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SN74LVT245B: Output Impedance ?

Part Number: SN74LVT245B
Other Parts Discussed in Thread: SN74LVC245A, SN74LVCR2245A

Dear Sirs,

I want to use the SN74LVT245B level shifter in order to protect my processor from backpowering on a RGMII interface. The processor, which has its own power supplies, is connected to a Ethernet PHY (which also has its own power supplies) through RGMII interface.

We saw that the SN74LVT245B can handle the RGMII interface speed (up to 250Mbps/line @3V3). Could you confirm ?

Then, we would like to know the output impedance of the SN74LVT245B in order to select the serie termination resistor of the line (R?) ?

Best Regards,

Stoffels K. 

  • Hi Kevin,
    The maximum speed rating of a logic device is a tricky subject - primarily because these devices aren't typically characterized for bit rate. The SN74LVT245B has _very_ strong output drivers (seen in the datasheet by the IOH/IOL ratings of 32/64mA respectively), so I would expect this transceiver to be able to operate well above 125 MHz (250 Mbps). I have more experience with the SN74LVC245A, which has weaker output drive (24mA for IOH and IOL), but can work at 100+ MHz as well. So, long story short, I think with low load conditions (~15pF) the SN74LVT245B or the SN74LVC245A would work for your application.

    The output impedance of the device can be approximated from the electrical specs in the datasheet. The VOH/VOL specs contain a current rating and can be used to get output impedance. Here's an example:

    At 3V (the minimum for a 3.3V supply), VOH = 2V with 32mA output, and VOL = 0.5V with 32mA output, 0.55V at 64mA output.
    Ohm's law gives:
    RH = (3-2)/.032 = 31.25 ohms
    RL1 = 0.5/.032 = 15.625 ohms
    RL2 = 0.55/.064 = 8.594 ohms

    Note that the output current affects the output impedance, so this isn't going to give you a simple answer. Also, typically the output will be lower impedance than what you calculate here, since the datasheet values are maximum ratings, and the device rarely operates near those ratings. Personally I would assume ~25 ohms.

    Also, you could try using a pin to pin device with built-in damping resistors like SN74LVCR2245A.