LED Constant-Current Boost — Open/Short, Line/Temp Comp, PWM & Analog Dimming

Use hybrid dimming: bias a small analog current (≈2–5% ILED) and gate with high-frequency PWM so the controller never idles at zero. Keep PWM ≥2–5 kHz to move flicker and audible content away; freeze/slow the integrator during off-time. Validate transient response and minimum on-time at the DCM/CCM boundary.

Set OVP slightly above nominal VOUT plus diode/sense margins, then implement timer-based soft shutdown instead of immediate restart. Add a controlled bleed to discharge the output safely. Rate the divider for open-LED voltage and consider EN latch only where mandated by safety or diagnostics.

Combine fast peak-current limit with blanking and a slower average-current/OCP comparator. Distinguish wire short from LED short by VOUT trajectory: near-instant collapse versus gradual deviation. Prefer hiccup with cool-down; reserve latch for safety. Record retry counts in validation to reveal oscillatory faults.

Choose 2–20 kHz depending on magnetics and mechanical resonance; stay above 2 kHz for flicker and avoid local acoustic peaks. For EMI, offset the PWM rate from switching frequency harmonics and enable small jitter. Validate conducted/radiated scans with both minimum and maximum duty cycles.

Filter the ADJ node just enough to reject noise without throttling bandwidth; keep its zero/pole outside the current-loop crossover. Avoid double-integrating: the error amplifier remains dominant. At very low ADJ, add a floor or hybrid bias to preserve loop authority and sensing signal-to-noise.

Sense VIN and proportionally bias the control reference so duty changes are pre-compensated. Start with a first-order coefficient from small-signal modeling, then trim using two-point calibration at VIN(min) and VIN(max). Re-check under cable drop and cold-crank profiles if applicable.

Use segmented mapping: constant current up to T1, gentle slope to T2, then strong reduction until shutdown. This protects LEDs and magnetics while preserving brightness in normal range. Place the NTC near the hot spot and validate thermal lag during rapid ambient steps.

Pre-charge the output with soft-start boost, then hand off to the current loop once the sense signal is reliable. Gate dimming until soft-start ends to avoid false OVP/CS trips. Cold starts demand longer soft-start and verified minimum duty to overcome elevated Vf.

Too small increases noise susceptibility and amplifier offset impact; thermal sense becomes coarse. Too large wastes power and heats nearby parts. Pick Rs = VCS/ILED that meets efficiency and ADC/noise budgets; Kelvin-route its traces and keep high-dv/dt nodes away.

High-side sense preserves ground integrity and simplifies short-to-ground detection but needs a higher-CM amplifier. Low-side offers simpler sensing and better noise margin but can disturb ground reference and short diagnostics. Choose based on layout constraints, protection thresholds, and companion interfaces.

Use per-string ballast (small series resistors) or active balancing with current sinks. Provide open-string detection and degrade gracefully by redistributing current within safe limits. Keep wiring symmetry, match LEDs thermally, and define a fault policy that avoids sudden brightness jumps.

Increase soft-start ramp and ensure minimum duty exceeds the threshold needed to overcome elevated Vf. Verify OVP margins at cold, and consider pre-charge or short pre-bias of the LED string. Validate start under worst-case VIN(min), lowest ambient, and longest cable drop.

Estimate drop using wire resistance and current, then compensate with line feed-forward or remote-sense near the LEDs. Keep the return path low-impedance and avoid common ground with switching currents. Consider local regulation or current sinks when strings diverge significantly in length.

SR improves efficiency at high current and low VOUT margin, but risks reverse conduction and instability at light load or PWM off-time. Use robust zero-current detection and disable SR during dimming off-windows. Validate thermal and EMI behavior across duty extremes.

For cold-crank, guarantee regulation or graceful dim during VIN sag using feed-forward and soft-start shaping. For load-dump, clamp VOUT and respect absolute ratings with OVP and surge components. Define clear recovery policy (auto-retry vs latch) consistent with diagnostic requirements.