Offline LED Driver Reference Design

Circuit introduction

This reference design is an offline (400V _DC ), flyback LED driver . This design can drive 27 WLEDs (white LEDs) at 400mA. After installing jumper J1, it can drive 6 amber LEDs at 400mA. The design uses the MAX16801 HB LED controller and a three-winding transformer (coupling coil). Since the current sense signal is fed directly into the IC control loop, a non-isolated architecture is used.

The transformer turns ratio is 18:6:1, the primary inductance is 800µH, the current rating is 750mA (peak), and the duty cycle is always less than 50%.

The operating frequency is fixed at 265kHz and cannot be adjusted. Provides 120V overvoltage protection (not latching), UV detection level is 310V. The startup delay time is approximately 43ms, after which the V _IN voltage rises to approximately 22V and the IC starts driving the external MOSFET. This will cause the capacitor voltage at V _IN to decay until the winding bootstrap voltage can support power supply to the device. Since the LED is in a high impedance state at low voltage, the main load on the secondary winding is just the output capacitance. The turns ratio from the secondary to the tertiary winding is 6:1, which means that once the voltage on the output capacitor reaches 60V, the bootstrap winding will provide 10V power to the IC. For applications with 6 LEDs in series (with J1 installed), the output capacitor must be charged to 10.7V before the IC is powered by 10V.