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LSF0108: Rising edge is slow, and high value is little low than other signals

Chase Jeong
Mastermind 7555 points
Part Number: LSF0108

Hello TI experts,

My customer designed a  PCB with LSF0108PWR, and there is some problems.

they use LSF0108 for MDIO and MDC, and there are 2 types, Clause22 and Clause45.

Here is the schematics as below;

line A1-B1 and A2-B2 are for MDIO Clause 22.

and line A5-B5 and line A6-B6 are for MDIO Clause 45.

we think that there is no problem on A1-B1 & A2-B2 CL22 line.

here is the pulse sequence on B2 pin below; (MDIO Clause 22)

but there is some problem in CL45, the high value is little low, and rising edge is lslow. (I remarked it in white box)

First I doubted pullup resistor on B side, we already tested 200 ohm, 250 ohm, 300 ohm.

the symptom was similar, just attaching 300 ohm pullup resistor showed best pulse sequence.

so, here is my request;

1) please review the schematic.

2) please let me know if there are any modifying points in schematic for improving this symptom.

3) please check that this LSF0108 device is suitable for both MDIO CL22 and CL45.

if it is not suitable, please recommend other device.

Thank you in advance.

Best regards,

Chase

  • 0
    TI__Guru* 97046 points

    Hi Chase,

    This video explains in detail how up-translation works for LSF family translators. I expect it will help you to understand the device operation.

    I would recommend to remove C1121 and C1122. These are likely the cause of the slow edges. Just doing a quick calculation, the expected rise time (10% to 90%) of a 300 ohm resistor + 1nF capacitor would be 660 ns (2.2 * R * C), which appears to be pretty close to what your scope shots are showing.

  • 0
    TI__Mastermind 45855 points

    Hey Chase,

    I recommend watching this series of training videos so you can get used to how the LSF functions. It will help you understand the functionality of the LSF translator which should help identify the cause of the slow edges and should give you an idea how to fix the problem. The LSF family is the best option we have for MDC and MDIO.