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TXS0104E: Input impedance

Roger Lin
Expert 4320 points

Part Number: TXS0104E
Other Parts Discussed in Thread: THVD8010,

Hi Jack,

The customer is wondering what's the input impedance of TXS0104?

They want to make sure that their application won't reach the maximum VIL

U1 = THVD8010 (B side, VCCB = 5V0):

 R pin (receiver output) VOL specification = 0.4V @ IOL = 4mA → Equivalent R_on = 100Ω

Actual sink current = 5V ÷ 10kΩ = 0.5mA

Conversion VOL = 100Ω × 0.5mA = 0.05V (Measured ≈ 0.06V)

 U7 TXS0104E(A side, VCCA = 3V3):

 VOL specification = 0.2 × IOVDD = 0.66V @ IOL = 20mA → Equivalent R_on = 33Ω

Actual sink current = 3.3V ÷ 10kΩ = 0.33mA

Conversion VOL ≈ 0.01V (Measured ≈ 0.05V)

 

Conclusion: In actual Under a 10kΩ pull-up load, the low-state outputs of U1 and U7 are significantly lower than the TXS0104 VIL max (0.15V). Measured values ​​(0.06V / 0.05V) also confirm a margin of ≥ 2.5× on the threshold. The original issue stemmed from a direct comparison between the "VOL max under rated high current" and the circuit's light load (0.3~0.5mA) conditions, and is not a functional problem. A waiver is recommended for closure.

 

Thanks 

Roger

  • 0
    Guru 325520 points

    The TXS actually does not have a VIL; see [FAQ] Why are the TXS01xx VIH/VIL specifications so stringent? If an input signal exceeds the specified VIL value, then the output voltage increases by the same amount.

    The A and B pins are connected directly; the switch resistance is less than 250 Ω. The pull-up resistors on both sides are visible to the driving device.

    The current through both pull-ups is about 3.3 V / 10 kΩ + 5 V / 10 kΩ = 0.83 mA. So the worst-case voltage at the B pin is the VOL of the THVD8010, i.e., it's 100 Ω × 0.83 mA = 83 mV. The worst-case voltage at the A pin is the input voltage plus the voltage drop over the switch (which depends on the current through the pull-up on the output side), i.e., 83 mV + (3.3 V / 10 kΩ) × 250 Ω ≈ 0.17 V.

  • 0
    Prodigy 30 points

    Hi


    Could you please review the analysis below and confirm whether it is correct? If so, we would like to confirm that this VIH/VIL & VOH/VOL issue can be closed.

    JP1 (TXS0104) VIL/VOL — Response

    We acknowledge the datasheet comparison: U7 VOL max (0.66 V) and U1 VOL max (0.4 V) both exceed the TXS0104 VIL max (0.15 V). However, this comparison applies the driver VOL specified at its rated sink current, which does not match the actual operating point of this circuit.

    1. Actual load and sink current

    The TXS0104 is a pass-gate architecture: the A and B pins are connected directly through a switch of less than 250 Ω, so the pull-up resistors on both sides are visible to the driving device. The total sink current is therefore:

    I = 3.3 V / 10 kΩ + 5 V / 10 kΩ = 0.83 mA

    This is far below the 20 mA / 4 mA conditions under which the driver VOL max values are specified.

    2. U1 (THVD8010) driving B-port — receiver low-level output

    • Datasheet: VOL max = 0.4 V @ IOL = 4 mA → equivalent R_on = 0.4 V / 4 mA = 100 Ω
    • Voltage at B pin = 100 Ω × 0.83 mA = 83 mV, versus TXS0104 VIL max of 0.15 V → PASS
    • Resulting voltage at A pin (output side) = 83 mV + (3.3 V / 10 kΩ) × 250 Ω ≈ 0.17 V, versus A-port VOL max of 0.4 V → PASS

    3. U7 (EFR32) driving A-port

    • Datasheet: VOL max = 0.2 × IOVDD = 0.66 V @ IOL = 20 mA → equivalent R_on = 33 Ω
    • Voltage at A pin = 33 Ω × 0.83 mA ≈ 27 mV, versus VIL max of 0.15 V → PASS
    • Resulting voltage at B pin (output side) = 27 mV + (5 V / 10 kΩ) × 250 Ω ≈ 0.15 V, versus B-port VOL max of 0.4 V → PASS

    4. Note on the 0.15 V threshold

    The 0.15 V limit is an input (VIL) requirement and applies only to the pin being driven. The opposite-side pin is an output and is governed by the A/B-port VOL max of 0.4 V. Both conditions are met in each direction.

    5. Measurement correlation

    Measured values (U1 ≈ 0.06 V, U7 ≈ 0.05 V) are consistent with the calculated worst-case bounds above.

    Conclusion: Under the actual loading conditions, all input low levels meet the TXS0104 VIL max (0.15 V) and all output low levels meet the 0.4 V VOL max, in both directions. The original issue arises from comparing the driver VOL max specified at rated sink current (20 mA / 4 mA) against a light-load operating point (0.83 mA). 

  • 0 in reply to Ryan Chen
    Guru 325520 points

    Correct.

    The VIL value of the TXS is not actually a requirement; the datasheet specifies this value only so that it can specify the VOL value. What matters is the difference between these two values; you can shift both values up by the same amount (e.g., VIL = 0.35 V, VOL = 0.6 V), and the TXS will still work.