Introduction — Scenario & Operating Stresses
This page focuses on automotive LDO behavior under three real events: cold-crank dips, load-dump surges (after TVS clamp), and reverse polarity/current. The design viewpoint is the LDO input pin, i.e., the post-TVS residual actually seen by the regulator.
The protection path is Vehicle bus → TVS clamp → LDO → regulated rail. ASIL-relevant pins (PG/RESET/EN) provide diagnosable power-up semantics to the MCU.
ISO 16750-2 Pulses — Interpreted at the LDO Pin
Vehicle-level pulses are defined on the bus; the LDO should be selected and validated against the post-TVS residual at its input pin. Compare that residual against Absolute Maximum VIN, the Recommended Operating VIN, and your dropout window.
Quick execution checklist
- Measure/derive the post-TVS residual at the LDO input (peak, plateau, width).
- Compare against Abs Max VIN and recommended operating range; ensure margin.
- Validate regulation across the dropout window at load and temperature corners.
- Log PG/RESET timing and any false-positive events during pulse playback.
Cold-Crank (3–6 V) — Hold-Up via Capacitance, Pre-Reg Assist & RESET Hysteresis
During cold crank the vehicle bus can dip into the 3–6 V window. Design from the post-TVS residual at the LDO input, then ensure continuity by combining storage capacitance, pre-regulator assist (boost/SEPIC/buck-boost), and RESET/PG thresholds with hysteresis.
Quick execution checklist
- Measure the post-TVS minimum VIN at the LDO pin across cables/connectors and temperature.
- Compute t_hold and estimate C_min; verify stability window vs ESR at −40/25/85 °C.
- If LDO alone cannot ride through, add a pre-reg (boost/SEPIC/buck-boost) to stay above dropout.
- Set RESET/PG thresholds and hysteresis to avoid chatter; log false-positive events.
Load-Dump — TVS Clamp, Series/π Filter, and LDO Headroom
Interpret ISO 16750-2 pulses at the LDO input: the post-TVS residual is what matters. Choose the TVS for energy and clamp level, use series/π filtering to shape the residual, and verify margin to the LDO’s Absolute Maximum VIN and recommended range.
Quick execution checklist
- Choose TVS for energy & clamp level; check thermal headroom at worst case.
- Use series/π filter to shape the residual; balance relief vs added drop/dynamic response.
- Confirm residual < Abs Max VIN with design margin; ensure it lies in the recommended range.
- Playback the residual; log PG/RESET timing, false-positive rate, temperature rise, and recovery time.
Reverse Protections — Reverse Battery vs Reverse Current
Distinguish reverse battery (VIN < 0 during jump-start or miswiring) from reverse current (VOUT > VIN due to parallel rails, pre-charge, or downstream energy sources). Choose blocking elements accordingly: internal MOSFET, external back-to-back FET, or a Schottky clamp. Coordinate EN/UVLO/PG so start-up order does not create backfeed.
Quick execution checklist
- Measure I_rev with VIN low and VOUT elevated; log peak and duration.
- For back-to-back FETs, verify gate control vs EN/UVLO timing and thermal headroom.
- Confirm start-up order: source stable → EN high → check PG/RESET without chatter.
- If using Schottky, estimate power loss & hot-spot; add copper area/thermal vias.
ASIL-Relevant Pins — PG / RESET / Watchdog / Monitor
Build a pin function matrix with polarity, electrical level, timing, and diagnostic semantics. Keep PG (power-good) and RESET independent at the MCU, and define a watchdog and monitor (MON/FLT/ERR) plan that can be exercised during cold-crank/load-dump playback without false trips.
Quick execution checklist
- Define polarity and level (open-drain vs push-pull, pull-up voltage) for PG/RESET/WD/MON.
- Measure tPG and tRST under cold-crank/load-dump playback; record false-positive rate.
- Route PG and RESET to separate MCU pins/clock domains; add periodic self-test.
- Confirm watchdog window and monitor thresholds match the rail’s ramp/hold behaviors.
