What is the purpose of a 1 Ω resistor at the output filter to ground?












12












$begingroup$


This is the LDO regulator I am analyzing. While I was a referring to an existing design based on LP2951, it is noted there is an additional 1 Ω resistor at the output filter capacitor to ground.



Enter image description here



What is the purpose of adding a 1 Ω resistor? How is this value selected?






resistors ldo







share|improve this question











$endgroup$








  • 8




    $begingroup$
    Some LDOs aren't stable with low ESR capacitors, so they generally recommend for example tantalum caps. Datasheets will sometimes have a graph showing ESRs which the regulator is stable with. You can look into that and add a series resistor to a ceramic cap for example to try to emulate that ESR. (The datasheet says that this LDO is stable with down to 0.01R, but I've been through situations where I make a PCB footprint compatible with multiple LDOs, so the resistor would be desirable)
    $endgroup$
    – Wesley Lee
    21 hours ago








  • 8




    $begingroup$
    @WesleyLee: That's an answer. Would you like to post it as such?
    $endgroup$
    – JRE
    20 hours ago










  • $begingroup$
    I love tantalum.
    $endgroup$
    – PCARR
    9 hours ago
















12












$begingroup$


This is the LDO regulator I am analyzing. While I was a referring to an existing design based on LP2951, it is noted there is an additional 1 Ω resistor at the output filter capacitor to ground.



Enter image description here



What is the purpose of adding a 1 Ω resistor? How is this value selected?






resistors ldo







share|improve this question











$endgroup$








  • 8




    $begingroup$
    Some LDOs aren't stable with low ESR capacitors, so they generally recommend for example tantalum caps. Datasheets will sometimes have a graph showing ESRs which the regulator is stable with. You can look into that and add a series resistor to a ceramic cap for example to try to emulate that ESR. (The datasheet says that this LDO is stable with down to 0.01R, but I've been through situations where I make a PCB footprint compatible with multiple LDOs, so the resistor would be desirable)
    $endgroup$
    – Wesley Lee
    21 hours ago








  • 8




    $begingroup$
    @WesleyLee: That's an answer. Would you like to post it as such?
    $endgroup$
    – JRE
    20 hours ago










  • $begingroup$
    I love tantalum.
    $endgroup$
    – PCARR
    9 hours ago














12












12








12


3



$begingroup$


This is the LDO regulator I am analyzing. While I was a referring to an existing design based on LP2951, it is noted there is an additional 1 Ω resistor at the output filter capacitor to ground.



Enter image description here



What is the purpose of adding a 1 Ω resistor? How is this value selected?






resistors ldo







share|improve this question











$endgroup$




This is the LDO regulator I am analyzing. While I was a referring to an existing design based on LP2951, it is noted there is an additional 1 Ω resistor at the output filter capacitor to ground.



Enter image description here



What is the purpose of adding a 1 Ω resistor? How is this value selected?






resistors ldo




resistors ldo






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 14 mins ago









Peter Mortensen

1,60031422




1,60031422










asked 21 hours ago









vt673vt673

3801513




3801513








  • 8




    $begingroup$
    Some LDOs aren't stable with low ESR capacitors, so they generally recommend for example tantalum caps. Datasheets will sometimes have a graph showing ESRs which the regulator is stable with. You can look into that and add a series resistor to a ceramic cap for example to try to emulate that ESR. (The datasheet says that this LDO is stable with down to 0.01R, but I've been through situations where I make a PCB footprint compatible with multiple LDOs, so the resistor would be desirable)
    $endgroup$
    – Wesley Lee
    21 hours ago








  • 8




    $begingroup$
    @WesleyLee: That's an answer. Would you like to post it as such?
    $endgroup$
    – JRE
    20 hours ago










  • $begingroup$
    I love tantalum.
    $endgroup$
    – PCARR
    9 hours ago














  • 8




    $begingroup$
    Some LDOs aren't stable with low ESR capacitors, so they generally recommend for example tantalum caps. Datasheets will sometimes have a graph showing ESRs which the regulator is stable with. You can look into that and add a series resistor to a ceramic cap for example to try to emulate that ESR. (The datasheet says that this LDO is stable with down to 0.01R, but I've been through situations where I make a PCB footprint compatible with multiple LDOs, so the resistor would be desirable)
    $endgroup$
    – Wesley Lee
    21 hours ago








