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Original question:

SN74LVC3G34: SN74LVC3G34DCUR driving capacity

SN74LVC3G34: Maximum current rating for combined three buffer driver

deepak naiknavare
Prodigy 140 points
Part Number: SN74LVC3G34
Other Parts Discussed in Thread: SN74LVC2G17, UCC27517, SN55451B, SN75451B, UCC27517A-Q1

Hello,

I am using the buffer to drive optical transmitter which can require a current upto 96mA in worst case. As per the datasheet the current could be 32mA (i assume it means for each channel), so combining 3 output pins can give me 96mA. Is my understanding correct?

But the absolute maximum current is 100mA for ground pin. I could not find the details on maximum group current in ground pin over all temperature range.

Is it ok to go with the IC or you recommend other. Attached is the snapshot of schematic

  • 0
    Guru 267960 points

    What is the purpose of R3? I guess an open-drain driver is not possible?

    Normal LVC inputs have a limit on the rise/fall time. With that R20/C32 filter, you need to use Schmitt-trigger inputs.

    100 mA is allowed over all temperatures, but you are near the limit, and you have to consider the output impedance of the buffer. It would be a better idea to use at least four buffers and multiple devices, e.g., 2× SN74LVC2G17 (with Schmitt-trigger inputs).

    Consider using device designed to drive high loads, e.g., UCC27517 with 300 mA.

  • 0 in reply to Clemens Ladisch
    Prodigy 140 points

    Hello

    Thanks for the feedback. I agree with your comments. Would it be a good idea to switch to SN55451B. Only issue i see is the high operating current upto 30mA

    Will it need an additional Schmitt trigger at input as well, as i don't want to use an additional Schmitt trigger

    The operating frequency is 8MHz

    Thanks,

    Deepak

  • 0 in reply to deepak naiknavare
    Guru 267960 points

    Why do you want to use the SN55451B instead of the UCC27517?

    Neither the SN55451B nor the UCC27517 have a limit on the rise/fall time at their inputs, so they do not need Schmitt triggers.

  • 0 in reply to Clemens Ladisch
    Prodigy 140 points

    The Switching frequency of the driver should be upto 10MHz, As per the datasheet of UCC27517, Figure 15, switching frequency data is mentioned upto 700kz. Is this correct? 

  • 0 in reply to deepak naiknavare
    Guru 267960 points

    Figure 15 shows the supply current when driving a gate capacitance of 1.8 nF. You would not want to drive such a MOSFET with a higher frequency.

    Your optical transmitter has a much lower capacitance. The propagation delay is at most 30 ns with such a high load, so it will be lower in your case. In any case, 1 / 10 MHz = 100 ns, so this is more than enough.

  • 0
    Prodigy 140 points

    Thanks for support, it resolves my issue. Just last comment: The SN75451B mentions its application as LED driver, the UCC27517A-Q1 mentions other application, do you think it makes any impact.

  • +1 in reply to deepak naiknavare
    Guru 267960 points

    The gate driver is optimized for driving highly capacitive loads, i.e., for peak currents up to 4 A. It still works fine for DC currents up to 300 mA, and is usually smaller and cheaper than other strong drivers.

    Please note that the SN7545x devices combine logic gates with drivers. If you need nothing but a strong open-drain output, you could just as well use a discrete MOSFET.