How it Works
A linear voltage regulator places a variable resistance (the pass transistor) in series between input and output. A feedback loop continuously adjusts this resistance so that V_out remains constant at the setpoint. The voltage difference (V_in − V_out) falls entirely across the pass transistor, which dissipates P_diss = (V_in − V_out) × I_load as heat.
The simulation shows: (left) efficiency vs load current curve — efficiency η = V_out/V_in, independent of load; (center) a thermal diagram showing junction temperature rise above ambient; (right) a heatmap showing how P_diss depends on V_in and I_load, with the 125°C limit marked.
η = V_out / V_in × 100%
T_j = T_amb + P_diss × θ_JA
V_out = V_ref × (1 + R1/R2)
Frequently Asked Questions
What is a linear voltage regulator?
A linear voltage regulator maintains a constant output voltage by varying the resistance of a pass element (transistor) in series with the load. It works by dissipating excess voltage as heat: P_diss = (V_in − V_out) × I_load.
What is an LDO regulator?
LDO stands for Low Drop-Out. An LDO regulator can function with very small difference between input and output voltage (as low as 100–300 mV), unlike older regulators that needed 2–3V headroom. This allows operation when supply voltage is close to the desired output.
What is dropout voltage?
Dropout voltage V_do is the minimum voltage difference V_in − V_out needed for the regulator to maintain regulation. Below this, the pass transistor saturates and output follows input. LDOs have V_do typically 100–500 mV.
How efficient is a linear regulator?
Efficiency η = V_out/V_in × 100%. A 5V regulator from 12V input is only 42% efficient. The rest is wasted as heat. This is the main disadvantage vs switching regulators which can achieve over 90% efficiency.
What is PSRR in a voltage regulator?
Power Supply Rejection Ratio (PSRR) measures how well the regulator rejects noise and ripple on the input. At DC, PSRR is typically 60–80 dB. It degrades at higher frequencies. LDOs generally have excellent PSRR at low frequencies compared to switching regulators.
What causes thermal shutdown in a regulator?
When the pass transistor junction temperature exceeds typically 150°C, a thermal shutdown circuit turns off the regulator. T_j = T_ambient + P_diss × θ_JA where θ_JA is the thermal resistance junction to ambient in °C/W.
What is the role of the feedback network?
A resistor divider from V_out feeds back to the error amplifier, which compares it to an internal bandgap reference (~1.2V). The error amplifier drives the pass transistor to maintain V_out = V_ref × (1 + R1/R2) constant regardless of load or input variations.
When should I use an LDO vs a switching regulator?
Use an LDO when low noise is critical (RF, audio, ADC supplies), the voltage difference is small, or simplicity matters. Use switching regulators when efficiency is critical, large voltage conversion is needed, or high currents are involved.
What is the quiescent current of an LDO?
LDOs have quiescent current I_Q flowing even at zero load (through error amplifier and bias circuits). This ranges from microamps in ultra-low-power LDOs to milliamps in older designs. Total I_in = I_load + I_Q, affecting efficiency at light loads.
How do I calculate the heatsink required for an LDO?
Power dissipated is P = (V_in − V_out) × I_load. With thermal resistance θ_JA, T_j = T_amb + P × θ_JA. If T_j exceeds 125°C, a heatsink is needed. Required heatsink thermal resistance: θ_SA = (T_j_max − T_amb)/P − θ_JC.