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Question of LMH7322

Alan Liu89
Prodigy 60 points
Other Parts Discussed in Thread: LMH7322

Hi


I have two questions:

1) Is the DAP a pad under the IC that need to connect through the pcb copper pad to the most negative voltage? Can I not have a pad for the DAP and let it be open?Can I just connect Vee pins to the -ve voltage?

2) My input is only signal that is +/-1V maximum, can I connect Vee=-3.3V, Vcci=+3.3V, Vcco=+2.5V for LVDS output? With these low voltage, do I still need the DAP solder to pcb for heat sink?

Thanks

Alan

  • 0
    Prodigy 60 points
    I forgot to ask:

    I know I need a resistor to connect for RHYSA to RHREFA, do I need a 0.01uF bypass cap on those pins?

    Thanks
  • 0 in reply to Alan Liu89
    TI__Mastermind 22825 points

    Hi Alan,

    To answer your questions about the LMH7322:

    1) The DAP connection to the PCB is for heat sinking purposes. You can eliminate the connection at the expense of higher junction temperature.

    2) With your conditions, each LMH7322 channel will dissipate about 150mW (300mW total). The datasheet Thermal resistance value of 38 deg . C/ W is based on the condition where the device DAP is tied to the PCB for heatsinking. I am not sure how to estimate the junction temperature if you do not apply the DAP heatsink!

    P_dissipation_each_channel= (3.3V - (-3.3V))*6.3mA + (2.5V - (-3.3V))*15.8mA + P_load

    P_load (power dissipated in device to drive the output termination resistors) = ~ 18mW

    P_dissipation_each_channel= 42mW + 92mW + 18mW = 151mW

    3) You do not need capacitors on the RHYSA and RHREFA pins.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 60 points

    Thanks for the reply


    I have another question. This comparator does not have power down feature. If I use only the A side, can I not connect VcciB and VccoB pins to safe power? Or I have no choice but to power them all up?

    Thanks

    Alan

  • 0 in reply to Alan Liu89
    TI__Mastermind 22825 points
    Hi Alan,
    If you don't need the 2nd channel, just leave its power pins disconnected as you have already pointed out and that would eliminate any unwanted power dissipation.

    Regards,
    Hooman
  • 0 in reply to Hooman Hashemi
    Prodigy 50 points
    One more question towards using only a single comparator instead of two:
    I simply leave all B pins unconnected? (or should I connect all input / power pins to ground and have the output pins floating?)
    I'm using +5/-5 V for comparator A.
  • 0 in reply to Michael SI
    TI__Mastermind 22825 points

    Hi Michael,

    Functionally, you can just leave ALL unused "B" side pins unconnected.

    If you are concerned with any ESD, the solution you noted ("connect all input / power pins to ground and have the output pins floating") also works. You could also ground the unused output pins. The "B" section is completely separate and would be unpowered.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 50 points

    Hi Hooman,

    from the datasheet VEE-A is connected to PAD and then to VEE-B via internal bond wires.

    So in the case where I have VEE-A connected to -5V probably grounding everything of channel B would be a very bad idea.

    I think the solution will be that I leave all the pins unconnected.

    Have a nice day

    Michael

  • 0 in reply to Michael SI
    TI__Mastermind 22825 points
    Hi Michael,
    You are correct.I had neglected to notice that the VEE pins (both A and B side) are tied to the DAP and if you ground VEEB, then you are essentially shorting VEEA to ground this way.

    If you still like to tie the unused pins to avoid floating them, I'd imagine that tying them to the VEEA node (instead of ground) would be an option.

    Regards,
    Hooman