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Original question:

LSF0204: pull up resistor

LSF0204: Higher push pull resistor

Masayuki Uchihara
Intellectual 1676 points
Part Number: LSF0204

Hi Team,

Do you have any concern if we use higher pull-up resistor? Now we set the resistor value is 10 kohm when VDPUs are 3.3V and 5 V.

Thanks,

Uchihara 

  • 0
    TI__Guru* 96947 points

    Hello Uchihara-san,

    Pull-up resistors should be selected based on desired operation speed and driver strength.  This video explains more:

    https://training.ti.com/translation-lsf-family-0?context=1134826-1139264-1134794

  • 0 in reply to Emrys Maier
    TI__Intellectual 1676 points

    Hi Emrys,

    When I confirmed the following training video (2.2 Understanding the Bias Circuit for the LSF Family), there were two events that I wanted to confirm.
    (1) 200kΩ is not connected to Vref_B (Since our configuration operates with EN = Vref_A = 3.3V, the MOS between Vref_A and VrefB does not turn on, so it is judged that 200kΩ for current limitation is unnecessary. ) If you have any concerns about operation, would you please teach me?
    (2) As described in (1), no current flows between Vref_A and VrefB in our configuration. If there is a factor that requires current to flow between Vref_A and VrefB, please tell us. (We have confirmed that this configuration allows steady operation)

    Thanks,

    Uchihara

  • 0 in reply to Masayuki Uchihara
    TI__Guru* 96947 points

    Hello Uchihara-san,

    It sounds like you are using the device as a voltage clamp, which is fine.  This mode of operation will require pull-up resistors on both sides of the devices to correctly operate, which will reduce your overall speed and increase drive current required -- but if it works in your system, then there is no problem.

  • 0 in reply to Emrys Maier
    TI__Intellectual 1676 points

    Hi Emrys,

    Can you tell me how much the speed reduction and increase drive current in the attachment schematics?

    In this case, the signal speed is around max 1Mbps..

     Thanks,

    Uchihara

  • 0 in reply to Masayuki Uchihara
    TI__Guru* 96947 points

    Hello Uchihara-san,

    I see a few issues in your schematic that I would like to address:

    1. The LSF0204 has a built-in 200 kohm resistor - there is no need to add an external 200kohm resistor to the VrefB pin, and this added resistor will cause an error in the clamping voltage.

    2. The internal structure of the LSF0204 is very different from what is shown here. It is actually this:

    Each channel has a single MOSFET and the gates are connected together to the bias circuit.

    3. Adding series resistors is not recommended as this can significantly impact the low-level output voltage of the device.

    4. The RC circuit shown will result in very slow edges that can be damaging to the device. The EN pin is a CMOS input that requires a fast transition from low to high, preferably under 100ns transition time.

    To directly answer your questions, the maximum sink current required at the 3.3V IC will be (3.3V / 5kohm) + (5V / 10kohm) =  1.16 mA for the top channel, and the maximum sink current required at the 5V MCU will be (3.3 / 10kohm) = 0.33 mA 

    The maximum up-translation speed in this configuration with a typical 15 pF load is estimated to be: 1/(6*10kohm*15pF) ~= 1.11 Mbps