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A linear regulator (like the 7805) works by dropping the difference between input and output across an internal pass transistor. That voltage difference × the load current becomes heat. Because the input and output currents are almost equal, the efficiency is roughly Vout/Vin — so a big input-to-output gap wastes a lot of power.
| Quantity | Formula |
|---|---|
| Power dissipated | Pdiss = (Vin − Vout) × Iout + Vin×IQ |
| Efficiency | η = (Vout×Iout) / (Vin×Iin) |
| Junction temp | TJ = TA + Pdiss×θJA |
| Max load current | Iout(max) = (TJ(max)−TA)/(θJA×(Vin−Vout)) |
To carry more current, add a heat sink (lower θ), reduce the input-output gap, or switch to a buck converter for high-power designs.
Linear regulators are cheap, quiet, and simple but waste the voltage difference as heat. Switching (buck) regulators are >90% efficient but noisier and more complex. Use linear for small drops/low current, switching for big drops or high power.
Yes — typically 0.33 µF on the input and 0.1 µF (often plus 10 µF) on the output to stay stable and quiet. LDOs are pickier and specify a minimum output capacitor and ESR range in the datasheet.
A Low-DropOut regulator works with a much smaller input-output difference (often <0.3 V versus ~2 V for a 78xx), so it wastes less power and suits battery circuits.
Not directly — small differences make one hog the load. Use a single higher-current regulator, add a pass transistor, or use a design made for current sharing.
All the input-output voltage difference times the load current turns into heat. 12V→5V at 0.5A dissipates 3.5 W — that needs a heat sink.
The minimum Vin−Vout the regulator needs to keep regulating (≈2 V for a 78xx, <0.5 V for an LDO). Below it, the output sags.
When efficiency or heat matters. A linear regulator wastes the voltage difference as heat; a switching (buck) converter can be >90% efficient.
P = (Vin − Vout) × Iout. Dropping 12 V to 5 V at 1 A dissipates 7 W in the regulator, which is considerably more than the 5 W delivered to the load.
The minimum input-to-output difference at which the regulator still regulates. A standard 7805 needs about 2 V; a low-dropout type may need only 100–300 mV, which matters when running from a battery whose voltage sags.
When the input-output difference or the current is large enough that the dissipated power becomes a thermal problem. Linear regulators suit low dropout, low current, or noise-sensitive rails where their low output noise is an advantage.
Usually because the output capacitor is unsuitable. Many LDOs require a minimum ESR for loop stability, so replacing a specified tantalum with a very low-ESR ceramic can cause oscillation. Check the datasheet stability region.
Approximately Vout/Vin, since the current in and out are nearly equal. Dropping 12 V to 5 V is at best about 42% efficient regardless of load.
Yes. The input capacitor supplies transient current and stabilises the source impedance; the output capacitor sets transient response and, for many LDOs, loop stability. Both values and types should follow the datasheet rather than general habit.