Layout — Short High-dV/dt Loops, Source-Side TVS, Thermal Spreading, Clean References
Keep the high-dI/dt / high-dV/dt loop minimal, place the TVS at the source, spread heat with copper + vias, and route PG/RESET/monitor to the same ground reference away from noisy nodes.
Validation — Cold-Crank / Load-Dump / Reverse / Thermal Cycling
Replay post-TVS residuals at the LDO input, sweep thresholds and hysteresis, and log tPG, tRST, false-positive rate, and thermal hotspots. Keep a unified CSV for comparison and A/B testing.
Validation playbook
- Cold-crank: replay 3–6 V dips; sweep duration/slope; log VOUT_min, tPG, chatter.
- Load-dump: compare raw vs post-TVS; verify residual < Abs Max with margin.
- Reverse: reverse battery & backfeed; measure I_rev_peak and isolation delay.
- Thermal cycling: −40/25/85/105 °C; insert pulses; track ΔT and recovery.
IC Selection (Mini-Matrix) — Off-Battery LDOs for Cold-Crank / Load-Dump
Quick matrix across the seven focus brands. Values are decision-grade to speed shortlisting for cold-crank sustain, off-battery VIN, protections, and diagnostics. Use Small-Batch Tips to secure cut-tape parts for prototypes.
| Brand / PN (fixed Vout) | VIN(max) / Off-battery | Cold-crank sustain | IOUT | Vdrop (typ.) | IQ (typ.) | Protections | Diagnostics / Pins | AEC-Q / Temp | Pkg | Small-Batch Tips |
|---|---|---|---|---|---|---|---|---|---|---|
| Texas Instruments — TPS7A16-Q1 (e.g., -50/-33) | 60 V class (off-battery) | Low UVLO; holds during 3–6 V dips (design-dependent) | 100 mA | ≈ 350–450 mV @100 mA | ~25–35 µA | Reverse-battery, OCP, TSD (check DS) | PG / EN / UVLO | AEC-Q100; −40…125 °C (grade dep.) | SOT-223, SOT-23/SC70, WSON (var.) | Prefer fixed 5 V/3.3 V; cut-tape common; use PG for ASIL hooks. |
| Texas Instruments — TPS7B86-Q1 (-50/-33) | 40 V class (off-battery) | Cold-crank friendly; robust post-TVS residuals | 150–300 mA (var.) | ≈ 300–500 mV @200 mA (typ.) | ~40–60 µA | Reverse-battery, OCP/OV, TSD | PG / RESET (family-dep.) / EN | AEC-Q100; −40…150 °C (grade dep.) | SOT-223 / TO-252 / HTSSOP (var.) | Pick common fixed codes first; broad disty stock. |
| STMicroelectronics — LDO40L (e.g., -50/-33) | 40 V class (off-battery) | Explicit cold-crank sustain (family note) | 400 mA class | ≈ 300–400 mV @300 mA | Low-IQ family (tens µA) | Reverse-battery, OCP, TSD; short-to-GND | PG / EN; reset options (var.) | AEC-Q100; −40…150 °C | DPAK / SOT-223 / DFN | Look for wettable-flank DFN for AOI. |
| Renesas — ISL78302 (dual LDO, 3.3/5.0 V options) | 40 V class (family dep.) | Low-IQ + POR; crank-tolerant topology | 2×300 mA (dual) | ≈ 250–400 mV @300 mA (per ch.) | ~30–60 µA (per ch., typ.) | Reverse-battery, OCP, TSD (check DS) | POR / PG; sequencing-friendly | AEC-Q100; −40…125/150 °C | QFN / TSSOP (var.) | Dual rail saves BOM; verify thermal per ch. |
| onsemi — NCV4274A/C (-50/-33 etc.) | 40 V class (off-battery legacy) | Cold-crank & load-dump proven (family) | 400 mA class | ≈ 400–600 mV @400 mA (typ.) | ~50–80 µA (variant) | Reverse-battery, foldback, TSD | RST/PG (variant) / EN | AEC-Q100; up to 150 °C (grade) | DPAK / SOT-223 / SOIC | Mature; wide stock; easy second-source. |
