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LM76005QRNPRQ1 testing UVLO issue

user5810002
Prodigy 40 points
Other Parts Discussed in Thread: LM76005

Hi Sir,

I would like to ask about the power IC (LM76005QRNPRQ1) for testing UVLO (Undervoltage-Lockout). When VIN is adjusted to 7.48V, VOUT & VIN have a dramatic variation (please refer to the pictures as beloew) What kind of causes is the phenomenon leaded? Is the behavior right? If the behavior is incorrect, how to modify the circuit?

1. Power _IN (12V) -> when Power_IN falls to 7.48V, 5V_VOUT & Power_IN have a dramatic variation. (Light Loading)

2. Power _IN (12V)  -> When The Power _IN fall to 6.92V, 5V_OUT turns off. (Light Loading)

<Measuring Point>

  • 0
    TI__Mastermind 30265 points

    Hi 

    The EN pin has hysteresis 150mV, so the output turn-on threshold is Vin=7.04V, turn-off threshold is Vin=6.1V, there is about 900mV hysteresis which is good enough to prevent the unexpected On/Off toggle at EN=1.2V threshold.

    the main reason should be bad input bypassing at VIN pin, there should be capacitor combination with high enough capacitance and low ESR, Please try it with 1x100uF E-cap and 2x4.7uF 50V ceramic cap, place it close to PVIN and PGND pin.

    B R

    Andy  

  • 0 in reply to Andy Chen
    Prodigy 40 points

    Hi Andy,

    Thanks for your reply. I get the VIn values based on the datasheet & our schematic (see picture 1.), doesn't it ?  Actually, I offer the DC source to the PWR_VIN (12V), so input should be clear & stable; however,  12V_VIN has the switching drop about 3.28V (as below picture 2.). Can adding 1x100uF E-cap. and 2x4.7uF 50V ceramic cap. restrain the switching drop ?

    I worry about the issues to affect the whole system power (Automotive Battery) because VIN has a dramatic variation.

    <picture 1>

    MIN

    TYP

    MAX

    6.6918 V

    7.06748V

    7.3375V

    <picture 2>

  • 0 in reply to user5810002
    TI__Mastermind 30265 points

    Hi,

    You need to check out why there is so big voltage drop from the Power Supply Output to PVIN pin of LM76005, is current limit of power supply set too low? or the wire has too big resistor?

    C714 47pF should not be placed.

    the UVLO calculation is correct.

    B R

    Andy

  • 0 in reply to Andy Chen
    Prodigy 40 points

    Hi Andy,

    I found out the root cause, the 6m cable. Due to our application, our DUT needs 12V power source by the 6m cable, but it causes the UVLO issue. When the 6m cable change to the 3m cable or power source directly connect our DUT, the issue doesn't appear. Also, I selected two different 6m cables, but the result is the same. However, I don't know why the 6m cable is the main cause. A long cable affects the obvious voltage drop/high peak to lead to the issue? Could you give me suggestions/comments?

    Thanks so much.

    Cordially,

    Woody

  • 0 in reply to user5810002
    TI__Mastermind 30265 points

    Hi Woody,

    You can program the UVLO with higher hysteresis by using the VCC or output voltage to bias the EN pin with proper resistor.

    B R

    Andy