Yesterday we covered BJT series-pass boosters for extending LDO current capability. Today we tackle the MOSFET version — which solves the fundamental problem that killed BJT boosters at high current: base current wastes power and drops out at low V_IN-V_OUT. A P-channel MOSFET booster draws essentially zero gate current, so 100% of the pass current comes from the booster and none is stolen from the LDO's output.

The circuit is deceptively simple: a P-MOSFET in parallel with the LDO, with its source at V_IN, drain at V_OUT, and gate pulled to V_IN through a sense resistor R_SENSE that sits in series with the LDO's input pin. When LDO current through R_SENSE creates enough voltage drop to exceed the MOSFET's V_GS(th), the FET turns on and shunts the excess current around the LDO. The LDO still regulates V_OUT through its feedback loop — the MOSFET just handles the bulk current.

Design rule of thumb: Size R_SENSE so the MOSFET starts conducting at roughly 60-80% of the LDO's rated current. For an LM317 rated at 1.5A with an IRF9540 (V_GS(th) ≈ 2-4V, use 2.5V for design):

• R_SENSE = V_GS(th) / I_turn-on = 2.5V / 1.0A = 2.5Ω
• At 10A total load: V_drop across R_SENSE = 2.5V (this is your dropout penalty!)
• Power in R_SENSE at 10A = I²R = 100 × 2.5 = 250W — clearly wrong

That calculation exposes the trap: a fixed R_SENSE burns catastrophic power at high current. The real circuit uses a small R_SENSE (~0.1Ω) plus a small-signal PNP transistor as a current-sense amplifier that drives the P-FET gate. The PNP's V_BE (0.6V) triggers gate drive at I = 0.6/0.1 = 6A, and the PNP provides gate current gain, so gate turn-on is sharp and R_SENSE power stays under 4W at 10A.

Real-world example: Bench supplies rated 0-30V/10A often use exactly this topology — an LM723 or discrete regulator handles the feedback loop and drives a 2N2907 sense transistor that gates a bank of paralleled TIP36C or IRFP9240 pass elements. This is why old Kepco and HP linear supplies are so reliable: the regulator IC never sees more than a few hundred milliamps.

Watch out for: The MOSFET has no current limit protection from the LDO — you must add external foldback or a current-limit transistor. Also, the MOSFET's V_GS(th) has a strong negative tempco (-6 mV/°C), so current-sharing between parallel FETs requires source ballast resistors, just like we discussed with parallel MOSFETs.