  • 8




    $begingroup$
    @WesleyLee: That's an answer. Would you like to post it as such?
    $endgroup$
    – JRE
    20 hours ago










  • $begingroup$
    I love tantalum.
    $endgroup$
    – PCARR
    9 hours ago








8




8




$begingroup$
Some LDOs aren't stable with low ESR capacitors, so they generally recommend for example tantalum caps. Datasheets will sometimes have a graph showing ESRs which the regulator is stable with. You can look into that and add a series resistor to a ceramic cap for example to try to emulate that ESR. (The datasheet says that this LDO is stable with down to 0.01R, but I've been through situations where I make a PCB footprint compatible with multiple LDOs, so the resistor would be desirable)
$endgroup$
– Wesley Lee
21 hours ago






$begingroup$
Some LDOs aren't stable with low ESR capacitors, so they generally recommend for example tantalum caps. Datasheets will sometimes have a graph showing ESRs which the regulator is stable with. You can look into that and add a series resistor to a ceramic cap for example to try to emulate that ESR. (The datasheet says that this LDO is stable with down to 0.01R, but I've been through situations where I make a PCB footprint compatible with multiple LDOs, so the resistor would be desirable)
$endgroup$
– Wesley Lee
21 hours ago






8




8




$begingroup$
@WesleyLee: That's an answer. Would you like to post it as such?
$endgroup$
– JRE
20 hours ago




$begingroup$
@WesleyLee: That's an answer. Would you like to post it as such?
$endgroup$
– JRE
20 hours ago












$begingroup$
I love tantalum.
$endgroup$
– PCARR
9 hours ago




$begingroup$
I love tantalum.
$endgroup$
– PCARR
9 hours ago










1 Answer
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31












$begingroup$

The first easy to use 'one component' regulators to be produced, like the 7805, were NPN follower based. This meant they had a high(ish) dropout voltage. The low output impedance, and the unavailability of low ESR capacitors at the time, meant that they were stable into any pretty much any output capacitor.



To meet the demand for lower dropout voltage, PNP output stages were then used. These had a higher output impedance. When used with high ESR aluminium electrolytics these were usually stable for a range of values. The ESR of the capacitor allowed some non-phase-shifted (or fast) feedback to the control circuitry, which allowed them to remain stable.



Unfortunately, shortly after the introduction of LDOs, ceramic capacitors started getting big enough to be usable after regulators, at the same time as miniaturisation was demanding them. Designers started to use LDOs with very low ESR caps. This removed the fast feedback, and some early LDO designs became unstable as a result.



The series resistor in your diagram suggests that LP2951 is one of these older LDO designs, that requires a minimum ESR on the output capacitor to be stable. It's not needed for an electrolytic output cap, but will be needed for ceramic.



Newer LDOs have been designed to be stable with low ESR caps. These can be identified by explicit claims on the data sheet that they are stable with ceramic capacitors.






share|improve this answer









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    31












    $begingroup$

    The first easy to use 'one component' regulators to be produced, like the 7805, were NPN follower based. This meant they had a high(ish) dropout voltage. The low output impedance, and the unavailability of low ESR capacitors at the time, meant that they were stable into any pretty much any output capacitor.



    To meet the demand for lower dropout voltage, PNP output stages were then used. These had a higher output impedance. When used with high ESR aluminium electrolytics these were usually stable for a range of values. The ESR of the capacitor allowed some non-phase-shifted (or fast) feedback to the control circuitry, which allowed them to remain stable.



    Unfortunately, shortly after the introduction of LDOs, ceramic capacitors started getting big enough to be usable after regulators, at the same time as miniaturisation was demanding them. Designers started to use LDOs with very low ESR caps. This removed the fast feedback, and some early LDO designs became unstable as a result.



