Introduction & Search Intent — Why “Energy First, Efficiency Second”
In intrinsic-safety (IS) supplies, the primary KPI is capped releasable energy (I×V×t) and safe temperature rise, not peak efficiency. A well-chosen LDO—paired with a protection stack and barrier—enables predictable current limiting, UVLO/OVLO windows, reverse/pre-bias safety, and thermal shutdown to remain non-igniting even under two-fault conditions.
- Audience intent: find IS-friendly LDOs with controlled ILIM modes (foldback/CC/hiccup), fast response, and barrier-friendly hooks.
- Decision lens: translate ILIM tolerance & response time into an I×V×t budget; size UVLO/OVLO & thermal margins first, efficiency second.
- Scope boundary: no deep audio-noise/PSRR or DDR/RF topics (covered on sibling pages).
Hazardous Area Power Constraints — I×V×t Window, Zone/Div, T-Class ↔ θJA
Translate standards into design knobs: express current limit modes and UVLO/OVLO as an I×V×t energy cap, map temperature classes (T1–T6) to ΔT = P·θJA, and validate under two-fault assumptions. This section provides the engineering lens—no certification claims; lab testing is still required.
- I×V×t budget: quantify with ILIM mode, tolerance, and response time; set UVLO/OVLO with hysteresis.
- Thermal margin: compute ΔT = P·θJA for steady vs. short-time faults; align with T-class limit.
- Two-fault: assume ILIM/TSD failures; the barrier and front-end must still bound energy.
Front-End Protection Stack — TVS / OVP FET / Surge & Reverse / Pre-bias & Backfeed
Sequence transients correctly: TVS clamps fast spikes, the OVP FET opens a controlled window, and reverse-path control prevents backfeed. These parameters must map to the IS I×V×t energy budget and to the LDO’s ILIM / UVLO / OVLO.
- TVS: choose VBR/VCLAMP/IPP compatible with the source and with OVP thresholds (clamp first, disconnect second).
- OVP FET: OVLO/UVLO + hysteresis; µs-scale response defines the energy window edge.
- Reverse / pre-bias: ideal-diode or back-to-back MOS; limit backfeed and enable soft, safe start.
LDO Topology for IS — Foldback / Constant-Current / Hiccup • SOA & Short-Circuit Power
Different current-limit behaviors shape the energy area and thermal stress. Pick the mode that caps the I×V×t window while meeting system reset and recovery needs; size UVLO/OVLO, soft-start, and reverse current limits accordingly.
- CC: predictable limit, higher steady thermal stress.
- Foldback: lowers average current and energy; check start/step-load compatibility.
- Hiccup: pulses energy; align with system reset cadence and logging.
Barrier / Isolation-Friendly Hooks — Zener Barriers & Downstream LDO Regulation
Barriers set the maximum transferable energy and withstand voltage; the LDO downstream provides a second cap using ILIM, UVLO/OVLO, reverse-current protection, and soft-start. Design for two-fault redundancy and avoid backfeed under pre-biased loads.
- Zener barrier: simple & low cost; energy dominated by series resistor and Zener dissipation.
- Isolation (transformer/digital/amp): high withstand and CMTI; respect creepage and clearance.
- Downstream LDO hooks: reverse-I limit, soft-start slope, UVLO/OVLO with hysteresis, predictable ILIM mode.
Selection Guide — Parameters & Thresholds
Choose the LDO by translating datasheet knobs into an I×V×t budget and thermal bounds. Prioritize predictable ILIM, usable UVLO/OVLO + hysteresis, robust reverse protection, and a thermal model consistent with T-class margins.
- VIN / VOUT / IOUT: cover worst case with barrier drop included.
- Iq: ensure standby energy doesn’t drift beyond the barrier window.
- Current limit (CL): mode (CC/foldback/hiccup), tolerance (±%), response time (µs/ms) → compute Emax.
