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SN74LVC1T45: 50ohm level shifter

Part Number: SN74LVC1T45
Other Parts Discussed in Thread: LMK00804B-Q1, SN74LVC1G34

Hello,

My goal is to to translate 2.5V logic level (from MAX10 FPGA) to 1.8V logic, driving a 50ohm load.

It means that I need a device capable of driving 1.8V/50ohm=36mA.

I didn't find any device capable to drive such current, so my question is:

1. Can you recommend about a device like this?

2. do you recommend using few devices (or one dual/quad..) by connected inputs and outputs in parallel, to increase drive capability? (As mentioned in https://e2e.ti.com/support/logic/f/151/t/437301?Suggested-Logic-Buffer-for-driving-TTL-50-ohm-load)

3. Is it possible to use a Clock buffer (like LMK00804B-Q1), which seems to be capable of driving 50ohm loads?

Thanks!

Max

  • Hey Max,

    Are you driving a 50 ohm resistor, or a 50 ohm transmission line?

    I assume the latter, since that's the most common reason to need 50 ohm termination. I would recommend sending 3.6V across the transmission line with an SN74LVC1G34 (or similar LVCxG device) and terminating the distant end at 50 ohms. The resulting waveform will be 1.8V at the distant end and any reflections (ringing) will be eliminated by the good impedance match.

    If it's the former, and you just need to drive static current of 36mA, then you can use a single SN74LVC1G34. The current limit for the device is 50mA. If the VOH/VOL specs aren't good enough for your application, you can parallel channels to double/triple drive strength (ie reduce output impedance by 1/2 or 1/3). I should note that the LVC family of logic has extremely strong output drivers -- one is more than enough for most applications.

  • Hey Max,

    For your third question, I would recommend posting a new thread with the part number LMK00804B-Q1. I don't support those parts, but if you make a new post it will be directed to the applications engineer who does.

  • Hi Maier,

    Thanks for the quick reply.

    I'm driving a 50 ohm resistor (conno't be taken off), but both cases that you described seems to be equivalent.

    About the "latter" case, I'm not sure I understood correctly.

    If the output impedance of the SN74LVC1G34 is 50ohm, and I connect a 50ohm load to it, then the 3.6V will be divided betweem the two 50ohm-s, each one will see 1.8V, am I wrong?

    But isn't the output impedance of the device about 20ohm or so? shouldn't I connect 30ohm resistor in series between the device and the 50ohm load, to ensure the equal division?

    Anyway, if I understand corrrectly, connection of mutiple devices in parallel is a safer solution..?!

    thanks!

    Max

  • Hey Max,

    You will always get some voltage drop at the output of a device - there's no such thing as driving 1.8V from a 1.8V devices and getting exactly 1.8V output - unless your drive current is extremely low (under ~20uA). If you need 1.8V output (not less), then you must have a supply higher than 1.8V.

    The output impedance of the SN74LVC1G34 is not typically 50 ohms -- typically it is closer to 8 ohms (at 5V). At 1.8V that's increased, but the driver is much stronger than what the datasheet says. For example, at 4.5V, the datasheet says the output is (4.5 - 3.8)/0.032 ~= 22 ohms -- that's a maximum under worst case (temperature, process variations) + a little headroom for liability.

    We can determine the exact worst-case output impedance from the datasheet specs for 1.8V. At 1.65V supply, the device is rated to output 1.2V (max) at 4mA current, From that we can calculate the on-state resistance of the output pFET:

    V(max) = (1.65 - 1.2) = 0.45V ; I = 0.4mA

    R = V / I = 112.5ohms

    So, the datasheet tells us the 1.8V output impedance is 112.5ohms max, but typically the real impedance will be less than half -- using the ratio from above, 8/22.5*112.5 =  40 ohms - which is surprisingly close to 50 ohms. Paralleling two of these would give you ~20 ohms, and 3 would give ~13 ohms.

    Another option would be to use power MOSFETs that have incredibly low r_on values to drive your circuit -- but that seems wasteful for only 36mA.

    What's the purpose of driving the 50 ohm load?  If I know more I might be able to help.