This design drives 20 white LEDs in a single string at 30mA to light a vanity mirror. The circuit boosts the input voltage in order to keep 20 LEDs in a single series string. This ensures that the same current flows through each device which results in equal brightness. The resistor Rset is used to set the load current in the following manner: Rset = 1.25V / Load current. To drive the string of LEDs at 30mA, a 41.2ohm resistor can be used for Rset.
The design features the LM5000 which is a monolithic integrated circuit specifically designed and optimized for flyback, boost or forward power converter applications. The internal power switch is rated for a maximum of 80V, with a current limit set to 2A. Protecting the power switch are current limit and thermal shutdown circuits. The current mode control scheme provides excellent rejection of line transients and cycle-by-cycle current limiting. An external compensation pin and the built-in slope compensation allow the user to optimize the frequency compensation. Other distinctive features include softstart to reduce stresses during start-up and an external shutdown pin for remote ON/OFF control. There are two operating frequency ranges available. The LM5000-3 is pin selectable for either 300kHz (FS Grounded) or 700kHz (FS Open). The LM5000-6 is pin selectable for either 600kHz (FS Grounded) or 1.3MHz (FS Open). The device is available in a low profile 16-lead TSSOP package or a thermally enhanced 16-lead LLP package.
Schematic
Example Schematic Showing Connection for all Components.
Example Schematic Showing Connection for all Components.
