assumption is that maybe the timing of VCC-flash starting could be the explanation for the more expansive solution with the addition LDO. Best regards Andrej
  • 0 in reply to Biser Gatchev-XID
    Intellectual 510 points
    My concern regarding BBB design is more about the different power domains created by the TL5209.
    As often mentioned the I/O pins of the AM335x are not fail safe. VDDHV rails of the AM335x are supplied with VDD_3V3A on the BBB board, so no I/O pin may rise above the voltage of VDD_3V3A rail.
    However some peripherals (Ethernet PHY, uSD, ...) are supplied by VDD_3V3B which is created by the TL5209.
    The TL5209 is enabled by VDD_3V3A and has a maximum ON threshold of 2V at its enable pin. So the VDD_3V3B is activated when VDD_3V3A reaches only 2V, which could damage the I/O pins if the TL5209 starts up too fast (and it does within 50µs according to the data sheet).
    On the other hand the Ethernet PHY and uSD are also not fail safe so in my opinion it must be guaranteed that VDD_3V3A and VDD_3V3B ramp up with no voltage difference between them.
    I can not imagine that the TL5209 supply fulfills identical ramp up at its enable pin and output. Did someone measure this? Or is there another mechanism in the BBB circuit which satisfies the fail safe requirements of the AM335x I/O pins?

    Best Regards
    tom
  • 0 in reply to Thomas Langhammer
    TI__Guru**** 393215 points
    BBB is not a TI design. It's designed and supported by www.beagleboard.org. Please ask there.
  • 0 in reply to Biser Gatchev-XID
    TI__Intellectual 1660 points
    Gerald will give you a more definitive answer on BeagleBoard.org/discuss. Note that new posters are moderated, so it may take a few hours to show up.

    Gerald's pretty conservative, but from my recollection there were a small number of other items on VDD_3V3B than just the flash and some current is provided to expansion boards.

    We should be able to answer about the power sequencing over there.