LM3500

The LM3500 is a fixed-frequency step-up DC/DC converter that is ideal for driving white LEDs for display backlighting and other lighting functions. With fully intergrated synchronous switching (no external schottky diode required) and a low feedback voltage (500mV), power efficiency of the LM3500 circuit has been optimized for lighting applications in wireless phones and other portable products (single cell Li-Ion or 3-cell NiMH battery supplies). The LM3500 operates with a fixed 1MHz switching frequency. When used with ceramic input and output capacitors, the LM3500 provides a small, low-noise, low-cost solution. Two LM3500 options are available with different output voltage capabilities.

The LM3500-21 has a maximum output voltage of 21V and is typically suited for driving 4 or 5 white LEDs in series. The LM3500-16 has a maximum output voltage of 16V and is typically suited for driving 3 or 4 white LEDs in series (maximum number of series LEDs dependent on LED forward voltage). If the primary white LED network should be disconnected, the LM3500 uses internal protection circuitry on the output to prevent a destructive overvoltage event.

A single external resistor is used to set the maximum LED current in LED-drive applications. The LED current can easily be adjusted using a pulse width modulated (PWM) signal on the shutdown pin. In shutdown, the LM3500 completely disconnects the input from output, creating total isolation and preventing any leakage currents from trickling into the LEDs.