I agree with others that switchers are a better choice in terms of efficiency, but they can be somewhat complicated to deal with if you're inexperienced, and there can be lots of weird effects that aren't immediately obvious (precharge sinking, beat frequencies, etc.) that can make life difficult. Assuming you've figured out your power dissipation and know how much current each rail can deliver, if the linears will work for you, stick with them (at least for the first pass).
If you're trying to achieve a variable-amplitude square wave output on your adjustable rail, the chopping may introduce noise into the main 24V rail, which could show up on the other rails. You may want to have an LC filter between the main 24V rail and the regulator input to provide high-frequency isolation, and will probably need extra capacitance on the adjustable regulator output (bulk electrolytic as well as low-impedance ceramic) if you expect the square wave edges to be sharp.
1, 5) There are some dangers with your scheme.
Power dissipation in the linear regulators will be
\$(V_{out} - V_{in}) \cdot I_{out} \$
which is significant, especially for the lower output rails. 78xx-type regulators have built-in thermal protection around 125°C, and (without heatsinking) a junction-to-air thermal resistance of 65°C/W. Your thermal management will be challenging.
Another potential problem - if the series-pass element in any of your low-voltage regulators fails or gets bypassed (shorted), you'll present the full 24V input to the output. This could be catastrophic to low-voltage logic. You should protect your low-voltage rails with SCR crowbars that can sink enough current to put the DC/DC brick into current limit and collapse the 24V rail (they'll need big heatsinks too). Fuses are unlikely to be good protection since the 24V brick likely isn't stiff enough to generate the \$I^2 \cdot t\$ needed to blow a fuse.
2) Whatever floats your boat.
4) Meters aren't huge loads. Just use one of your rails.
3) Correct - all regulators have headroom requirements. If you want the maximum 24V out, you'll need a direct connection, and will have to rely on whatever intrinsic protections the brick will provide you.
Turn the problem around: Once you know R1, then R2 is easy to solve for. R1 is pretty easy to decide upon.
It is a bit of a balancing act, really, but here's how it goes:
- The datasheet suggests R1 be kept below 400 kOhms for stability. Lower the value of R1, higher the quiescent current required by the voltage splitter R1+R2. Higher the value of R1, higher the instability of the output.
- We know that the upper leg of R1 in the diagram is biased at 3.75 Volts for steady state.
- Hence, let us start with the maximum standard E12 series resistor value for R1 within the datasheet constraints, i.e. 390 KOhms
- IR1 can be calculated thus:
I = V / R = 3.75 / 390,000 = 9.61538 uA
- Current through R2 is given as the sum of current through R1, and bias current 150 nA. IR2 is thus
9.61538 - 0.15 = 9.46538 nA
- For a desired output voltage of 4.0 Volts, R2 must thus develop
4.0 - 3.75 = 0.25 Volts for the above current.
- Therefore
R2 = 0.25 / 9.46538e-6 = 26412 Ohms. Closest E12 value = 27 kOhms.
- Vo with R1 = 390 k and R2 = 27 k is 4.01367 Volts, less than 0.5% deviation from target voltage (assuming perfect resistor values, of course).
If stability is more desirable than saving quiescent current, try the above sequence with a starting value of R1 as 22 kOhms.
- IR1 = 170.455 uA
- IR2 = 170.305 uA
- R2 = 1468 Ohms, nearest E12 value 1.5 kOhms
- Vo = 4.00591 Volts.
Using the above calculation steps, choose any value for R1 as long as it is less than 400 kOhms, to obtain the value of R2.
Best Answer
This shows a common misconception about regulators. You can't control the current, and voltage at the same time, and this is why:
Think about your regulator like a water valve. The water flow is current, and the pressure is voltage. You can buy a regulator that controls current, or voltage; but either way it's just a "valve" and the most it can do is go all the way open, or all the way closed.
Switching regulators are more complicated, but we won't get into that now. Suffice to say, you can't control output current and voltage with those either.
So back to the "valve". From this analogy, you can see there's a few things your regulator simply cannot do.
Side note: A voltage regulator will also have a current rating. This doesn't mean it controls the current, it just means that if more than that current flows trough it, it will be damaged. You have to make sure that doesn't happen.