    The series resistor in your diagram suggests that LP2951 is one of these older LDO designs, that requires a minimum ESR on the output capacitor to be stable. It's not needed for an electrolytic output cap, but will be needed for ceramic.



    Newer LDOs have been designed to be stable with low ESR caps. These can be identified by explicit claims on the data sheet that they are stable with ceramic capacitors.






    share|improve this answer









    $endgroup$


















      31












      $begingroup$

      The first easy to use 'one component' regulators to be produced, like the 7805, were NPN follower based. This meant they had a high(ish) dropout voltage. The low output impedance, and the unavailability of low ESR capacitors at the time, meant that they were stable into any pretty much any output capacitor.



      To meet the demand for lower dropout voltage, PNP output stages were then used. These had a higher output impedance. When used with high ESR aluminium electrolytics these were usually stable for a range of values. The ESR of the capacitor allowed some non-phase-shifted (or fast) feedback to the control circuitry, which allowed them to remain stable.



      Unfortunately, shortly after the introduction of LDOs, ceramic capacitors started getting big enough to be usable after regulators, at the same time as miniaturisation was demanding them. Designers started to use LDOs with very low ESR caps. This removed the fast feedback, and some early LDO designs became unstable as a result.



      The series resistor in your diagram suggests that LP2951 is one of these older LDO designs, that requires a minimum ESR on the output capacitor to be stable. It's not needed for an electrolytic output cap, but will be needed for ceramic.



      Newer LDOs have been designed to be stable with low ESR caps. These can be identified by explicit claims on the data sheet that they are stable with ceramic capacitors.






      share|improve this answer









      $endgroup$
















        31












        31








        31





        $begingroup$

        The first easy to use 'one component' regulators to be produced, like the 7805, were NPN follower based. This meant they had a high(ish) dropout voltage. The low output impedance, and the unavailability of low ESR capacitors at the time, meant that they were stable into any pretty much any output capacitor.



        To meet the demand for lower dropout voltage, PNP output stages were then used. These had a higher output impedance. When used with high ESR aluminium electrolytics these were usually stable for a range of values. The ESR of the capacitor allowed some non-phase-shifted (or fast) feedback to the control circuitry, which allowed them to remain stable.



        Unfortunately, shortly after the introduction of LDOs, ceramic capacitors started getting big enough to be usable after regulators, at the same time as miniaturisation was demanding them. Designers started to use LDOs with very low ESR caps. This removed the fast feedback, and some early LDO designs became unstable as a result.



        The series resistor in your diagram suggests that LP2951 is one of these older LDO designs, that requires a minimum ESR on the output capacitor to be stable. It's not needed for an electrolytic output cap, but will be needed for ceramic.



        Newer LDOs have been designed to be stable with low ESR caps. These can be identified by explicit claims on the data sheet that they are stable with ceramic capacitors.






        share|improve this answer









        $endgroup$



        The first easy to use 'one component' regulators to be produced, like the 7805, were NPN follower based. This meant they had a high(ish) dropout voltage. The low output impedance, and the unavailability of low ESR capacitors at the time, meant that they were stable into any pretty much any output capacitor.



        To meet the demand for lower dropout voltage, PNP output stages were then used. These had a higher output impedance. When used with high ESR aluminium electrolytics these were usually stable for a range of values. The ESR of the capacitor allowed some non-phase-shifted (or fast) feedback to the control circuitry, which allowed them to remain stable.



        Unfortunately, shortly after the introduction of LDOs, ceramic capacitors started getting big enough to be usable after regulators, at the same time as miniaturisation was demanding them. Designers started to use LDOs with very low ESR caps. This removed the fast feedback, and some early LDO designs became unstable as a result.



        The series resistor in your diagram suggests that LP2951 is one of these older LDO designs, that requires a minimum ESR on the output capacitor to be stable. It's not needed for an electrolytic output cap, but will be needed for ceramic.



        Newer LDOs have been designed to be stable with low ESR caps. These can be identified by explicit claims on the data sheet that they are stable with ceramic capacitors.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 20 hours ago









        Neil_UKNeil_UK

        74.7k277165




        74.7k277165






























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