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LM3481EVAL: R30 and R36 in Ref Design: TIDA-01352

Mark Liang
Prodigy 40 points
Part Number: LM3481EVAL
Other Parts Discussed in Thread: TIDA-01592, TIDA-01352, LM5030

Tool/software:

Hello,

On Page 11 of the Reference Design Guide, TIDA-0159, a red cross is on R30 and R36, does the red cross mean no fit or 0 ohm, as on Figure 15 on Page 21, components were on those locations? 
  • 0
    TI__Mastermind 30160 points

    Hi Mark,

    I looked through the TIDA-0159 design guide but did not find the exact schematic you are referencing to.
    Maybe you could send me a link to the document or make a screenshot?

    In general, if a component in the schematic is marked with a red cross, it normally means this component is not populated.
    So there is a footprint present in the layout, but there is no component soldered on this place.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 40 points

    Hi Niklas,

    Thank you for your reply!  It should be TIDA-01592, sorry about that.TI_25W_High-Voltage_Power_Supply_tiduds2.pdf

    Best regards,

    Mark

  • 0 in reply to Mark Liang
    TI__Mastermind 30160 points

    Hi Mark,

    Thanks for the attachment.
    I agree with you that the board picture and the schematic do not align here.
    If there is a red cross in the schematic, it means the component is no fit. I assume they tested on the physical board both with and without the resistors fitted, and took the picture while the resistor is placed.

    The function of these resistors is to snubber the secondary side switch node. If strong voltage overshoots are visible on this node, the snubber will reduce the overshoots, but the component is not mandatory for the application to run.

    If you are interested in building a similar design, I would recommend to place the footprint and start with not fitting the component. If bench measurements show that there are strong overshoots, you can place the snubber components later on.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 40 points

    Hi Niklas,

    Thank you for your reply.

    So C22 and C31 are not used?  Could you please confirm that?

    Reading another similar document: TIDA-01352: on page 11, two kinds of red cross were used, a small one and a big one (please see attached document).  It looks that the big cross was no fit, but the small one was 0 ohm connection.  Any thought?

    Best regards,
    MarkTI_400W_Programmable_Power_Supply_Ref_Design_tiducd8.pdf

  • 0 in reply to Mark Liang
    TI__Mastermind 22720 points

    Hi Mark,

    The engineer is out of office. Please expect a reply till Monday.

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    TI__Mastermind 30160 points

    Hi Mark,

    With R30 and R36 not fitted, the according capacitors C22 and C31 lose their function and therefore are not used, you are correct.

    In the other document you have the same meaning of the red cross. Both large and small cross mean that the component is not fitted.
    The difference in the size of the cross seems to be just a visual decision.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 40 points

    Hi Niklas,

    If R64 is not fitted (Figure 8) in TIDA-01352, then LM5030 will not work, right?

    Best,
    Mark

  • 0 in reply to Mark Liang
    TI__Mastermind 30160 points

    Hi Mark,

    I am not an expert for the LM5030 device, but based on the pin description in the datasheet, the device should still work if the VCC pin is supplied.

    For more details on the LM5030 device, I would recommend to set up a new thread here in e2e.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 40 points

    Hi Niklas,

    I don't think that LM5030 will work, since the internal regulator has no input.

    Best,

    Mark

  • 0 in reply to Mark Liang
    TI__Mastermind 30160 points

    Hi Mark,

    Instead of connecting the input of the regulator, you can directly connected the output, which is the VCC pin.
    The advantage is that you do not have the losses of the regulator and you can set a higher driver voltage level, but you need to be aware that the VCC pin has much tighter abs max limits than VIN and the voltage should be very stable as well.

    Best regards,
    Niklas