Linear Regulators#

The simplest way to get a stable voltage: dissipate the difference between the available input and the desired output as heat. A linear regulator is a controlled variable resistance between the input and output that continuously adjusts to maintain a fixed output voltage regardless of load current and input voltage variations.

How They Work#

A linear regulator is fundamentally a feedback loop:

1. A reference voltage sets the desired output
2. A feedback network (voltage divider) samples the actual output
3. An error amplifier compares the feedback to the reference
4. A pass element (transistor) adjusts its resistance to keep the output at the target voltage

If the load draws more current, the output starts to droop. The error amplifier detects the droop and drives the pass transistor harder, reducing its resistance and restoring the voltage.

Dropout Voltage#

The minimum voltage difference between input and output required for the regulator to maintain regulation. This is the headroom.

Standard regulators (e.g., 78xx series): ~2 V dropout. A 7805 (5 V output) needs at least 7 V input.

LDO (Low-Dropout) regulators: 100-500 mV dropout. An LDO producing 3.3 V might work from 3.4 V input.

Why LDO matters: In battery-powered designs, every millivolt of dropout is wasted headroom. An LDO squeezes more useful life out of a battery by working closer to the minimum voltage.

What happens below dropout: The output follows the input minus the dropout. The regulator stops regulating and just passes the input through with a small drop. The output is unregulated and drops with the input.

Dissipation and Heat#

The power dissipated in the regulator:

P_diss = (V_in - V_out) x I_load

This is the core tradeoff. A 12 V to 5 V regulator at 500 mA dissipates (12 - 5) x 0.5 = 3.5 W. That’s serious heat — it needs a heatsink.

The efficiency: eta = V_out / V_in. For 12 V to 5 V, that’s 42%. The other 58% is heat. Worse, the efficiency drops as the input-output differential increases.

This is why linear regulators are impractical for large voltage drops at high current. A 24 V to 3.3 V conversion at 1 A dissipates 20.7 W. A switching regulator is needed for that (see Switching Regulators).

When linear regulators win: Small voltage drops, low current, or when the clean output (no switching noise) is worth the efficiency penalty. LDOs post-regulating after a switching regulator combine switching efficiency with linear quietness.

Noise and PSRR#

Linear regulators produce much cleaner output than switching regulators:

• No switching noise — There’s no clock, no switching transients, no fundamental ripple frequency
• PSRR (Power Supply Rejection Ratio) — How well the regulator suppresses input noise. Good LDOs provide 60-80 dB of PSRR at low frequencies, meaning input noise is attenuated by 1000x-10000x
• Output noise — The regulator’s own noise, generated by the reference and error amplifier. Specified in uV_rms, typically 10-100 uV_rms for standard parts, down to a few uV_rms for low-noise LDOs

PSRR degrades with frequency. At DC and low frequencies, PSRR is high. As frequency increases, the error amplifier’s loop gain drops and PSRR decreases. At high frequencies (above the loop bandwidth), the regulator provides little rejection. This is why output capacitor selection matters — the cap takes over filtering where the regulator’s active rejection drops off.

Output Capacitor Requirements#

Linear regulators need output capacitors for stability and transient response:

• Stability — Some regulators (especially older designs) require an output capacitor with minimum ESR for stability. Too little ESR = oscillation. Too much ESR = poor transient response
• Transient response — When the load suddenly changes, the output cap supplies (or absorbs) current until the feedback loop catches up. Bigger cap = less voltage excursion during transients
• Modern LDOs — Many recent LDOs are stable with low-ESR ceramic capacitors. Always check the datasheet — the stability requirements vary widely between parts

Thermal Design#

Since a linear regulator converts excess voltage directly to heat, thermal management is often the design constraint:

• Calculate P_diss for worst-case conditions (maximum V_in, maximum I_load)
• Look up junction-to-ambient thermal resistance (theta_JA) for the package and PCB layout
• Verify: T_junction = T_ambient + P_diss x theta_JA < T_max

See Thermal Reality for the thermal model details.

Tips#

• Use LDOs for post-regulation after a switching converter to get both efficiency and low noise
• Calculate worst-case power dissipation at maximum V_in and maximum I_load, then verify thermal design meets this
• Choose output capacitor type carefully — check the datasheet for ESR requirements to avoid oscillation
• For battery-powered designs, select LDOs with low quiescent current for sleep mode efficiency

Caveats#

• Dropout is not constant — Dropout voltage increases with load current. The datasheet spec is usually at full rated current. At light loads, dropout may be much less
• Quiescent current — The regulator itself draws current even with no load. Ranges from microamps (modern LDOs) to milliamps (older parts). This affects battery life in sleep modes
• Reverse current — If the output voltage ever exceeds the input (e.g., during power sequencing), current can flow backwards through the regulator, potentially damaging it. Some regulators have internal protection; many don’t
• Input capacitor matters too — The input cap provides instantaneous current during load transients and prevents input-side oscillation
• Ground pin current — The current flowing out the ground pin is the quiescent current, not the load current (for positive regulators). The load current flows through the pass element from input to output. For some LDO architectures, ground current increases with load — check the datasheet
• Thermal shutdown — Most regulators have thermal shutdown that turns off the output if the junction temperature gets too high. This protects the part but drops the output, potentially causing downstream problems. Design so thermal shutdown is never reached in normal operation

In Practice#

• A regulator that oscillates typically has the wrong output capacitor ESR — try a different capacitor type or add a small series resistor
• Output voltage below spec at high load indicates the regulator is in dropout — increase input voltage or reduce the input-output differential
• A regulator cycling on and off under load is hitting thermal shutdown — improve heatsinking or reduce power dissipation
• Output that follows input minus a small drop indicates operation below dropout — the regulator is not regulating