Voltage Regulators

Every circuit needs stable supply rails. Learn linear regulators (series pass, LDO dropout, 78xx, LM317), efficiency and heat, thermal design, switching regulators (buck, boost, buck-boost) with duty cycle and inductor ripple, noise and layout, and choosing between linear and switching.

Batteries sag, adapters vary and chips need precise 3.3 V, 1.8 V or 0.9 V rails. Regulators use feedback to hold the output constant. Linear regulators are quiet but waste power as heat; switching regulators are efficient but noisier and more complex.

An analogy

A linear regulator is like a tap you keep adjusting to keep the flow steady — the excess pressure is simply wasted. A switching regulator is like filling a bucket in quick bursts and letting a buffer smooth it out — much less waste.

Linear regulators

A pass transistor drops the excess voltage under control of an error amplifier comparing the output with a reference. 78xx fixed, LM317 adjustable, LDOs (low dropout 0.1–0.3 V) for battery systems. Clean output, low noise, but all excess voltage becomes heat.

Thermal design

Dissipation × thermal resistance gives temperature rise. SOT-23 ≈ 200 °C/W, DPAK ≈ 50 °C/W on copper, TO-220 with heatsink ≈ 5–20 °C/W. Keep T_J below ≈ 125 °C with margin.

Switching regulators

A MOSFET switches at 100 kHz–3 MHz; an inductor and capacitor filter the pulses. Buck steps down (D = V_out/V_in), boost steps up, buck-boost/SEPIC either way. Efficiencies 85–95 %.

Noise, layout and control

Hot loops with fast di/dt must be tiny; ground planes and input capacitors close to the switch. Output ripple from ESR and ΔI_L. Control modes: voltage mode, current mode, constant-on-time; compensation for stability. Post-regulate noisy rails with LDOs for ADCs and RF.

Formulas

• Linear dissipation: P = (V_in − V_out) I_out, η ≈ V_out/V_in
• Junction temperature: T_J = T_A + P θ_JA
• LM317 output: V_out = 1.25 V (1 + R₂/R₁)
• Buck duty / ripple: D = V_out/V_in, ΔI_L = (V_in − V_out) D / (L f_sw)
• Boost duty: D = 1 − V_in/V_out

Worked example: Linear vs buck

Given: 12 V → 5 V at 0.5 A

Find: Losses.

• Linear: P = 7 × 0.5 = 3.5 W, η = 41.7 %
• Buck at 90 %: P_in = 2.78 W, loss 0.28 W

Answer: 3.5 W vs 0.28 W wasted

Worked example: LM317 and thermal check

Given: R₁ = 240 Ω, R₂ = 720 Ω; 3.5 W, θ_JA = 50 °C/W, T_A = 25 °C

Find: V_out and T_J.

• V_out = 1.25 × (1 + 3) = 5.0 V
• T_J = 25 + 3.5 × 50 = 200 °C

Answer: 5.0 V but 200 °C — needs a heatsink or a switcher

Common mistakes

• Wrong: Using a linear regulator for large drops at high current. — Right: Check dissipation; use a switcher.
• Wrong: Long switcher traces. — Right: Keep hot loops short; follow reference layouts.

Applications

• Battery devices: LDOs and bucks for long battery life.
• Processors: Multi-phase bucks at < 1 V and 100+ A.
• Precision analog: Low-noise LDOs for ADCs and RF.

Key takeaways

• Linear: simple, quiet, P = ΔV × I.
• T_J = T_A + Pθ_JA.
• Buck D = V_out/V_in; boost D = 1 − V_in/V_out.
• Switchers: efficient, mind layout and noise.

Going further

• Current-mode control: Inner current loop for easy compensation.
• PSRR: Supply rejection of LDOs vs frequency.
• EMI filtering: Input filters and spread-spectrum clocks.