The LM3501 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 integrated synchronous
switching (no external schottky diode required) and a low feedback voltage (515 mV), power
efficiency of the LM3501 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 LM3501
operates with a fixed 1 MHz switching frequency. When used with ceramic input and output
capacitors, the LM3501 provides a small, low-noise, low-cost solution.
Two LM3501 options are available with different output voltage capabilities. The
LM3501-21 has a maximum output voltage of 21V and is typically suited for driving 4 or 5 white LEDs
in series. The LM3501-16 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 LM3501 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 by varying the analog control voltage on the
control pin or by using a pulse width modulated (PWM) signal on the shutdown pin. In shutdown, the
LM3501 completely disconnects the input from output, creating total isolation and preventing any
leakage currents from trickling into the LEDs.
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