Panasonic FC, Nichicon Fine Gold and KZ muse have all the same effect.
Removing all decoupling at the buffer (2x 47u) does not solve it.
I still have about 150MHz oscillation.
When I touch my finger on led pin from the GLED431 connected to the negative rail has effect, also placing a cap (both film or electrolytic) over the LED has impact. Oscillation frequency remains the same but amplitude halves.
Removing all decoupling at the buffer (2x 47u) does not solve it.
I still have about 150MHz oscillation.
When I touch my finger on led pin from the GLED431 connected to the negative rail has effect, also placing a cap (both film or electrolytic) over the LED has impact. Oscillation frequency remains the same but amplitude halves.
What you describe says to me the buffer is the unstable culprit. Not the SR.
I have seen ztx951s act squirrley. Don’t really see why it is needed as part of the current source.
If you must keep it, bypass the entire gled431 ckt with a 10uF or higher electrolytic. Or, maybe try another type of current source.
Walt
I have seen ztx951s act squirrley. Don’t really see why it is needed as part of the current source.
If you must keep it, bypass the entire gled431 ckt with a 10uF or higher electrolytic. Or, maybe try another type of current source.
Walt
Yes, I had exactly the same idea.
I've taken out the ztx951 and changed the necesarry resistors to have about 13mA current.
Result: still oscillation but lower amplitude.
Solution: 1x metalized polyester (MMK) on positivel rail decoupling.
Adding an extra on on the negative rail did NOT start the oscillation again.
Enclosed working as build circuit.
I've taken out the ztx951 and changed the necesarry resistors to have about 13mA current.
Result: still oscillation but lower amplitude.
Solution: 1x metalized polyester (MMK) on positivel rail decoupling.
Adding an extra on on the negative rail did NOT start the oscillation again.
Enclosed working as build circuit.
Attachments
Why not install an RC lowpass filter between the VREF generator (junction of R1 & R59) and the base of the CCS transistor, Q14? The very small DC voltage drop across the filtering resistor can be easily accommodated by simply readjusting the trimmer potentiometer R56. You simply twirl the knob of R56 until the voltage across the 100 ohm current sensing resistor (R62) equals 1.60 volts. Presto! The constant current source is 16 milliamps, exactly as desired.
Any noise that might be produced by the forward biased LED, is attenuated by 10X at deep bass frequencies and 100X at midrange frequencies. At the cost of only two components: 470R and 470uF. If you select a Premium Audio Grade UKZ-type capacitor from Nichicon's "MUSE" series (here is a datasheet), it only costs a dollar {sales page}
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Any noise that might be produced by the forward biased LED, is attenuated by 10X at deep bass frequencies and 100X at midrange frequencies. At the cost of only two components: 470R and 470uF. If you select a Premium Audio Grade UKZ-type capacitor from Nichicon's "MUSE" series (here is a datasheet), it only costs a dollar {sales page}
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Bensen, you could implement a more stable source by using the 2 halves of the 389 in series. One as a follower, one on the bottom as the Isource. I assume you are running close to idss, for zero offset. But, the dissipations of the two series halves must be matched for low drift. This isn’t trivial! Must make vdss the same, plus tracking. Think on it.
Walt
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