Mosaic
Well-Known Member
Hi all:
I have a possible case of random LM358 (12V single supply) noise causing instability on a PWM control circuit voltage.
the setup use 2 LM358 internal opamps.
1st stage measures current difference across a 0.1 ohm current sense via a low pass filter and amplifies by 10 (1k input & 10K FB)
2nd stage takes noninverting input from 1st stage and a target current reference voltage on the inverting input. Gain is x10 again with 1K input & 10K FB resistors.
The output of the 2nd stage passes thru a 1n4148 forward biased and is divided by half with a pair of 39K resistors and a .01uf filter cap to ground before feeding the PWM ref voltage to a TL494. The Opamp outputs in the range of 6 to 7.2V to control the PWM reference in the 2.65V to 3V region.
What is happening is I am occasionally getting instability in the regulation of the PWM and resultant current control, small deviations and occasional significant ones which would amount to a net OPA noise voltage of perhaps 0.01V to .05V. Sometimes its fine for hours, other times for seconds, op temps are <35C. Supply voltages are rock solid.
By breaking the control loop and feeding the TL494 with a trimmer controlled ref V....the instability goes away.
Thus I am wondering if the LM358 is operating in class B with only a 78K load on the output?
I am going to decrease this to around 5K, but I am just guessing here....
What current LOAD does the LM358 require to do Class A outputs?
I can consider an MC33172 replacement as a trial but I only have SOIC on hand...and the app. is DIP with underlying traces.
I have a possible case of random LM358 (12V single supply) noise causing instability on a PWM control circuit voltage.
the setup use 2 LM358 internal opamps.
1st stage measures current difference across a 0.1 ohm current sense via a low pass filter and amplifies by 10 (1k input & 10K FB)
2nd stage takes noninverting input from 1st stage and a target current reference voltage on the inverting input. Gain is x10 again with 1K input & 10K FB resistors.
The output of the 2nd stage passes thru a 1n4148 forward biased and is divided by half with a pair of 39K resistors and a .01uf filter cap to ground before feeding the PWM ref voltage to a TL494. The Opamp outputs in the range of 6 to 7.2V to control the PWM reference in the 2.65V to 3V region.
What is happening is I am occasionally getting instability in the regulation of the PWM and resultant current control, small deviations and occasional significant ones which would amount to a net OPA noise voltage of perhaps 0.01V to .05V. Sometimes its fine for hours, other times for seconds, op temps are <35C. Supply voltages are rock solid.
By breaking the control loop and feeding the TL494 with a trimmer controlled ref V....the instability goes away.
Thus I am wondering if the LM358 is operating in class B with only a 78K load on the output?
I am going to decrease this to around 5K, but I am just guessing here....
What current LOAD does the LM358 require to do Class A outputs?
I can consider an MC33172 replacement as a trial but I only have SOIC on hand...and the app. is DIP with underlying traces.