A linear regulator is a voltage regulator that controls output voltage by continuously adjusting the impedance of a pass element (typically a BJT or MOSFET) connected between input and output, dissipating the excess voltage as heat. It produces a stable DC output voltage that is lower than its input voltage by at least the required dropout voltage.
In practice
Linear regulators appear in nearly every embedded system, most commonly to supply a clean, low-noise rail for sensitive analog circuitry, reference voltages, or RF front ends. Classic fixed-voltage parts like the 7805 (5 V, 1 A) and the LM317 (adjustable) have been in use for decades. Low-dropout (LDO) variants such as the AMS1117, MCP1700, and TLV700x series are more common in modern designs because they function with smaller input-to-output differentials, sometimes as low as 100-200 mV, which matters when stepping down from a Li-ion cell (nominally 3.7 V) to a 3.3 V rail.
Efficiency is the primary drawback. The power dissipated in the pass element equals (V_in - V_out) multiplied by the load current. Dropping 5 V at 500 mA means 2.5 W turned into heat, which requires careful thermal management: heatsinking, package selection (TO-220 vs. SOT-23), and PCB copper pour. Battery-powered designs with significant voltage headroom or heavy loads typically favor switching regulators for this reason. Linear regulators are still preferred when load currents are low, when switching noise would compromise ADC or RF performance, or as a post-regulator after a switcher.
Output capacitor selection matters more than many designers expect. LDOs in particular can be conditionally stable depending on the ESR of the output capacitor. Older LDO designs required a minimum ESR (typically 0.1-10 ohms) for stability, meaning low-ESR ceramic capacitors would cause oscillation. Many modern LDOs are ceramic-capacitor-stable, but the datasheet's recommended capacitor type and value should always be followed. Placing the capacitor close to the regulator's output pin keeps parasitic inductance low and improves transient response.
Quiescent current (IQ) is a critical spec in battery-operated systems. Standard LDOs can draw anywhere from tens of microamps to several milliamps of quiescent current regardless of load, while ultra-low-IQ parts such as the TPS7A02 or MAX8893 draw microamps, which matters significantly when the system spends most of its time in sleep mode. For more on bypass capacitor placement and role, the EmbeddedRelated post "The Other Kind of Bypass Capacitor" provides useful context.
Frequently asked
What is the difference between a standard linear regulator and an LDO?
A standard linear regulator commonly uses an NPN
BJT or N-channel
MOSFET as the pass element and requires a relatively large input-to-output voltage difference (often 2-3 V) to regulate properly. An
LDO (low-dropout) regulator often uses a PNP BJT or P-channel MOSFET, though topology and process vary across vendors; what defines the LDO class is that the minimum required headroom can be as low as a few hundred millivolts. For systems running from a single Li-ion cell or regulated bus close in voltage to the target rail, the LDO's lower dropout is essential.
When should I choose a linear regulator over a switching regulator?
Prefer a linear regulator when: load current is low (tens of milliamps or less) so thermal dissipation is manageable; output noise must be minimal, as for
ADC references, PLLs, or
RF circuits; board space or BOM cost rules out the
inductor and extra passives a switcher needs; or as a post-regulator to clean up a switcher's output. Prefer a
switching regulator when efficiency matters, input-to-output differential is large, or load currents are high.
How do I calculate the power dissipated in a linear regulator?
P_dissipated = (V_in - V_out) x I_load, plus V_in x I_Q where I_Q is
quiescent current. For example, regulating from 9 V to 3.3 V at 200 mA dissipates (9 - 3.3) x 0.2 = 1.14 W in the pass element. That heat must be conducted away through the package, PCB copper, or an external heatsink. Check the regulator's thermal resistance (theta_JA) and maximum junction temperature to confirm the design is safe across the full ambient temperature range.
Why can ceramic output capacitors cause instability in some LDOs?
Many older
LDO topologies use the output
capacitor's ESR to introduce a zero in the loop response, which is necessary for phase margin. Ceramic capacitors have very low ESR (often under 10 milliohms), which removes that zero and can cause the regulator to oscillate. Always check the datasheet's stability requirements. Modern LDOs designed for ceramic capacitors include internal compensation to handle low-ESR loads, but substituting a different capacitor type or value without verifying stability is a common source of subtle power rail problems.
What is quiescent current and why does it matter?
Quiescent current (I_Q) is the current the regulator itself consumes independent of the load, flowing from input to ground through internal bias circuitry. In always-on or low-power systems where the
MCU spends most of its time sleeping, I_Q can dominate the average current draw. A regulator with 2 mA of I_Q continuously drains about 48 mAh per day, which is significant for coin-cell or small LiPo designs. Ultra-low-IQ LDOs (1-10 uA range) are available from multiple vendors and are worth selecting specifically for battery-powered applications.
Differentiators vs similar concepts
Linear regulators are often contrasted with switching regulators (buck, boost, buck-boost). Switching regulators transfer energy primarily through an
inductor using a switched waveform, achieving efficiencies of 80-95% or more but generating switching noise and requiring more external components. (Charge-pump converters transfer energy via
capacitors but are a distinct special case not typical of most embedded power designs.) Linear regulators are simpler, quieter, and cheaper in low-current applications, but waste the input-output voltage difference as heat, making them inefficient when headroom or load current is large. Within the linear regulator family, standard regulators (e.g., 7805 series) are distinguished from LDOs by their pass element topology and minimum required dropout voltage.
