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LSF0101: LSF0101 for unidirectional application

vinod sharma
Intellectual 430 points
Part Number: LSF0101
Other Parts Discussed in Thread: SN74AHCT1G125, SN74LV1T34,

Hi,

Please tell how to use this device for unidirectional data.

Input (3.3V, 10nsec pulse from an FPGA)  ---> Output (Input pin of a sensor at 5V)

1.Can we remove pullup from 3.3V side (Vref_A)?

2. Max sink current for FPGA pin is 10mA only. Is this part suitable for this application?

 

regards

Vinod Sharma

 

 

 

  • 0
    TI__Guru* 96681 points
    Hi Vinod,
    I would recommend using a device such as the SN74LV1T34 or SN74AHCT1G125 to complete this translation.

    The LSF0101 would be capable of this translation with a higher drive current (assuming a very low load capacitance), however with a 10mA driver, it's unlikely to work. Here's the equations I used to figure it out:

    Maximum Driver (FPGA) Sink Current (10mA) (RB1 = B1 side pull-up resistor, assuming A1 has no pull-up)
    I_OL = 10mA >= VCCB / RB1 = 5 / RB1 -----> RB1 >= 500 ohms

    Maximum Data Rate (CB1 = parasitic line capacitance on B1 port)
    100Mbps <= 1/( 3* 2*RB1*CB1) --> RB1 * CB1 <= 1/(6*100*10^6)

    Assuming your line capacitance is ~ 15pF (very minimal), we can get a specific required pull-up value of:
    RB1 <= 111 ohms

    Note that to get the speed you need, the pull-up resistor must be less than 111 ohms, but the drive current limits you to a 500 ohm or larger pull-up resistor. Since these two don't intersect, the LSF0101 won't be able to translate for this application.

    We are planning to release a series of videos explaining all this for the LSF in the next week or so - keep an eye out for The Logic Minute :)