Recipe

Choose a regulator for a board or module

Use when: a circuit needs a steady voltage from a supply or battery at a different voltage

Goal: an LDO or a switching regulator that keeps the output steady at the lowest input and stays cool at the highest

Taught in Voltage regulators

1. 1

   Note the output voltage, the largest current the load draws, and the lowest and highest input voltages the supply can really give.

   Example

   3.3 V at 200 mA from a lithium cell that runs from 4.2 V full down to 3.2 V near empty.

   Why: a regulator fails at the edges of its input range, not at the label value.

2. 2

   For an LDO, the lowest input must be at least V_{out} plus the dropout from the datasheet.

   Example

   3.3 V + 0.3 V = 3.6 V needed, but the cell reaches 3.2 V: the LDO drops out near the end.

   Why: below this point the output follows the input down.

3. 3

   P = (V_{in,max} - V_{out})\,I, and compare it with what the regulator's package can shed.

   Example

   (4.2 - 3.3) \times 0.2 = 0.18 W: comfortable.

   Why: the hottest case is the largest voltage drop at full current.

4. 4

   Little heat and enough headroom: an LDO. Too much heat, or a battery you want to drain fully: a switching regulator.

   • the heat is small and headroom holds — LDO.
   • heat is large, or efficiency decides battery life — buck (or boost, if the output must be above the input).

   Why: an LDO's efficiency is at best V_{out}/V_{in}; a switcher's is in its datasheet and rarely below 80%.