Mic29752 resistors help

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#22
sgrossklass said:
What is the problem you are trying to solve by using a linear power supply to begin with?
What sort of IEC appliance class are you aiming for?

Since the regulator is fairly noisy, there is little point in making R1 + R2 any lower than required to draw the specified minimum load current, 10 mA for this particular chip. That would be a total of 2 kOhms.

Exactly 20 V seems a bit hard to hit without going at least E96, at which point you could use e.g. 1.82k and 121R (note: rated power dissipation on 1.82k should be at least 0.4 W, so maybe a 1/2 watt type or two 1/4W in parallel).
Click to expand...

Ok thank you. What minimum power dissipation on 121ohm resistor?
 
#24
Ooops - my bad 😱 - thanks for setting me straight, sgrossklass ..

.. and I thought I'd read enough of that rascally PDF ..

One of my favorite things about this forum is having smart other folks to 'check my work'. Since this is already my 2nd error on this relatively short thread, maybe it's time for me to push my chair back and see what else I can learn.😉

Cheers
 
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#26
hi guys i need some more advice. the controller datasheet says:
"Where the regulator is powered from a source with high AC impedance, a 0.1 µF capacitor connected between Input and GND is recommended. This capacitor should have good characteristics to above 250 kHz".
What is "a source with high AC impedance"? do i need this capacitor?
Thank you so much.
 
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#27
OK, no other takers, yet? Guess I'll take a stab ..

AC impedance depends on the frequency of interest. In this case, they probably mean *more than a few centimeters of wiring/PCB trace from the bulk capacitor*. Since it isn't a terribly expensive part, and an oscillating regulator isn't very fun to diagnose -- or put up with, maybe just fit one anyway. Then you have one less thing to go wrong and worry about.

Pushing chair back again ..

Cheers
 
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