| onsemi — NCV8161 (low-noise high-PSRR) | 24–30 V class (check family) | Use after pre-reg for crank immunity | 450 mA | Low dropout (tens–hundreds mV) | ~20–40 µA (typ.) | OCP, TSD (use ideal-diode for reverse) | PG (var.) / EN | AEC-Q100; −40…125/150 °C | DFN / XDFN | Great behind a buck; RF-friendly PSRR. |
| Microchip — MCP1799-xx (e.g., -50/-33) | 45 V class (off-battery) | Low UVLO; good for 3–6 V dips (design-dep.) | 80 mA | ≈ 300–450 mV @80 mA | ~20–35 µA | Reverse-battery (DS), OCP, TSD | EN; simple PG by MCU sampling | AEC-Q100; −40…150 °C (grade) | SOT-223 / TO-252 / SOT-23 | High VIN with modest current; easy stock. |
| Microchip — MAQ5300-xx (fixed) | 36–40 V class (family) | Automotive crank/load-dump use cases | 300 mA class | ≈ 300–500 mV @200–300 mA | Low-IQ family (tens µA) | Reverse-battery, OCP, TSD | EN / PG (variant) | AEC-Q100; −40…125/150 °C | DFN / SOT-223 / DPAK | Check wettable-flank options for AOI. |
| NXP — VR5510 (PMIC w/ LDOs) / FS6600 (SBC) | Battery-front architectures (with pre-clamp) | System-level crank/load-dump strategy | LDO rails (mA-hundreds mA) | Family-dep. | Low-IQ modes (system) | Reverse, OV/UV, watchdog, FS/ERR | PG/RESET/WD/FS ASIL-friendly | AEC-Q100/-Q106 (var.) | QFN/EP (var.) | Pick PMIC/SBC when WD/FS is mandatory. |
| Melexis — Pairing slot (use with above LDOs) | — | — | — | — | — | Use back-to-back FETs / ideal-diode for reverse paths | Combine with MLX sensors/WD monitors at system level | Automotive portfolio (system ICs) | — | Pair with TI/ST/onsemi LDOs for regulated rails. |
Choose 5 V / 3.3 V suffixes (e.g., -50/-33) for better cut-tape availability.
Grade-1/0 and −40…125/150 °C; align with ECU location & derating.
Prefer DPAK/SOT-223/DFN with wettable flanks for automotive AOI.
Keep 2× compatible PNs (same pinout/voltage) to hedge lead-time risk.
Replay post-TVS residual + cold-crank curve + reverse-event before PO.
← Back to Low Dropout Regulators (LDOs)
Submit BOM (48h) — Automotive LDO for Crank / Load-Dump
Get a short-listed pair of automotive LDOs (A/B fallback) for off-battery use with cold-crank, load-dump residual, and reverse considerations. We verify AEC-Q grade, package, PG/RESET/WD hooks, and thermal margins—then reply within 48 hours.
- Attach your cold-crank profile (min V, duration, slope) and post-TVS residual.
- Specify VIN(max) bin (40/45/60 V), target Vout and Iout, and preferred package.
- Note required diagnostics (PG/RESET/WD) and temperature grade (G1/G0).
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Frequently Asked Questions
How do I keep 3.3 V alive through a 3–6 V cold-crank dip?
Size the hold-up capacitor from ΔQ = Iload × Δt and place UVLO below the dip plateau with adequate hysteresis. If dip time is long, add a pre-reg buck ahead of the LDO or relax Vout during crank via PG-gated loads. Validate recovery time and brownout count under the worst curve.
UVLO and hysteresis for crank—what’s a safe window?
Set UVLO just below the expected dip plateau, not the minimum spike, with 0.3–0.8 V hysteresis to avoid chatter. Ensure PG/RESET thresholds align, so loads remain deterministic. Sweep the UVLO pair across temperature and measure false-positive resets per hour during curve playback.
Do I need a 60 V LDO if a TVS is already on the input?
