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Original question:

input voltage of LM317A

LM317A: External capacitance

Nariaki Ogasawara
Intellectual 1215 points
Part Number: LM317A

Hi Team,

Q1. The datasheet mentions about  the External Capacitance at section 8.4.1, the customer can not use tantalum and 
 and aluminum electrolytic capacitor. It is also written that  LM317A is not necessary output capacitors. 
 If the customer wants to add the ceramic capacitors at output, how many capacity values is better?
Condition : 
Vin : 24V
Vout : 15V
Iout : 500mA (Max)

Q2 : The bypass capacitor
 Is the bypass capacitor (C2) at Figure 17 recommended? Because the TLV317A's EVM has the same circuit. In case of the above condition, the C2 is mandatory?

Regards,
Ogasawara

 

  • 0
    TI__Guru 55940 points

    Hi Ogasawara,

    The amount of capacitance required for your application will depend on your application specific conditions and application specific performance requirements.  LM317A will achieve the best performance with solid tantalum capacitors due to how the capacitance will derate; however, as you noted, an output capacitor is not required for the control loop to achieve stability.

    In order to determine how much and what kind of capacitor will allow you to achieve your application specific requirements, we recommend performing load transients while monitoring your output voltage on a scope during your prototype phase.  LM317A-337N-EVM can be used early in your prototype phase as you make these measurements.

    The bypass capacitor between the adjustment terminal and ground is not required; however, this capacitor can improve the LM317A ripple rejection.  The LM317A-337N-EVM was designed with this capacitor as well as the protection diodes so that you can use the EVM as a tool early in your prototyping phase to determine if your application will need these components.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    TI__Intellectual 1215 points

    Hi Ryan,

     Thank you for your response.
     My understanding is if output does not add external capacitor, even in this state LM317A does not self-oscillation.
     If lord transient wants to improved, there is need output capacitance. If the customer uses Ceramic capacitance for improving load transient, it has risk of self-oscillation for small ESR.
     
    My questions is

    1. Does LM317A never self-oscillate without external capacitance?
    2. What is recommended ESR value for non self-oscillation for Ceramic capacitance?

    Regards,
    Ogasawara

  • 0 in reply to Nariaki Ogasawara
    TI__Guru 55940 points

    Hi Ogasawara,

    LM317A is stable without any external capacitors.  Application conditions such as a line or load transient will cause a temporary dip or increase in the output voltage; however, the control loop of LM317A will regain regulation.  Our datasheet demonstrates this in the line and load transient response curves.  The solid lines show the LM317A without any external capacitors while the dotted lines show the response to the same transient with external capacitors.

    Ceramic capacitors generally have low ESR values; however, keep in mind that there will be resistance in the trace and mounting of the capacitor which will add to the ESR.  The best method to validate your intended output capacitors is to perform your worst case expected load transients while monitoring the output voltage on a scope. This will help you understand what performance you can expect in your application. LM317A-337N-EVM can be used early in your prototyping phase.

    Very Respectfully,

    Ryan