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Original question:

LP3470: custom order value

LP3470: RESET_N line goes to 1.4V (expected < 0.4V) when VCC > 0.5V

Pierre-Olivier Lessard
Prodigy 105 points
Part Number: LP3470
Other Parts Discussed in Thread: TPS3808, TPS3890

We are facing an issue on some systems using the LP3470M5-2.63/NOPB; the RESET_N output line goes to 1.4V instead of going below 0.4V when VCC is around 0.8V. Is there something wrong with our implementation, please refer to the uploaded figure below. Simulation does not show any problem.

Regards,

Pierre-Olivier

  • 0
    TI__Genius 12630 points
    Pierre-Olivier,

    Does the issue happen everytime with multiple LP3470 devices? Has the circuit worked in the past? One initial observation I have is when the output isn't going all the way to 0V during output logic low, I check the value of the pull-up resistor. Small value pull-up resistors lead to higher output voltage when logic low. Try to increase the pull-up resistor to 20kohm and check if this has any effect.

    Also if this issue happens everytime or sometimes or only on this specific unit can help narrow if the issue is an application issue or device issue. Look forward to hearing your response. Thanks

    -Michael
  • 0 in reply to Michael_DeSando
    Prodigy 105 points
    Hi Michael,

    Yes we noticed that we increasing the value up to 2.2k (before asking on the Forum) seemed to help decrease the output voltage but we don't what is the minimum Rpu value that will guaranty a output low voltage < 0.4V. Is there any graph showing what is the maximum output voltage vs Rpu value?
  • 0 in reply to Pierre-Olivier Lessard
    Prodigy 105 points
    Hi Michael,

    Just another question: If the Vcc voltage would be higher, let’s say, 2V instead of 0.8V, would it help to make the output voltage lower, with 1k resistor?
  • 0 in reply to Pierre-Olivier Lessard
    Prodigy 105 points
    Hi Michael,

    Do you have any update regarding our questions?
  • 0 in reply to Pierre-Olivier Lessard
    TI__Genius 12630 points
    Increasing the pull-up voltage will increase the VOL linearly. There is a recommended external pull-up resistor range found in table 6.5 but we recommend always choosing the correct value for your specific application. Notice the VOL specs are tested under specific output current conditions and the pull-up resistor will set the output current. The typical recommended value both in the EC table and in the typical application circuit recommends 20kohm.

    The lower the resistor value, the stronger the pull-up because as the pull-up resistor approaches 0 ohms, this would be a direct pull-up to the supply voltage. As you increase the pull-up resistor value, there will be a larger voltage drop during the logic low condition and ideally you want the output to drop all the way to 0V when the output is logic low. The pull-up resistor should be selected specifically so that when the output is logic low, the actual output meets your applications logic low requirement.

    Please refer to section 7.3.3 Pullup Resistor Selection for more information.

    Please let me know if you have any additional questions. Thanks!

    -Michael
  • 0 in reply to Michael_DeSando
    Prodigy 50 points

    Hello Michael,

    I am involved in the design with this part. It works fine with high-value pull-up like 10k. We wanted to make the reset trace more immune to the noise, so we decided to make it with lower impedance. That is why we decided to change the pull-up resistor from10k to 1k. It seems that it is within the values of the spec. However, in practice it did not work, as the output logic low voltage was 1.4V.

    This result contradicts the datasheet, Or, as the minimum, the datasheet does not specify the value of pull-up resistor (or the output current) for the whole operational range of Vcc. It does it for VCC =2.4 to 5V (Section 6.5). The interpretation of this table should be that the 0.68k resistor can only be used when the Vcc is 2.4V or more. Nothing is said about the resistor for the rest of the Vcc range, from 0.5V to 2.4V.

    Section 7.7.7 does not say anything about the VCC and how it affects the minimum pill-up value, which it obviously does.

    Regards,

    Dmitri.

  • 0 in reply to Dmitri Golovanov73
    TI__Genius 12630 points
    Dmitri,

    Since this device was created in 1999, the standards of our device definition and supporting data and characterization has much improved. I would recommend for LP3470 to simply increase the pull-up resistor within the range to meet your application needs. You can also try a newer device with better performance and characterization such as TPS3890 or TPS3808. I believe the lower pull-up resistor value will allow transients on the supply to propagate to RESET pin easier, where a large resistance will minimize the transients. You can also add capacitors to the /RESET pin although this is will impact on the /RESET delay. Lowering the pull-up resistor, shrinks the output range from 0V to VDD to some higher VOL voltage to VDD. It is recommended to choose a resistor such that at your specific operating voltage, temperature range, and under all load conditions, the output logic low meets the application requirements.

    If you need any additional support or have any questions, please let me know. Thanks!

    -Michael