PTN78000WAH Power Module Design Guide for 24V Industrial Step-Down Rails (2026)

Bottom Line: When selecting a wide-input DC-DC power module for 24 V industrial step-down conversion, the PTN78000WAH from Texas Instruments delivers 6 A across a 7–36 V input range with output adjustable from 0.9 V to 5.5 V using a single resistor — making it the workhorse choice for 24 V → 3.3 V or 24 V → 5 V point-of-load (POL) rails on factory floor controllers, motor drive auxiliaries, and telecom shelf supplies. Three design factors dominate every PTN78000WAH layout: (1) input filter sizing for 24 V rail transients, (2) the Rset resistor that programs the output voltage, and (3) thermal anchoring of the SIP package through PCB copper pour. Get those three right and the module's 94% peak efficiency translates to a quiet, drop-in solution that needs no compensation network and no external switching components.

This application note walks through the design considerations, recommended topologies, and common pitfalls for industrial step-down rails using the PTN78000WAH and its closely related family members.

Wide-Input Range and Transient Tolerance

The 7–36 V input range of the PTN78000WAH covers the entire IEC 61131-2 industrial 24 V DC supply class, including the +20% / –15% steady-state tolerance band (20.4 V to 28.8 V) plus the surge and brownout margins typical of factory automation environments. This eliminates the need for a pre-regulator stage that 5–28 V class modules would require for 24 V industrial use.

The trade-off is larger input filter design. At 36 V max input, ripple voltage sees a wider gap between supply and the module's internal switching reference, which translates to higher peak inductor current during input-side transients. The PTN78000WAH datasheet (SLTS269) specifies a minimum 100 µF aluminum electrolytic at the input pin, plus a 10 µF X7R ceramic for high-frequency decoupling. For 36 V applications, derate the electrolytic to 50 V working voltage minimum.

Surge protection is the integrator's responsibility. A 1.5KE36CA TVS diode at the input pin clamps 24 V supply transients to safe levels (per IEC 61000-4-5 surge immunity test). Without surge protection, a single rail-side switching transient on a contactor coil can latch the module's input UVLO and force a power cycle.

Output Voltage Programming via Rset

The PTN78000WAH outputs an internal 0.9 V reference and uses a single external resistor (Rset) connected from the SET pin to GND to program any output between 0.9 V and 5.5 V. The formula from the datasheet is:

Rset = 10 kΩ × (0.9 V / (Vout - 0.9 V)), with Rset placed on a star-grounded pad at the module pin.

Common output settings:

Vout	Rset (kΩ)	Use Case
1.2 V	22.5	FPGA core rail
1.8 V	10.0	DDR memory I/O, MCU low-voltage
2.5 V	5.6	Legacy logic, intermediate rail
3.3 V	3.74	Microcontroller VDD, sensor analog
5.0 V	2.20	USB host, op-amp supply

Use 1% tolerance metal-film resistors. The output voltage tolerance contribution from a 1% resistor is approximately ±1.4% on the regulated output, which stacks with the module's ±2% reference accuracy for a worst-case ±3.4% total — acceptable for digital rails but tight for analog ADC reference applications. For ADC reference rails requiring better than 1% accuracy, post-regulate with a precision LDO or use a dedicated reference like the REF2030AIDDCR.

Output Capacitor Sizing and Transient Response

The PTN78000WAH integrates the output inductor and FETs but requires external output capacitance to set transient response. The datasheet specifies a minimum 100 µF aluminum electrolytic plus a 10 µF X7R ceramic at the output. For loads with fast transient steps (FPGA core supply, switching ASICs), increase the ceramic content:

• Static load: 100 µF electrolytic + 10 µF ceramic — meets datasheet minimum
• Moderate transient (≤1 A/µs): 100 µF electrolytic + 22 µF + 10 µF ceramics
• Aggressive transient (≥3 A/µs): 100 µF electrolytic + 47 µF + 22 µF + 10 µF, plus 0.1 µF at each load decoupling pin

The module includes TI's TurboTrans® technology — an external pin (TT) accepts an optional resistor to ground that boosts loop bandwidth and improves transient response by ~2× at the cost of slightly higher quiescent current. For most industrial rails where the load steps are slow (under 100 mA/µs), leave TT open and accept the default response.

Thermal Anchoring of the SIP Module

The PTN78000WAH ships in a 5-pin SIP (single in-line package) — physically a vertical PCB-mount module with through-hole leads. Power dissipation at full load is roughly (1 − η) × Pout = (1 − 0.94) × 6 A × 3.3 V ≈ 1.2 W under nominal conditions, which the SIP package dissipates through its leads into the PCB copper pour.

Three layout rules apply:

1. The two ground leads (pins 2 and 3) must connect to a copper pour of at least 1 cm² on the same layer, with thermal vias to inner ground planes. Traces alone do not dissipate enough heat.
2. Maintain ≥3 mm clearance between the module and adjacent components on the PCB. The SIP body reaches ~75 °C at +25 °C ambient under full load.
3. For ambient temperatures above +60 °C, derate output current per the datasheet thermal curve — at +85 °C ambient, max continuous output drops to ~3.5 A.

