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eur

Member Since: March 10, 2008

Country: Netherlands

  • Here is the original document from 1997 that explains the bi-directional function of the MOSFETs:

    http://ics.nxp.com/support/documents/interface/pdf/an97055.pdf

    (Google for an97055.pdf if it moves again).

    2.3.1 Description of the level shift operation. For the level shift operation three states has to be considered:

    • State 1. No device is pulling down the bus line and the bus line of the “Lower voltage” section is pulled up by its pull-up resistors Rp to 3.3 V. The gate and the source of the MOS-FET are both at 3.3 V, so its VGS is below the threshold voltage and the MOS-FET is not conducting. This allows that the bus line at the “Higher voltage” section is pulled up by its pull-up resistor Rp to 5V. So the bus lines of both sections are HIGH, but at a different voltage level.

    • State 2. A 3.3 V device pulls down the bus line to a LOW level. The source of the MOS-FET becomes also LOW, while the gate stay at 3.3 V. The VGS rises above the threshold and the MOS-FET becomes conducting. Now the bus line of the “Higher voltage” section is also pulled down to a LOW level by the 3.3V device via the conducting MOS-FET. So the bus lines of both sections become LOW at the same voltage level.

    State 3. A 5 V device pulls down the bus line to a LOW level. Via the drain-substrate diode of the MOS-FET the “Lower voltage” section is in first instance pulled down until VGS passes the threshold and the MOS-FET becomes conducting. Now the bus line of the “Lower voltage” section is further pulled down to a LOW level by the 5 V device via the conducting MOS-FET. So the bus lines of both sections become LOW at the same voltage level.

    Note that the gate has to be connected to the lower voltage. Yes, they wrote MOSFET with a dash in 1997.

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