I'm reading the LM3466 datasheet (page 9) and am confused about the symbol downstream of the SEN/SRC pins and R(sen). Is this a ground symbol? If so, why do they use multiple different ground symbols in this schematic?

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Cheers, Fin

  • \$\begingroup\$ Ok. That's weird. Waiting to see the explaination for it. \$\endgroup\$ – JRE Jan 26 '18 at 19:31
  • \$\begingroup\$ I think this may have something to do with it "To minimize the effect of noise, the ground connections of the LM3466 and the sense resistor RSEN should be closed" (p13 on the datasheet). Does this mean that the ground paths should be isolated? \$\endgroup\$ – bigFin Jan 26 '18 at 19:33
  • 2
    \$\begingroup\$ Possibly related... electronics.stackexchange.com/questions/67894/… \$\endgroup\$ – Brian Drummond Jan 26 '18 at 19:39

The datasheet says that \$R_{SEN}\$ connects to ground. It does not clearly explain the two symbols. The intent of using different symbols is to control the physical realization of the circuit. There is no universal standard. In this case, a different symbol for \$R_{SEN}\$ implies that it is to be connected to ground separately from the other ones.

The datasheet says that all LED current flows through \$R_{SEN}\$. Since this current may be significantly large and noisy, it can induce voltage drop in the ground wire/plane through which it returns to the source. Such voltage drop in the ground plane can interfere with the operation of more sensitive analog circuitry. Suppose you have a node referenced to ground, but a large current pulse through the ground plane induces a voltage drop such that the sensitive 'ground' point is now at some non-zero voltage. Now your sensitive node isn't referenced to ground, but to ground+noise.

In a complex system of these devices it would be critical to ensure that every \$R_{SEN}\$ is referenced to the same ground potential, or they will behave differently because of the ground voltage drop problem.

(As you noted in your comment, page 13 of the datasheet does mention that the \$R_{SEN}\$ ground connections should be "closed" for this reason--though it is not clear what 'closed' means. Maybe they meant 'close.')

  • \$\begingroup\$ Thanks for the thorough answer. I'd upvote you if I had the rep to do so. \$\endgroup\$ – bigFin Jan 26 '18 at 19:36
  • \$\begingroup\$ You should have enough to upvote now @bigFin ;) \$\endgroup\$ – KingDuken Jan 26 '18 at 19:40
  • \$\begingroup\$ @KingDuken Yeehaw \$\endgroup\$ – bigFin Jan 30 '18 at 20:11

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