- UVLO/OVLO + hysteresis: define the window, avoid edge chatter.
- Reverse protection: ideal-diode/back-to-back MOS; no backfeed under pre-bias.
- Thermal: ΔT ≈ P·θJA; distinguish steady vs short-time; align to T-class with TSD margin.
Validation Playbook (IS-Oriented)
Convert intrinsic-safety goals into a repeatable script: assume two faults, integrate cold/hot-start energy, log brown-out behavior, and verify reverse discharge is blocked. Pass when E = ∫ I·V·t stays within the window and thermal margins meet the target T-class.
- Two-fault matrix: ILIM high, TSD off, UVLO/OVLO err (pick two).
- Energy: integrate cold/hot start; report E_peak, t_rise, event timeline.
- Brown-out: log duration, recovery voltage/time, no chattering.
- Reverse path: block backfeed; provide a controlled discharge path.
Layout & Clearance
Keep creepage/clearance, isolate hot zones, and route sensing cleanly. Split barrier vs low-voltage domains, use Kelvin sense at the load, and place the RC filter where it won’t create an inner-loop pole/zero.
- Creepage/Clearance: keep a visible split; avoid slots/edges reducing distance.
- Thermal: move hot devices away from barrier; use copper spread and thermal vias.
- Partition: only cross the split via isolators; return paths short and single-point.
- Kelvin Sense: sense at the load; short, symmetrical pair; star to AGND.
- RC Filter: near sense pins; avoid creating an inner-loop pole/zero.
- Reverse Path: block backfeed; provide a controlled discharge (soft-off).
Mini IC Matrix — Cross-Brand Substitutions & Pin-Compat Tips
Focus on energy window (I×V×t), thermal (ΔT ≈ P·θJA), and threshold window (UVLO/OVLO + hysteresis). Prefer pin-compatible or near-window swaps to minimize IS re-validation time. Rows list concrete part numbers from the seven brands.
| Brand | PN / Series | Package / Pinout | VIN (min/max) | VOUT Range | IOUT Max | CL Mode | UVLO/OVLO (+Hys) | Reverse-I | Iq (typ) | θJA / TSD | IS Notes (Emax path) | Pin-Compat Group / Second-Source |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| TI | TL783 (HV adjustable) | TO-220 / TO-263 · LM317-style | ~15 V / up to ~125 V | Adj (external R) | ~700 mA | CC (linear) | — | External diode path recommended | — | θJA pkg-dep / TSD yes | Use for HV post-barrier rails; compute P=(VIN−VOUT)·I; short-time E via start profile | TO-220 LM317 group → Drop-in with adjust network |
| TI | TPS7A8300 (2 A, low-noise) | VQFN 3×3 / 4×4 | ~1.4–6.5 V | 0.8–3.6 V (adj) | 2 A | CC / foldback (datasheet-dep) | UVLO yes | Reverse block ext path | Low | TSD yes | Choose foldback for tighter Emax; validate hot-start pre-bias | QFN 3×3 group → Minor tweak |
| ST | LD1117 (1 A) | SOT-223 / DPAK | ~4.75–15 V | Fixed/Adj | 1 A | CC (classic) | — | Diode path recommended | — | TSD yes | Legacy but predictable; compute ΔT; check barrier drop | SOT-223/DPAK group → Drop-in |
| ST | LDBL20 (200 mA, tiny) | DFN 1×1 | ~1.5–5.5 V | Fixed | 0.2 A | Foldback (family-dep) | UVLO yes | — | Ultra-low | TSD yes | Use for small post-isolator rails; tiny θJA area → derate | DFN micro group → Small rework |
| Renesas | ISL80101A (1 A) | QFN | ~2.2–6 V | Adj/Fixed | 1 A | CC / foldback (family-dep) | UVLO yes | Reverse block ext path | Low | TSD yes | Well-behaved for reset-sensitive loads; log E_peak at start | QFN 3×3 group → Minor tweak |
