U1, LM5020, is a 100V Current Mode PWM Controller. The oscillator frequency is set by resistor R3.

At the beginning of each oscillator cycle, FET M1 will be driven on. This is because Vfb, the voltage feedback pin, is grounded. Note that the COMP pin is left open, since the voltage control loop is not used for this design. With M1 ON, L1 inductor current increases. The inductor current is conducted directly through the LED, so the LED current is likewise increasing.

The LED current is sensed by the parallel combination of R5 and R11, and this signal is filtered to remove high frequency switching noise by the low pass filter consisting of R6 and C4. Current increases until the current sense input, CS, reaches its threshold of 0.5V. At this time, M1 is turned off.

With M1 OFF, the inductor current must decay. When the inductor voltage reverses, the catch diode, D1, turns ON. The current decays in the loop consisting of LED, L1, and D1 until the next oscillator cycle begins, at which time M1 turns ON and the operating cycle repeats. The operation is at a fixed frequency, and the peak current through the LED is controlled.

The LM5020 includes an under voltage lockout (UVLO) feature, so that a circuit implemented with this controller will be held in the off state until Vin reaches an acceptable voltage. This value is determined by the R1 and R2 voltage divider.

In addition, a PWM control input is coupled into the UVLO function by R4. The designer can select R1, R2, and R4 values so that a PWM logic level signal will gate the converter's operation. This can be accomplished with very little phase delay, since the propagation delays are quite fast, and the control loop decisions are driven by voltage levels with no loop filter delay. Also, because the LED current is controlled, and there is no directly controlled output voltage, which one would usually find in a voltage converter, no output capacitor is used. Therefore, there is no capacitive discharge delay when the converter is turned off by the PWM signal.