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SN74HCS164-Q1: Output current

Youhei Miyaoka
Expert 4025 points
Part Number: SN74HCS164-Q1
Other Parts Discussed in Thread: SN74HCS164, SN74AC164-Q1, CD74AC164, TLC6C598-Q1,

Tool/software:

Hi team,

My customer need to check circuit feasibility. They will use SN74HCS164 as LED indicator and its schematic is below.

LED is 15mA/ch and total maximum current is 75mA. Can this device handle this current?

Could you also teach me how to read maximum current in datasheet?

Regards,

Youhei MIYAOKA

  • +1
    Guru 317260 points

    The test condition for VOH is 6 mA; for larger currents, the output voltage is not guaranteed. The absolute maximum rating for the total output current is 70 mA; 75 mA can destroy the chip.

    The CD74AC164/SN74AC164-Q1 allows higher currents.

    In general, for higher loads, you invert the signal and use a low-side driver like the TLC6C598-Q1.

  • 0 in reply to Clemens Ladisch
    TI__Expert 4025 points

    Thank you Clemens,

    They try to set total 70mA with SN74HCS164-Q1. One thing I want to confirm - 15mA/ch is possible? If you have maximum current per channel please let me know.

    Regards,

    Youhei MIYAOKA

  • +1 in reply to Youhei Miyaoka
    Guru 317260 points

    15 mA on one channel will not damage the device, but there is no guarantee what VOH might be in this case. You might need to use a smaller resistor to get 15 mA.

    5× 15 mA is more than 70 mA and can damage the device.

  • 0 in reply to Clemens Ladisch
    TI__Expert 4025 points

    Hi Clemens,

    Thank you. What is the maximum voltage the customer shoud keep lower at VOH?

    Regards,

    Youhei MIYAOKA

  • +1 in reply to Youhei Miyaoka
    TI__Genius 9851 points

    Hi,

    VOH is the output voltage of the device. As the device drives more current, the VOH will drop. This would affect the current through the LED, since I = (VOH - Vf)/R, where Vf is the forward voltage of your LED. Effectively, this means you will need a smaller resistor than expected to get 15 mA on the output, but TI does not guarantee VOH at that high of a current draw, so the exact resistor size would need to be determined experimentally.