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LSF0108: abnormal rise edge of output of LSF0108

Prodigy 700 points

Replies: 4

Views: 45

Part Number: LSF0108

Dear TI,

Here we meet a issue when using LSF0108. Below is the schematic. V1.8 side is input, while V3.3 side is output.

We observed the slew rate of 3.3V output rise edge is slowed sometime. In below waveform, Yellow is for A1 pin and blue is for B1 pin. Could you help explain this abnormal phenomenon? Thanks.

  • Those overshoots look as if the 3.3 V device is actively driving the line high. Isn't the SD protocol bidirectional? I suspect that you get slow edges only when the 3.3 V side is the output.

  • In reply to Clemens Ladisch:

    Hi Ladisch,

    But why only this bit's edge is slow while others are fast. But what will cause this to be slow?

  • In reply to Jiayu Ye:

    You get a fast edge when the 3.3 V device is driving the line high.
    You get a slow edge when the 1.8 V device is driving the line high, because voltages over 1.8 V are only weakly driven by the pull-up resistor.

  • In reply to Clemens Ladisch:

    Hello,

    Yes, Clemens is correct. 

    When the line is being actively drive high from the 3.3V side to the 1.8V side, you will get a fast edge.

    When you are trying to operate in the opposite direction and do up translation from 1.8V to the 3.3V side, the 3.3V side will only be pulled up through the external pull up resistor so will have an RC curve response as opposed to an edge you would see from a low impedance driver.

    Best,
    Michael

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