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LM1117: LM1117 voltage at input and output drops when GSM module is transmitting data

Omkar Inamdar1
Prodigy 100 points
Part Number: LM1117

Hello,

       I am using LM1117 for powering micro-controller  my design with input 5V from SMPS and output of LM1117 set at 3.3 V. I also have GSM module on same 5V rail with different regulator. I have observed on the oscilloscope that at particular interval when GSM is transmitting the data, both 5V and 3.3 V droops by 0.4 V which causes reset of one of analog IC operating at 3.3 V in my design as it has narrow range of input voltage. I am using 10 uF tantalum capacitor at the o/p of regulator. How can this voltage drop be avoided ?

  • 0
    TI__Guru 55940 points
    Hi Omkar,

    The GSM is causing a transient on the 5 V rail which results in a line transient for the LM1117. You can reduce the magnitude of the line transient response of the LM1117 by increasing the output capacitance. Alternatively you can attempt to decrease the magnitude of the line transient by increasing the input capacitance.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 100 points

    Hello Ryan,

                We will make the changes in capacitor and observe the result. I wanted to know whether any specific type of capacitor ( Tantalum , Electrolytic or MLCC ) should be used to suppress transient effect ?  

    Regards,

    Omkar

  • 0 in reply to Omkar Inamdar1
    TI__Guru 55940 points

    Hi Omkar,

    You will want to look at how your intended capacitor(s) derate over voltage and temperature.  This information should be provided by the capacitor manufacturer(s).  As you increase the capacitance, the magnitude of the transient response will decrease regardless of capacitor type; however, it is important to ensure that the ESR requirements for LM1117 are still met.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 100 points

    Hello Ryan,

                Thank you for the answer. This has been helpful .

    Regards,

    Omkar