JanThere are some laws that just can't be bent. Ohms law is one of those. It dictates the law of conservation of power, which means: P_in = P_out.
So, you can't just expect the small linear regulator in a SOT23-5 case to convert 40V to 5V and 100mA without getting hot.
The power dissipated by the regulator will be:
With a full 8S (yes, I want to extend to this cell-count) battery at 33.6V this means around 0.3W of power which the regulator needs to get rid off.
SOT-23 cases feature very bad thermal design with around 170°C heat-up per W dissipated power. That's the "Junction to ambient thermal resistance" value in the datasheet. The value is so high because the plastic case has no metallic pad which could transfer the heat into the PCB much more efficiently.
So, 0.3W x 170°C = 51°C heat on top of the ambient temperature. This would be okay on paper but it creates a hot-spot on the PCB which spreads to other parts which have a drift with temperature...
The paper linked above has good design techniques for this style of case. I'll definitely try that anyway as the DC/DC chips I want to try are in that case too. Richtek has some good info on this as well.
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I remember doing one of those Internet courses on switching mode power supply design. One slide had types of components and succinct summary of efficiency:
Resistor ✘ power loss
Capacitor ✔ but cannot lower voltage
Inductor ✔ definitely
Active component, e.g. transistor ✔ definitely
I see that course https://www.coursera.org/specializations/power-electronics is still being offered.
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Yes, that's why I ordered a test-board for a DC/DC circuit I want to try: https://hackaday.io/project/164742-universal-lipoli-ion-uvlo-undervoltage-lockout/log/165041-06-new-dcdc-buck-design
Those aren't too efficient in the lower current region, though. Some behaviors need to do be tested under live conditions :)
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BTW strictly speaking Ohm's law isn't about conservation of energy. It just connects voltage, current and resistance.
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True, strictly speaking I should've written "laws of thermodynamics"... What I really meant was: there is no magic way of transforming higher to lower voltages at current xy without having to deal with the heat generated by the "conversion"...
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