Hoping wrote on 2021-10-19, 20:44:
So if the 486 dx4 100 consumes 5.22w maximum (cpu-world) and the mic 29152bt can give 3v3*1.5 around 4.95w, something is wrong. […]
So if the 486 dx4 100 consumes 5.22w maximum (cpu-world) and the mic 29152bt can give 3v3*1.5 around 4.95w, something is wrong. So I'll need the Mic 29302, right? What power rating for the resistors? Most ones I've seen are 400mw, is this low?
But I'm not used to this, and I'm having trouble to find the resistors with the correct values (:, but with patience and time maybe I'll get then.
Or maybe look for other values that are available, using the formula posted above (1.24*(1+R1/R2)). I guess that if this formula is correct the values that Chicony used doesn't need to be the only ones that works, R1 is fixed and R2 is the variable.
For example, 1.24*(1+160/95)=3.328, very close to 3.3.
Again if I'm right, never did this.
Let's see. What's the current through the CPU? Power=Current * Voltage (P=IV). Max power is 5.22W and Voltage is 3.3V, so current = 1.6A (on average). So the first thing is that the regulator needs to be able to supply that. The 29152 datasheet says max current of 1.5A. So that could be a problem, certainly borderline if the CPU is maxed out. The 29302 can supply 3A so that'd be ok. Except...
There's also the power lost in the regulator to be considered. The input voltage is 5V, the output 3.3V, so it's dropping 1.7V. P=IV=1.6A * 1.7V = 2.7W being burnt by the regulator.
The datasheet gives a formula for working out how good the heatsink needs to be: (TjMax - Tambient)/P - (Junction to Case + Case to Sink). Junction to Case for TO-220 is 2C/W (from datasheet), Case to Sink might be up to 2C/W (suggested by datasheet)
TjMax is 125C, and let's put Tambient at 50C (inside case, limited airflow). So (125-50)/2.7 - (2+2) = 28-4 = 24 C/W
So the heatsink temperature must only rise by 24C per Watt emitted (that's a measure of thermal resistance, the lower the better as it means power can be dumped without raising temperature much). 24C/W is probably lower than can be reached using just the copper pad on the PCB surface, so there's a chance of the regulator overheating if the CPU is drawing its full current for too long. It'd be nice to see a heatsink on the regulator to definitely get the thermal resistance below 24C/W.
The resistors aren't actually handling much current, I think mA, so their power handling isn't a problem. They're just providing a feedback signal to the regulator.
Assuming I've gotten things right (I think the voltages based on Deskor's board look about right, it'll be good to see what the manual says), then you're right that the value don't need to be the same. I'd probably stay around the same values though, so keeping R1 between 100k and 200k. That said, the closest I can find to 3.3V using common resistor values is R1=56, R2=33, giving 3.34. So maybe 56k and 33k?