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LSF0108: Query on LSF0108RKSR Level translator

Nandhini KS
Prodigy 250 points
Part Number: LSF0108
Other Parts Discussed in Thread: TXS0108E

Hi, We are using LSF0108RKSR part in our application. We have connected 200K resistor to the VREF_B (3.3V) Power pin and we have connected system reset signal to EN pin. Is this connection is OK? or we have to connect Pull Up 200K resistor with VREF_B supply to EN pin as mentioned in datasheet ?

  • 0
    Guru 317280 points

    The EN pin is not a digital input, but is connected directly to the gate of the reference transistor:

    When active, the EN pin must have the same voltage as Vref_B.

    To deactivate the LSF, you can pull the EN pin low with an open-drain/open-collector driver (SN74xxx07) or transistor; see the training video Using the Enable Pin with the LSF Family.

  • 0 in reply to Clemens Ladisch
    Prodigy 250 points

    Thank you for the response and sharing the video link.

    Our level Translator circuit connection is shown below for reference

    Is it recommended to use 0.1uF capacitor at Vref_B pin? In our circuit, we are driving the Enable pin with Open drain output using 1K pull up value and there is no capacitor at Vref_B.  Is this fine?

  • 0 in reply to Nandhini KS
    Guru 317280 points

    Section 9.2.1.2.1 of the datasheet says that

    the EN input must be connected to Vref_B and both pins must be pulled up to the HIGH side Vpu through a pull-up resistor (typically 200 kΩ). […]
    A filter capacitor on Vref_B is recommended for a stable supply at the device.

    You must connect EN and Vref_B together, and you must use the existing 200 kΩ resistor as pull-up.

    (If there is a capacitor at Vref_B/EN, it will slow down enable/disable operations. If that becomes a problem, consider making it 0.01 µF or less.)

    And the TXS0108E would be easier to use.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 45995 points

    Hi Nandhini,

    If you go to the 1 minute mark of that video, it shows the correct way to control the EN pin with an open drain driver. There is also a bias circuitry video that covers why the 200 k resistor is necessary.