Offline LED driver reference design

This application note presents a reference design for a non-isolated LED driver intended to operate directly from a 400V_DC input. This reference design is a flyback LED driver for offline environments (400V_DC). The design can drive 27 white LEDs (WLEDs) at 400mA. With the jumper, J1, installed, the design drives 6 amber LEDs at 400mA. The design uses the MAX16801 HB LED controller and a three-winding transformer (coupled inductor). There is no electrical isolation as the current sense is fed directly into the IC control loop.

The transformer has an 18:6:1 turns ratio. Primary inductance is 800?H with a current rating of 750mA (peak) and a duty cycle always less than 50 percent.

The frequency of operation is 265kHz and is nonadjustable. Overvoltage protection (nonlatching) is at 120V. The UV detect level is 310V. The turn-on delay time is about 43msec, after which VIN will be about 22V and the IC will begin to drive the external MOSFET. This will, in turn, cause the VIN capacitor to decay until the bootstrap winding can provide support. Because of the high impedance of the LEDs at low voltage, the main secondary load will initially be only the output capacitor. The secondary-to-tertiary turns ratio is 6:1, which means that the bootstrap winding will supply 10V to the IC as soon as 60V develops across the output capacitor. For the 6-LED string option (i.e., with J1 installed), the output capacitor must, obviously, charge to 10.7V before 10V is available to the IC.

View the PDF document for more information.