This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LSF family pull up resistor question

Other Parts Discussed in Thread: LSF0204

Dear experts,

Is it possible to use for example the LSF0204 for an SPI application where the i have a 3.3V microcontroller and a 5V SPI device. On the 5V side i have a pull up resistor with 3.3kOhm. The clock frequency is at max. 40MHz. Is there any equation to calculate the maximum pull up resistor?


Best regards

  • The pullup value will be directly related to the drive strength of your input device. You should size the resistor for the maximum current that your input device can sink -- for example, if the input can sink 10mA, the resistor will be calculated by R = 5V / 10mA = 500 ohms. The reason you want to maximize this current is to also maximize speed on the output. The output capacitance will effect the up-translation maximum frequency because the output pulls up through the resistor and basicall you have an RC circuit output.