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LM1085: Decoupling Capacitors

Sangeet Yadav
Prodigy 20 points
Part Number: LM1085

I am using LDO (LM1085) in my design for conversion of 24V supply to 12V and it perfectly suits for the output current requirement.

I know that in order to ensure that the LDO works properly, I need to choose proper decoupling capacitors for that and the datasheet shows that it should be 10uF at the input and 10uF at the output of LDO.

My question is that if I use bypass capacitors (0.1uF and 100uF) for my input Supply Voltage then the effective capacitance at the input of the LDO will change, will the LDO work fine, if not, how to select the bypass capacitors for the supply and, is there any range of decoupling capacitors for the LDO? Also do the size of capacitors matter (like 0805, 0603, 1206 etc)?

In addition to that I want to ask that if I have to convert the 12V further to 5V then again the effective capacitance at the output of LDO will no longer be 10uF because of the addition of another capacitance used at the input of the 2nd LDO, how to select the capacitance values now?  

I will appreciate the efforts for clearing my doubts. 

Thanks.

  • 0
    TI__Mastermind 23361 points

    Hi Sangeet, 

    Normally in the LDO's datasheet, the required output cap and the recommended input cap are the local ones that are sitting very close to the input/output pins. As long as you meet the local capacitors' recommendation, you should be fine. The LDO is designed to drive a load with capacitance. It's good practice to have a decoupling capacitor for the supplies, and the decoupling will help bring down the source impedance which is a good thing for the input of the LDO. 

    As for your 2nd question on the 5V rail, as you may have noticed, most of the input of the output capacitors are mentioned as "higher than" a certain value, and as I stated before so there is no concern to have remote capacitors. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Prodigy 20 points

    Hi Jason,

    Also, the ESR of capacitor plays an important role and it changes with the size and material of capacitor like Ceramic or Tantalum.

    Just want to know about the size and type of decoupling capacitors to choose and in the datasheet nothing is mentioned about that.

    Thanks 

    Sangeet Yadav

  • 0 in reply to Sangeet Yadav
    TI__Mastermind 23361 points

    Hi Sangeet, 

    For the input cap, you would want lower ESR and this will help to reduce the source impedance, which converts to better AC transient performance. The input cap does not normally relate to the stability of the LDO, but the output cap may require certain ESR for stability and the ESR requirement from the output cap is valid for the local cap and normally up to the LDO's bandwidth which is 1-2MHz. 

    Regards,
    Jason