(1) NO regulator needed?
IF you mean that the motor can run on any voltage from 3.5 to 6.4 then Vbattery of 3.6 to 4.2 is always in the range and you do not need a regulator. ie
- Vbat_min < Vmotor_Min < Vbat_max < Vmotor_max
(2) Buck Boost:
If you mean that you want to be able to set a motor voltage anywhere in the range 3.5V to 6.4V at any time regardless of battery voltage then you need a buck-boost converter. A boost only or buck only converter will not work in all cases.
If maximum efficiency is not crucial (and it may not be for 3 seconds of opoeration) Then the 48 cents MC34063 data sheet here will do your job. See fig 17 for one of several ways to achieve buck boost - efficiency can be better than what they say.
For better efficincy the eg TPS63060 is an example of an IC that uses syncronous rectification data sheet here About $4/1 at Digikey. This can provide over 1A out in buck or boost mode.
Example circuit only:
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.
Best Answer
Yes, but it isn't the best way. There will be losses with the LDO. Use an adjustable DC-DC converter to go from 12V directly to 4.2V. TI makes lots of parts that are easy to design around.
This will be too slow. By the time the overvoltage is detected, it's likely your camera will be dead. There are several questions about overvoltage protection on here already, do a search.
That is reasonable. I would recommend a high side switch on the voltage divider so you can turn it off when not taking a measurement. Otherwise, it's a constant load on the battery that will slowly drain it.