Controller for automotive LED strings — what it is and where it fits
The Texas Instruments LM3409QMYX/NOPB is a DC DC controller designed to drive LED strings in a step-down (buck) topology. It is qualified to AEC-Q100, making it suitable for automotive exterior and interior lighting modules — headlamps, daytime running lights, taillights, and ambient lighting — where the supply rail is 12V or 24V with transients up to 42V. The part operates over a -40°C to 125°C junction temperature range, covering under-hood and cabin environments. Dimming is supported via both analog and PWM methods, giving the system designer flexibility to adjust brightness without a separate dimming IC.
External MOSFET — design trade-off
The LM3409QMYX/NOPB has no internal switch. This means the designer selects an external N-channel MOSFET sized for the LED string current and thermal dissipation. For a 1A string at 12V, a small SOT-23 FET may suffice; for a 3A string at 24V, a DPAK or TO-252 FET with a heatsink pad is typical. The controller's gate drive is designed for standard logic-level FETs. This external-FET approach adds two or three BOM lines but lets the thermal load sit off the controller package, which helps in compact 10-HVSSOP layouts.
Supply range and transient headroom
Minimum supply is 6V, maximum is 42V. On a nominal 12V automotive system, the 42V ceiling provides margin for load-dump events (typically clamped to 35V–40V in most ECUs). On a 24V truck or bus system, the same margin holds. The controller's undervoltage lockout ensures clean startup and shutdown behavior on the 6V low end.
Lifecycle and compliance
It is ROHS3 compliant, meaning no restricted substances above exemption thresholds. The package is a 10-lead HVSSOP (3.00 mm × 3.00 mm body), supplied in Tape & Reel for automated assembly. Cut Tape quantities are available for prototyping.