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LMZ31520: Unity gain

Part Number: LMZ31520
Other Parts Discussed in Thread: TPS53355

Can you tell me what the unity gain is on these two parts. Marvell uses a TPS53355 part with a feed back loop to keep the voltage level stable at the switch chip. Their application note says any regulators can be used that have a unity gain bandwidth in the range of 15-300Khz. Do these have that as I'm having issues with the TPS53355 part and i have used these two other one with no issues.

  • Brad,
    Are you asking about the closed loop bandwidth of the converter or the open loop bandwidth of the error amplifier? What type of issues are you having with TPS53355? TPS53355 does not use an error amplifier, rather it has a hysteretic comparator internally at the FB pin.
  • power supply info.docxi'm trying to send a copy of part of my schematic to better describe my issue. I do a screen shot of it to word and copy it here. please let me know if you get this attachment as it seems to mot work.

  • okay this seems to have attached. So i have 1vp-p ripple on the input of this regulator. It is not causing any issues but I don't like it and have not been able to squelch it. I also have popped a few of these TPS53335 regulator trying to figure out what's going on with them. They seem very sensitive. So i was looking for another solution like the simple switchers that I have used and have had no issues with. The 3,5,8 amp ones have a voltage divider adjustment but the 20 and 30 amp ones I can't tell what's inside the v-adjust circuit. Can I use these regulators in the feedback type circuit Marvell needs for their voltage adjust feedback loop?
  • Can you post some waveforms of the input voltage ripple? Also can you tell me the characteristics of the AVS_FDDFB signal?
  • Do you have any additional information on your AVS condition? What range of output voltages do you need to support? What is the range of the AVS_VDDFB voltage level? Does it change rapidly or is it a fixed level that is set individually for each Marvell device?
  • I need to support .98V and the feedback voltage is set as required by the dynamic use of the switch chip.

    if more ports come on or as Ethernet traffic picks up there could be a bit more of a voltage drop so the chip adjusts the voltage regulator to correct the voltage it sees back to .98V.

  • If you just need a steady 0.98 V with no voltage adjustment, then you should not need that connection at all. The TPS53355 will regulate across a wide range of load current just with its own FB pin. I don't see how applying a voltage at the AVS node as shown can work if the series resistor is 0 ohm. Of course I am not familiar with that Marvell device. I would need more details to comment further, but if you just need a constant 0.98, I would just disconnect AVS and set R1024+R1017+R1018 = 6.333k.
  • Did you ever resolve your issues? Do you need further assistance?