Linear regulators

Part 8 · Voltage Regulation

Key point: while \(V_{\mathrm{in}}\) stays above \(V_{\mathrm{set}}+V_{\mathrm{do}}\), the pass element holds \(V_{\mathrm{out}}\) at \(V_{\mathrm{set}}\). It still burns \(P_{\mathrm{heat}}=(V_{\mathrm{in}}-V_{\mathrm{out}})I\), so linear wins when that drop is small.

\(V_{\mathrm{in,min}}=V_{\mathrm{set}}+V_{\mathrm{do}}\). In dropout, \(V_{\mathrm{out}}=\max(0,\,V_{\mathrm{in}}-V_{\mathrm{do}})\). \(\eta=V_{\mathrm{out}}/V_{\mathrm{in}}\).

1
The pass FET holds 3.30 V.
Preset 12→3.3 (Part 2 rail). In regulation. VOUT is 3.30 V. Headroom is 8.70 V (Vdo 0.30 V). Vin,min is 3.60 V. Pheat is 4.35 W. η linear is 27.5%.
2
2.00 A. Heat is 17.4 W.
Preset Harsh 12→3.3 @ 2 A. In regulation. VOUT is 3.30 V. Headroom is 8.70 V (Vdo 0.30 V). Vin,min is 3.60 V. Pheat is 17.4 W. η linear is 27.5%. The rose trace is Pheat. It meets the 20 W line as VIN climbs past this marker. The figure shows 2.00 A.
3
7805. Vin,min is 7.00 V.
Preset Classic 7805. In regulation. VOUT is 5.00 V. Headroom is 7.00 V (Vdo 2.00 V). Vin,min is 7.00 V. Pheat is 7.00 W. η linear is 41.7%. The amber line is Vin,min.
4
Battery sag. Dropout.
Preset Battery sag. Dropout. VOUT is 3.20 V. Headroom is 0.25 V (Vdo 0.25 V). Vin,min is 3.55 V. Pheat is 100 mW. η linear is 92.8%. VIN is below 3.55 V. Pass FET fully on; VOUT follows VIN − Vdo.
5
Post-reg. Pheat is 340 mW.
Preset Post-reg 5→3.3. In regulation. VOUT is 3.30 V. Headroom is 1.70 V (Vdo 0.25 V). Vin,min is 3.55 V. Pheat is 340 mW. η linear is 66.0%.

Constant \(V_{\mathrm{do}}\) at this current. No PSRR, SOA, or thermal foldback. The error amp is Part 9. Above \(V_{\mathrm{in,min}}\), extra VIN becomes extra heat.

Pass element · dropout · heat

Gold marker is this VIN. Amber line is \(V_{\mathrm{in,min}}\).

\(V_{\mathrm{IN}}\) (V) \(V_{\mathrm{set}}\) (V) \(I_{\mathrm{load}}\) (A) Dropout \(V_{\mathrm{do}}\) (V)

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Series pass · VIN → P-FET → VOUT → load · rose glow is heat in the drop

\(V_{\mathrm{OUT}}\) vs \(V_{\mathrm{IN}}\) · Y fixed 0–12 V

Power vs \(V_{\mathrm{IN}}\) · teal load · rose heat · Y fixed 0–20 W

V_OUT—

Headroom—

V_in,min—

P_heat—

η linear—

\(V_{\mathrm{out}}=V_{\mathrm{set}}\)

→ —

\(V_{\mathrm{out}}=\max(0,\,V_{\mathrm{in}}-V_{\mathrm{do}})\)

→ —

\(P_{\mathrm{heat}}=(V_{\mathrm{in}}-V_{\mathrm{out}})I\)

→ —

\(\eta=V_{\mathrm{out}}/V_{\mathrm{in}}\)

→ —