Not necessarily. Decide by the post-TVS residual at the LDO pin, not the raw bus. If residual peak or plateau plus tolerance can exceed the LDO absolute maximum, choose a higher VIN(max) part or tighten clamp/layout. Always measure residual with short loops and realistic wiring.
How do I verify the post-TVS residual stays below Abs Max?
Replay standardized load-dump pulses into your clamp, then probe at LDO-IN with a short ground spring. Log peak, plateau, width, and repeat at temperature. Compare results against the device absolute maximum minus margin. Failures usually indicate excessive loop area or undersized clamp.
Reverse-battery vs reverse-current—what’s the practical difference?
Reverse-battery is VIN polarity inversion at the connector. Reverse-current is backfeed from a powered VOUT into an unpowered VIN or neighbor rail. The former is solved with series elements or blocking FETs; the latter needs an ideal-diode path or body-diode isolation to stop unintended discharge.
Single diode, Schottky, or ideal-diode FET for backfeed blocking?
Schottky is simple but wastes headroom and heats at current. A back-to-back FET ideal-diode gives low drop and controlled turn-off, preferred for tight crank margins. Use a supervisor to hold loads off until rails settle, and confirm reverse-current peaks and shut-down delay in test.
PG vs RESET vs watchdog—what’s minimal for ASIL-B diagnostics?
Typically PG plus RESET gives voltage-valid and reset-timing observability. Add a watchdog (or SBC with FS/ERR) when MCU safety goals require supervision. Keep all these pins referenced to the same ground point, and time-stamp events during crank and residual playback to prove diagnostic coverage.
Can an SBC/PMIC replace discrete LDO plus watchdog pins?
Yes when you need consolidated power, WD, fail-safe, and communication diagnostics. SBCs simplify safety pin routing and start-up ordering but constrain rails and BOM flexibility. If IQ or noise at an always-on analog block is critical, keep a high-PSRR discrete LDO after the SBC pre-reg.
How do I balance PSRR, dropout, and IQ for always-on rails?
Start from IQ budget over key-off days, then check dropout against crank dip margins and heater loads. Pick high PSRR only where ADC/RF needs it; behind a buck, PSRR may relax. Validate noise and ripple at the load pins, not just the regulator, across temperature and load transitions.
When is a pre-reg buck mandatory ahead of the LDO?
Use a buck when load power or crank duration makes hold-up caps impractical, or when VIN residuals would exceed an LDO’s absolute maximum. The buck drops stress and heat, while the LDO cleans noise. Confirm thermal rise, transient recovery, and EMI with your exact wiring harness model.
Where should the TVS and sense RC sit to avoid inner-loop issues?
Place TVS at the source connector and close the return locally before running to the LDO. Keep any sense RC tied at the regulator with short traces; do not insert RCs in a way that forms a hidden inner loop. Probe VIN, VOUT, and PG while stepping load and injecting standard pulses.
What should I log to compare A/B parts under crank and residuals?
Capture VIN_peak/plateau/width at the LDO pin, VOUT_min and recovery, PG/RESET delays, chatter count per hour, reverse-current peaks, and thermal ΔT. Repeat across temperature. Use a single CSV schema so A/B and layout changes are directly comparable without reworking scripts.
Why are fixed-voltage bins (-50/-33) easier to source in small batches?
Fixed bins match common ECU rails and ship broadly in cut-tape, so distributors stock more. Adjustable variants are versatile but scarcer in AEC-Q bins. When schedule matters, shortlist fixed versions first and keep a pin-compatible alternate with the same voltage code ready.
How do I pick two pin-compatible fallbacks to hedge lead time?
Keep the same package and thermal pad footprint, match voltage code and PG/RESET polarity, and ensure UVLO/dropout align with your crank profile. Validate both under identical pulse playback and temperature. Document differences in IQ and PSRR so firmware and EMC checks are predictable.
Do I need wettable flanks and which grade for under-hood ECUs?
Wettable flanks improve AOI reliability and are recommended for dense DFN/QFN in automotive production. For under-hood locations, Grade-0 or Grade-1 temperature ratings are typical. Confirm airflow and copper pour for junction limits, and include hot-soak measurements in your validation plan.
