SLM4120

The SLM4120 can drive LED strings with total forward voltage up to 40V (limited by internal switch rating). Number of LEDs = 40V / Vf per LED. For typical white LEDs with Vf = 3.0-3.3V: (1) 3V LEDs - maximum 13 LEDs (39V). (2) 3.3V LEDs - maximum 12 LEDs (39.6V). Leave 1V margin for regulation. For RGB LEDs with different forward voltages (R=2V, G/B=3.3V), calculate based on total string voltage. For parallel LED strings, each string needs its own current limiting. The 40V output also allows driving long LED strips designed for 24V or 36V operation. Always verify actual LED forward voltages at operating current as they vary with temperature and manufacturing.

Maximum 12-13 white LEDs; verify actual LED Vf at operating conditions.

The SLM4120 has a minimum input voltage of 2.5V, making it suitable for: (1) Single Li-ion battery - operates from 3.0V (fully discharged) to 4.2V (fully charged). The 2.5V rating provides margin for voltage droop under heavy load. (2) Two alkaline/NiMH cells - operates from 1.8V (discharged) to 3.0V (fresh). Note: at 1.8V, output current capability is reduced. (3) 5V USB input - plenty of headroom for 5V ±5% tolerance. (4) 12V automotive - operates during cold crank when battery voltage drops. Below 2.5V, the UVLO circuit disables the device to prevent unstable operation. For single alkaline/NiMH cell (1.5V nominal), consider a different driver with lower minimum input.

Suitable for single Li-ion, two AA/AAA, 5V USB, and 12V automotive.

Inductor selection for SLM4120 boost LED driver: (1) Inductance value - typically 10-47μH depending on switching frequency and current. Higher inductance reduces ripple but increases size. (2) Current rating - select inductor with saturation current >1.5× peak switch current. For 1A LED current at 50% duty cycle, peak current is ~2A, so use 3A rated inductor. (3) DCR - lower DCR improves efficiency. Target <50mΩ for 1A applications. (4) Core material - ferrite core for high frequency operation. Shielded inductors reduce EMI. (5) Example: 22μH, 3.5A saturation, 30mΩ DCR shielded inductor works well for most applications. (6) Size - balance between electrical performance and physical size. Larger inductors have lower DCR and higher saturation current.

Use 22μH 3A shielded inductor for general applications; adjust based on specific requirements.

The SLM4120 is designed as a constant current LED driver, not a constant voltage regulator. However, it can be adapted for CV output: (1) LED load - normal operation with LED string provides constant current to LEDs. (2) Resistive load - can drive resistive load but regulation is current-mode, not voltage-mode. (3) CV modification - add voltage divider from output to FB pin to regulate voltage instead of current. This requires recalculation of compensation. (4) Output capacitor - for CV operation, larger output capacitor (100μF+) is needed for stability. (5) Load regulation - CV performance is not as good as dedicated voltage regulators. For proper CV applications, consider using Silan's DC-DC converters like SLM3400 series designed for voltage regulation. The SLM4120 excels at LED driving where constant current is desired.

Use SLM4120 for LED current regulation; use dedicated DC-DC converter for CV applications.

The SLM4120 operates at a fixed 1MHz switching frequency. This frequency provides: (1) Component size - 1MHz allows use of small inductors (10-22μH) and capacitors, reducing solution size. (2) EMI - frequency is above AM radio band (530-1700kHz) reducing interference concerns. (3) Efficiency - good balance between switching losses and component size. Higher frequencies reduce size but increase switching losses. (4) Bandwidth - control loop bandwidth of ~100kHz provides good transient response. (5) Audible noise - 1MHz is well above audible range (20Hz-20kHz), preventing acoustic noise from inductor or capacitors. The fixed frequency simplifies EMI filtering design compared to variable frequency converters. External synchronization is not supported - the device uses internal oscillator.

Fixed 1MHz operation; no external frequency adjustment possible.

Output overvoltage protection in SLM4120: (1) Open LED protection - built-in OVP circuit limits output voltage to ~42V if LED string opens. This is the primary protection. (2) Zener diode - add external 39V Zener from output to GND as additional protection. Size for 1W dissipation. (3) Output capacitor voltage rating - use capacitors rated for at least 50V to handle OVP condition. (4) Layout - minimize parasitic inductance in output path to reduce voltage spikes. (5) Soft start - SLM4120 has internal soft-start to prevent inrush current and voltage overshoot. (6) Testing - verify OVP function by temporarily disconnecting LEDs during testing. Output should clamp to safe level. The built-in OVP is usually sufficient, but external Zener provides extra safety margin for critical applications.

Built-in OVP is sufficient for most applications; add external Zener for extra safety margin.