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SN74AHCT244-Q1: Buffer with higher output current

Part Number: SN74AHCT244-Q1
Other Parts Discussed in Thread: UCC27517A-Q1, SN74ACT244-Q1, UCC21750-Q1

Hi Team,

I am Andy Robles working as application engineer for High Power Drivers. We have a customer using the SN74AHCT244-Q1 as a buffer to convert 3V MCU PWM signal into 5V PWM signal to drive our gate drivers.

We ran into an issue with the gate driver than can be fixed with increasing the capacitance at the gate driver PWM input signal, but in order to keep similar RC constant we need to reduce the resistance for which the buffer is starting to become a limiting factor. In the SN74AHCT244-Q1 datasheet abs max section it is mentioned that the device is rated for max +/- 25mA continuous output current.

  • Since the output of the buffer is a PWM driven RC the peak current would not be DC, but rather be a pulse peak current of up to 250mA at up to a 50kHz frequency. Would this buffer be okay with such pulse current?
  • Is there any other P2P device with higher current ratings that we could position here instead of the SN74AHCT244-Q1?

Best regards,

Andy Robles

  • The absolute maximum rating must not be exceeded, not even for a short time.

    Isn't the current limited by the R of the RC?

    The SN74ACT244-Q1 has stronger outputs. If you need even more current, a gate driver with TTL-compatible inputs like the UCC27517A-Q1 can be used for level shifting. (Or just use it as the actual gate driver …)

  • Hi Clemens,

    The current is limiting the RC, but higher resistance is causing the issue on the gate driver side to appear more easily. The UCC27517A-Q1 is not an option here as the gate driver being used is an isolated gate driver (UCC21750-Q1) with protection features other gate drivers don't have.

    To make sure I have it correct:

    • Maximum output current
      • SN74AHCT244-Q1 = +/-25mA
      • SN74ACT244-Q1 = +/- 50mA
    • Although the above are spec'd as maximum continuous output current it should also be followed as the maximum pulse current. Is this correct?

    Is there a good way to find the equivalent output resistance of the buffer?

    Best regards,

    Andy Robles

  • Hey Andy,

    Yes - the maximum continuous output current has to be used also as the maximum pulse current because we do not specify a pulse current in the datasheet - ie you have to go by the safe limit.

    The output resistance of the device is fairly easy to calculate (at least the worst case) from the datasheet provided specs.

    The VOL is measured at a particular test current, IOL, and the ratio of those is the resistance:

    ROL = VOL / IOL = 18.33 ohms (maximum).

    Typically we would expect the resistance to be about half of the maximum, so I would use 9 ohms for general / typical calculations.

  • Hi Emry,

    Could we find out if the output current is limited by the internal transistors impedance or if it's a thermal limitation?

    If it's a thermal limitation then the buffer should be able to allow a much higher pulse current to have the equivalent thermal impact as the continuous current.

    Is the max pulse current something we could find out either through testing or looking at the design, or is there any other newer buffer that specs this(different package is okay)?

    • Requirements:
      • 3-state outputs
      • Will operate with signals up to 50kHz
      • 4-channel acceptable, but prefer 8-channel
      • If we can get close to ~200mA great
        • If it's lower then just the higher the better to allow more margin in the design

    Is there anyone from your team we could loop into our customer email thread discussion? This would allow us to get the feedback to the customer faster.

    Best regards,

    Andy Robles

  • Hi Andy,

    I'm afraid these logic gates aren't designed to drive a large amount of current, and we don't specify them for that.

    You can exceed the max specs in the datasheet at your own risk. To be clear - if there's a failure, TI holds no liability if you're exceeding the max rating in the datasheet.