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LSF0204-Q1: Input abnormal after running under -40C

Chuqiang Wen
Expert 5865 points
Part Number: LSF0204-Q1
Other Parts Discussed in Thread: LSF0204, TXU0202, TCA9800, TXU0102

Tool/software:

Hi team, 

Customer met one issue which need your help. You can find the schematic, description and waveform as below. 

Description: 

After running under -40C, we found 4 pcs input low level signal become abnormal that the input signal from A1 channel(PIN 2) cannot be pulled a normal low level. We can still find about 0.837V when A1 logic is low, see picture 1 for waveform(CH1=A1 channel, CH2=A2 channel). After de-soldering PIN 12 & 13 to leave it open(disconnect output), you can find that the low level signal is back to normal, see picture 2 for waveform. 

Picture 3 is the normal input waveform before putting device run at -40C, low level signal shows normal. 

Questions: 

1. Output load equivalent capacitive value is about 60pF. Is this phenomenon related to the Cload requirement? What is the output capacitive load capability of LSF0204-Q1? 

2. Do you know why and how low temperature cause this abnormal? 

Picture 1 abnormal waveform

Picture 2 input waveform after disconnect output

Picture 3 input waveform before put it at -40C

Schematic: each input and output channel have 2Kohm pull up resistor which not show in the screenshot. 

Thanks!

Ethan Wen

  • 0
    Guru 317280 points

    There is no Cload requirement.

    Does the problem go away when you replace the chip?

  • 0 in reply to Clemens Ladisch
    TI__Expert 5865 points

    Hi Clemens, 

    Customer didn't try to replace a new one. Before doing AB, do you have any suggestions to debug? 

    Thanks!

    Ethan Wen

  • 0 in reply to Chuqiang Wen
    TI__Mastermind 41036 points

    Hi Chuqiang,

    VOL voltage of 0.8V is high- are there any pullups on B-side (output) not shown in current schematic? If so, please remove them to see if B-side is causing VOL to float up. 

    Regards,

    Jack 

  • 0 in reply to Jack Guan
    TI__Expert 5865 points

    Hi Jack, Clemens, 

    Clemens Ladisch said:
    Does the problem go away when you replace the chip?

    As feedbacks from customer, the problem still exist when they replace the chip. 

    Jack Guan said:
    VOL voltage of 0.8V is high- are there any pullups on B-side (output) not shown in current schematic? If so, please remove them to see if B-side is causing VOL to float up. 

    There have a 2Kohm pull up resistor on B-side which not show in the schematic. When customer removed this pull up resistor, the waveform shows better(low level signal got better). After the investigation, customer found that the GPIO of SOC driving capability was changed after operating at -40C, which may cause the abnormal on LSF0204. 

    We want to check with you how the pull up resistor/output load impact the low level signal at input side? 

    Thanks!

    Ethan Wen

  • 0 in reply to Chuqiang Wen
    Guru 317280 points

    The LSF is a passive switch. For high input voltages (VREF_A or higher), the switch is open, and the output voltage is determined only by the pull-up resistor. For low input voltages (below VREF_A), the switch is closed, and the corresponding Ax and Bx pins are connected directly.

    So when the SoC pulls the line low, it must be able to sink the current through all pull-up resistors on both sides.

    The LSF is cheap, but does not buffer the signals. To improve the VOL, use an I²C buffer like the TCA9800. For the other signals, you would usually use a buffered, unidirectional translator like TXU0102/TXU0202.