| Renesas | ISL8013A (3 A) | QFN | ~2.2–6 V | Adj | 3 A | CC (fast) | UVLO yes | Ext path | Low | TSD yes | High I; evaluate ΔT vs T-class; consider foldback alt | QFN high-I group → Small rework |
| onsemi | NCP1117 (1 A) | SOT-223 / DPAK | ~4.75–20 V | Fixed/Adj | 1 A | CC | — | Diode path recommended | — | TSD yes | Classic post-barrier rail; compute ΔT and E_peak | SOT-223/DPAK group → Drop-in |
| onsemi | NCP785A (HV linear) | TO-252 | HV (family-dep) | Fixed | — | CC | — | Ext path | — | TSD yes | Use where HV margin is needed; validate surge + SOA | TO-252 HV group → Minor tweak |
| Microchip | MCP1799 (70 V, 100 mA) | SOT-223 / SOT-223-3 | ~4–70 V | Fixed | 0.1 A | CC / foldback (family-dep) | UVLO yes | — | Low | TSD yes | Great for HV post-isolator rails with small load | SOT-223 HV group → Drop-in |
| Microchip | MIC29302 (3 A LDO) | TO-263 / TO-220 | ~3–26 V | Adj/Fixed | 3 A | CC (fast) | — | Ext path | — | TSD yes | High current; confirm ΔT and E_peak; consider foldback alt if tight | TO-263/220 group → Minor tweak |
| NXP | VR5510 (PMIC; includes LDO rails) | QFN | Platform-dep | Multiple rails incl. LDO | — | Programmable (PMIC) | Configurable | Integrated policies | — | Diag/TSD | Use LDO rail downstream of barrier; map PMIC limits to Emax | System PMIC group → Small rework |
| NXP | PF5020 (PMIC; includes LDO) | QFN | Platform-dep | Multiple rails incl. LDO | — | Programmable | Configurable | Integrated | — | Diag/TSD | Consider when SoC already uses PF series; validate barrier window | System PMIC group → Small rework |
| Melexis | MLX81115 (LIN RGB driver; 5 V LDO inside) | SOIC / QFN | Automotive domain | Internal 5 V LDO | — | Integrated policies | Integrated | No backfeed via device path | — | Diag/TSD | Use downstream of barrier in LIN lighting nodes; external LDO optional | System node group → Small rework |
| Melexis | MLX81206 (BLDC driver; regulator inside) | QFN | Automotive domain | Internal regulator | — | Integrated | Integrated | — | — | Diag/TSD | For motor nodes; if not using internal LDO, pair with external IS LDO | System node group → Small rework |
Usage tips: For each candidate, compute E_max using its current-limit mode/tolerance/response and confirm ΔT against θJA. If swapping across brands, keep package group constant to stay “drop-in / minor tweak” and re-run cold/hot-start + brown-out tests.
FAQs — Intrinsic-Safety (Energy-First) for LDOs
How does current-limit mode affect Emax in intrinsic safety?
Emax depends on the limit waveform and timing. Constant-current keeps I high until UVLO/TSD acts; foldback reduces I as VOUT drops, shrinking ∫I·(VIN−VOUT)·dt; hiccup limits duty of stress by timed retries. Use the vendor’s tolerance and response times in the integration. See selection thresholds.
Where should UVLO/OVLO + hysteresis be set to avoid brown-out chatter?
Align UVLO slightly above the lowest safe VIN after barrier drops, and add hysteresis ≥2–5% of VIN to prevent oscillation. If the front-end OVP trips, ensure OVLO is coordinated so only one device “owns” the window. Validate with slow VIN ramps and step transients. See selection and validation.
How do I compare cold-start vs hot-start energy?
For cold-start, integrate I·(VIN−VOUT) from 0 to regulation during soft-start. For hot-start with pre-bias, verify reverse-current blocking and capture the shorter inrush profile. Report Epeak, trise, and any ILIM/UVLO events from the data logger. Match ambient and load to make A/B comparable. See validation.
