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TXS0108E-Q1: 1V8 CMOS compatibilty

Ivan Goncharov
Prodigy 120 points
Part Number: TXS0108E-Q1

Hi TI experts!

I'm using TXS0108E-Q1 to provide level shifting between 1V8 modem and 3V8 MCU. But when I was checking levels I saw values that confused me a lot.

Here is comparison of 1V8 CMOS levels and TXS0108 input thresholds:

As I see 

  • The worst case output high voltage (VOH) of the driver is lower than the minimum input high voltage (VIH) of the TXS10180.
  • The worst case output low voltage (VOL) of the driver is higher than the maximum input low voltage (VIL) of the TXS10180.

Does it mean that TXS0108E-Q1 is incompatible with 1V8 CMOS logic? Or am I missing something?

  • 0
    TI__Guru* 97046 points

    Hi Ivan,

    I'll ask my translation expert to get back to you tomorrow when he returns to the office.

  • 0
    Guru 317370 points

    The VOH of your 1.8 V CMOS device is specified for a certain output current, but the TXS does not actually sink any current. So the actual output voltage of the modem will be near 1.8 V.

    Ignoring the edge accelerators, the TXS works like an analog switch. When the input voltage is less than VIH, then the switch is closed, and the A/B pins are directly connected, which means that the output voltage follows the input voltage (with a small voltage drop due to the on-resistance). When the input voltage is at 1.8 V, the switch is open, and the output voltage is determined by the pull-up resistor. This means that the TXS's VIL specification is not a requirement for switching; it is there only as a requirement for the VOL specification. In other words, if the input voltage is higher than VIL, then the output voltage can be higher than VOL (by the same amount), but the device still works. And again, the current through the two 40 kΩ pull-up resistors will be smaller than the test current for the modem's VOL specification, so the actual VOL will be lower.