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Original question:

LSF0108-Q1: Threshold at which the switch is turned on or off

LSF0108-Q1: About Vref_A-0.5 variability

Koji Hayashi92
Mastermind 6970 points
Part Number: LSF0108-Q1

Hi TI-team

I confirmed the "Voltage-Level Translation With the LSF Family"

https://www.ti.com/lit/an/slva675b/slva675b.pdf?ts=1601948050943&ref_url=https%253A%252F%252Fe2e.ti.com%252Fsupport%252Flogic%252Ff%252F151%252Fp%252F945739%252F3493950

There is the following sentence on P.2.

Remember that the LSF device acts as a passive switch when the input signal is below Vref_A – 0.5 V.

Does this mean that the Vgs (th) of the internal FET is 0.5V ?

Is it not necessary to consider the variation of 0.5V ?

Is the input from the VinB side correct with the recognition that it is Vref_A-0.5 ?

 

Best Regards,

Koji Hayashi

  • 0
    Guru 243220 points

    At Vref_A, the MOSFET is already partially on, but it allows only a few microamperes to pass. (The biasing circuit ensures that the gate voltage is approximately VGS(th) above Vref_A.)

    Going 0.5 V below that ensures that the MOSFET is fully on.

  • 0 in reply to Clemens Ladisch
    Mastermind 6970 points

    Hi Clemens-san

    Thank you for quick response.

    I understand.

    Can you also provide answers to the following questions ?

    Is it not necessary to consider the variation of 0.5V ?

    Is the input from the VinB side correct with the recognition that it is Vref_A-0.5 ?

      

    Best Regards,

    Koji Hayashi

  • 0 in reply to Koji Hayashi92
    Guru 243220 points

    VGS(th) + 0.5 V is a safe value. (The LSF is not intended for high currents anyway.)

    That application note talks about analog signals, which is not the intended application. The LSF acts as an analog switch for voltages below Vref_A − 0.5 V, but digital signals will be above Vref_A. What the application note says is correct, but probably not useful for your circuit.

  • 0 in reply to Clemens Ladisch
    Mastermind 6970 points

    Hi Clemens-san

    Thank you for response.

    My customers will be using Vref_A=3.3V and Vref_B=5V.

    Is my understanding below correct ?

    1. Even if 3.3V is given to A1, it will not be completely turned off, but B1 will be pulled up to 5V.

    2. Even when the input signal is B1⇒A1, the switch turns on when the voltage of B1 is 2.8V or less.

    >VGS(th) + 0.5 V is a safe value. (The LSF is not intended for high currents anyway.)

    Does this mean Vref_A- (VGS(th) + 0.5 V) ?

    Best Regards,

    Koji Hayashi

  • 0 in reply to Koji Hayashi92
    Guru 243220 points

    1. As shown in the app note, a very small current flows out of 5 V, through the 200 kΩ resistor, into Vref_B, through the reference transistor, out of Vref_A, and into the 3.3 V supply. With A1 at 3.3 V, the same amount of current can flow through the switch transistor. But the pullup at B1 is smaller than 200 kΩ, so most of the voltage is dropped over the transistor, and B1 is near 5 V.

    2. When B1 is 5 V, the switch is completely off. When B1 is 0 V, the switch is completely on. The switch will turn on somwhere between 3.3 V and 2.8 V, but for digital signals, the exact voltage does not matter.

    With "VGS(th) + 0.5 V", I meant the difference between the gate voltage and the source voltage. The gate voltage is VGS(th) above Vref_A, so this is the same as Vref_A − 0.5 V.

  • 0 in reply to Clemens Ladisch
    Mastermind 6970 points

    Hi Clemens-san

    Thank you for response.

    >2. When B1 is 5 V, the switch is completely off. When B1 is 0 V, the switch is completely on. The switch will turn on somwhere between 3.3 V and 2.8 V, but for digital signals, the exact voltage does not matter.

    There is a Vref_A-0.4V as an output variance in the Vref_A side IC that my customer is using.

    If the switch recognizes Hi, there is no problem, but they are concerned that it will recognize Lo and turn on.

    Does this matter ?

    Do they need measures?

     

    Best Regards,

    Koji Hayashi

  • 0 in reply to Koji Hayashi92
    Guru 243220 points

    At Vref_A − 0.4 V, the switch is likely to be turned on.

    The footnote of table 3 says that "Vref_A have to be the lowest voltage level across all of inputs and outputs."

    Where does that variance come from? Is it strong enough to pull the signal on the other side down?

  • 0 in reply to Clemens Ladisch
    Mastermind 6970 points

    Hi Clemens-san

    Thank you for response.

    >Where does that variance come from?

    The 3.3V side IC has ”VOH(min) ＝ Vcc-0.4V” specifications.

    >Is it strong enough to pull the signal on the other side down?

    Does that mean you have enough Sink to pull the (5V-Vref_A-0.4)/Rpu ?

    If the 3.3V IC doesn't have that capability, does it matter ?

     

    Best Regrds,

    Koji Hayashi

  • 0 in reply to Koji Hayashi92
    Guru 243220 points

    That VOH specification has a certain sourcing current as test condition. In this application, the output does not source any current, so it will be at 3.3 V. (See [FAQ] What is the output voltage (VOH or VOL) when the output current is X or the supply voltage is Y?)