Why not rely on a large output capacitor instead of soft-start?
A large capacitor can push inrush above the energy window and cause upstream sag or reverse discharge when rails collapse. A controlled soft-start shapes I and dV/dt, making Emax predictable and repeatable. Keep discharge paths explicit for safe power-down and brown-out recovery. See barrier hooks.
What guarantees reverse-current blocking under pre-bias?
Prefer devices with defined reverse-current limits or pair the LDO with an ideal-diode or back-to-back MOSFET. Test with a charged output capacitor and battery-like source. During hot-start, confirm zero backfeed and a controlled discharge path to avoid violating the barrier budget. See barrier hooks and layout.
What are the edges for high-voltage LDOs used post-barrier?
Confirm VIN max with margin to surges, then model SOA under short-time faults. Compute P=(VIN−VOUT)·IOUT and ΔT≈P·θJA for steady state, and evaluate thermal time constant for transients. Validate surge, reverse conditions, and clamp interactions with the front-end. See selection thresholds.
Can Iq slowly breach the energy window during standby?
Yes—long dwell with non-negligible Iq can integrate significant energy in tight budgets. Use Iq(max) at the worst temperature and include any housekeeping loads. For sleep modes, log cumulative I·V·t over realistic duty cycles and confirm headroom remains for start-up events. See selection.
How do I execute a two-fault test and define pass criteria?
Combine two independent faults (e.g., ILIM high and TSD disabled). Recreate cold/hot-start, short, and brown-out while logging I, VIN, VOUT, T. Pass if Epeak ≤ window, ΔT stays within T-class margins, and no uncontrolled reverse discharge occurs. Repeat for consistency. See validation.
Where should Kelvin sense and the RC filter be placed?
Sense directly at the load with a tight, symmetric pair to AGND. Place the RC near the sense pins so it filters noise without creating an inner-loop pole/zero. Keep returns short and avoid crossing barrier splits except through isolators. See layout and clearance.
What should brown-out logs include, and how is “no chatter” judged?
Log timestamps, depth and duration of VIN dips, recovery voltage/time, and any UVLO/OVLO toggles. “No chatter” means a single clean exit and re-entry into the window without repeated toggles within the hysteresis band. Correlate with load steps and temperature. See validation.
How do I triage drop-in vs minor-tweak vs small-rework substitutions?
Keep package and pin order when possible. Match ILIM mode/tolerance, UVLO/OVLO levels, and thermal path to maintain the same energy, threshold, and ΔT windows. Minor tweaks adjust Radj/soft-start. Small rework changes pad or pin sequencing and needs full re-validation. See Mini IC Matrix.
How do I design a safe discharge path for large output capacitors or batteries?
Provide a defined discharge element—soft-off circuitry or a bleed resistor sized for the energy and thermal limits. Ensure reverse-current protection prevents backfeed toward the barrier. Validate with power-down sequences and long hold-up scenarios across temperature. See barrier hooks and layout.
How do θJA and thermal time constant map to T-class margin?
Use ΔT≈P·θJA for steady operation and a first-order thermal model for short events. Compare peak temperature plus margin to the target T-class. Include board copper and vias, and check that repeated hiccup cycles don’t cause temperature ratcheting. See selection.
How do I split responsibilities between the barrier and the LDO?
The barrier enforces the primary energy and voltage withstand; the downstream LDO applies a second cap using ILIM, UVLO/OVLO, reverse protection, and controlled soft-start/shutdown. Validate timing so only one device governs each edge of the window. See barrier hooks.
Submit BOM (48h)
Small-batch validation and cross-brand substitution within 48 hours. We provide a drop-in and a near-window alternative, checked against Emax, ΔT, and UVLO/OVLO windows.
Submit BOM (48h)