You picked a nice LM317 or LT1085 for your bench supply, but the load turned out to be 5 A and your regulator maxes out at 1.5 A. Rather than switching topologies, you can bolt on an external pass transistor and let the regulator IC handle the loop while a beefy PNP or P-channel MOSFET carries most of the current.

The classic circuit: Insert a small "sense" resistor R_SC between V_IN and the regulator's input pin. Tie a PNP power transistor (say a TIP42 or MJ2955) with its emitter to V_IN, collector to the regulator's output, and base to the junction of R_SC and the regulator input. When regulator input current rises high enough to drop ~0.6 V across R_SC, the PNP turns on and shunts additional current around the IC to the load.

How the loop stays honest: The regulator still senses its output pin voltage and adjusts its internal pass element. If the load pulls harder, the regulator tries to source more current, R_SC drops more voltage, and the PNP delivers proportionally more. The regulator effectively becomes the error amplifier for the composite pass device.

Sizing R_SC: Rule of thumb — pick R_SC so the PNP starts conducting at roughly half the IC's rated current. For a 1.5 A LM317, target turn-on at 750 mA:

• R_SC = V_BE(on) / I_threshold = 0.6 V / 0.75 A ≈ 0.82 Ω
• Power dissipation at 5 A total (with IC handling ~750 mA, PNP handling ~4.25 A): P_RSC ≈ (0.75)² × 0.82 = 0.46 W → use a 1 W resistor

Concrete example: A 12 V → 5 V @ 5 A supply. Voltage across the PNP is 7 V, so at 4.25 A it dissipates ~30 W. That transistor needs a serious heatsink (θ_JA under ~3 °C/W with a 25 °C ambient to stay below 125 °C junction). The LM317 dissipates only about 5 W — a modest TO-220 heatsink handles it.

Gotchas:

• No current limit on the PNP. The IC's internal foldback protects only its own path. Add a base resistor plus a small current-sense circuit if short-circuit survival matters.
• SOA matters more than average power. A dead short at 12 V into a saturated PNP will vaporize it long before thermal limits engage. Bipolars have secondary breakdown; MOSFETs handle SOA better for this trick.
• Stability: The added PNP adds a pole. Keep the output cap ESR in the range the regulator's datasheet requires, and add 10 nF from base to emitter of the PNP to damp oscillations.