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LM358B: Capacitive load drive capability

Hideki Hayashi1
Expert 8770 points
Part Number: LM358B
Other Parts Discussed in Thread: OPA994, LM358

Hi team,

Question 1

The datasheet for LM358BIPWR includes an item on capacitive load driving capability, which is listed as 100pF. Customer understand that if a capacitive load greater than 100pF is connected to the LM358BIPWR, the phase margin in the feedback will decrease, increasing the risk of oscillation.

On the other hand, phase margin correlates with frequency, and I believe that if the actual operating conditions are DC or very low frequency, the risk would tend to be lower even if a large capacitive load is connected to the LM358BIPWR.

Is this understanding correct?

Question 2

Could you please recommend some operational amplifiers that don't have limitations on capacitive load driving capability? I'm considering using them as a voltage follower for a DC+1.65V power supply.

Question 3

Customer is concerned about phase margin when external noise is applied and is considering whether they can measure this in-house. Does TI conduct measurements that include external noise? If you have any insights, I would appreciate your guidance on measurement conditions and methods.

 

Best regards,

Hayashi

 

  • 0
    TI__Mastermind 25648 points

    Hayashi,

    1. Question 1 answer: The stability of an op amp is not dependent on the input frequency.  Even if the input is DC and op amp with poor phase margin can oscillate continuously of be marginally stable.  For a marginally stable op amp with a DC input, the output can be triggered into oscillation by small transients in the power supply or by output current demand transients.  In summary, you need to make sure the op amp is stable even for DC inputs.
    2. Question 2 answer: The OPA994 is one device that is specifically designed to drive capacitive loads.  There are many other options that are better than the LM358, but the OPA994 is especially good at driving capacitive load.
    3. Question 3 answer: For amplifiers that have very poor phase margin (PM<10°) it can be difficult to measure because the op amp may oscillate continuously.  In this case you just know that your phase margin is very poor.  For phase margins greater than 10°, you can normally measure the phase margin by applying a small signal step to the input of the op amp and looking at the overshoot.  The ANALOG-ENGINEER-CALC software tool can be used to calculate phase margin based on the percentage of overshoot to the small signal step.  A similar test would be to look at gain peaking versus frequency.  

    This topic is covered in detail in Stability app note.  The app note is pretty long, but you can use the table of contents to find the section on indirect stability methods.  This will cover how to measure using a small signal step.  Also, any op amp (including LM358) can be compensated to drive a capacitive load.  The simplest method is to place an isolation resistance between the output of the op amp and the capacitive load.  The stability app note covers many different compensation methods.

    Best regards, Art