For surface-mount alternatives where SIP through-hole is impractical, see the LMZ31710RVQT — a 10 A LMZ31 module in a QFN footprint that fits low-profile chassis.

Recommended Solutions

The PTN78000W series and its PTH08 cousins cover most industrial 24 V step-down topologies. Pick by output voltage, current, and form factor:

Module	Vin	Vout	Iout	Package	Best For
PTN78000WAH	7–36 V	0.9–5.5 V (adj.)	6 A	SIP-5 TH	Default 24 V industrial POL
PTN78000HAH	7–36 V	11.85–22 V (adj.)	1.5 A	SIP-5 TH	Step-down to logic intermediate rails
PTH08000WAH	4.5–14 V	0.9–5.5 V (adj.)	5 A	SIP-5 TH	12 V rail step-down (lower voltage class)
PTH08T231WAZ	4.5–14 V	0.7–3.6 V (adj.)	6 A	SMT module	Surface-mount 12 V step-down
PTH08T250WAZ	4.5–14 V	0.7–3.6 V (adj.)	50 A	SMT module	High-current FPGA core supply
LMZ31710RVQT	4.5–17 V	0.6–6 V (adj.)	10 A	QFN module	Modern SMT replacement

Default recommendation: PTN78000WAH for 24 V industrial. Step up to PTH08T240WAD or PTH08T250WAZ when output current exceeds 6 A or when surface-mount assembly is required. Step down to LMZ31710RVQT for new designs where the LMZ31 family's smaller QFN footprint and lower BOM cost matter more than backward compatibility.

For broader switching regulator selection across topologies (modules vs. discrete IC + inductor), see our switching voltage regulator selection guide.

Common Pitfalls

1. Floating SET pin with no Rset. The module powers up at the internal 0.9 V reference, which most loads will misinterpret as undervoltage. Always populate Rset, even for the rare 0.9 V output case (use a low-value resistor or short SET to GND through a shunt).
2. Undersized input electrolytic. Below 100 µF, the input ripple at 36 V exceeds the module's specified ripple rejection and triggers internal current limit during heavy load steps. Symptom: random output dropouts under transient load.
3. Skipping the TVS diode on input. Industrial 24 V rails carry surge events from contactor coils, motor brakes, and switched-mode loads. A failed module post-surge often shows blackened input pins and burned PCB copper near the input pad.
4. Inadequate ground copper pour. Connecting the two ground pins through traces alone leads to thermal runaway above 50 °C ambient. The module's lead temperature climbs to 110 °C and the internal switch shuts down on overtemperature.
5. Using oversized output ceramics without electrolytic. Pure ceramic output capacitance can underdamp the loop and cause oscillation. Always include the specified aluminum electrolytic alongside the ceramics.

FAQ

What is the difference between the PTN78000WAH and PTH08000WAH? The PTN78000WAH accepts a wide 7–36 V input range, making it suitable for 24 V industrial supplies. The PTH08000WAH covers 4.5–14 V, targeting 12 V rails. Both produce 0.9–5.5 V output but at different current ratings (6 A vs 5 A). For 24 V applications, the PTN78000WAH is the correct choice; using the PTH08000WAH on a 24 V rail will cause immediate damage.

Can I use the PTN78000WAH on a 48 V telecom rail? No. Maximum input is 36 V. For 48 V telecom supplies, use a separate 48 V → 12 V or 48 V → 24 V intermediate bus converter, then feed the PTN78000WAH from the intermediate bus.

Why is the PTN78000WAH still in production despite being a 2009-era part? TI maintains the PTN78000W family under its product longevity program with extended availability commitments — important for industrial customers with 10–15 year support cycles. As of 2026, the part has no announced obsolescence and lead times typically range 8–16 weeks through authorized distributors.

How does TurboTrans® affect quiescent current? With TT pin open (default), no-load quiescent current is approximately 8 mA. With TT enabled via a resistor to ground, quiescent rises to 12–15 mA depending on the resistor value. For battery-backed industrial loads where standby current matters, leave TT open.

Where can I source the PTN78000WAH during allocation periods? Submit an RFQ on FindMyChip — we cross-check live availability and lead times across 200+ verified distributors and return ranked sourcing options within 24 hours, with 5-point authentication on every shipment. For real-time stock checking on alternate variants, use our search index to confirm availability before committing your BOM.

Conclusion

The PTN78000WAH remains a production-proven choice for 24 V industrial DC-DC step-down rails despite its decade-plus age. Its wide input range, integrated magnetics, and single-resistor output programming make it the lowest-effort path from a 24 V industrial supply to a 3.3 V or 5 V POL rail. Where the SIP form factor or 6 A current ceiling don't fit modern designs, the PTH08T family (higher current, surface-mount) and LMZ31 family (smaller QFN, modern silicon) offer drop-in upgrades that share the same design principles covered above.

Submit your RFQ to confirm pricing and stock across the PTN78000W and LMZ31 families before locking your industrial power BOM.