AN2961

Test results

Doc ID 15559 Rev 1

9/14

LED current regulation

In Figure 5, the LED current regulation versus AC input voltage is shown. As illustrated, the

system is capable of regulating the current between a 90 VAC and 265 VAC input mains with

little variation.

Figure 5.

LED current regulation vs. input voltage

Open load operation

Open load operation is an abnormal condition, and the system is capable of protecting itself.

When this condition occurs, the circuit regulates the output voltage at a typical value of 39 V.

In this situation the constant current circuit does not operate, but the D1 Zener diode does.

When diode D1 conducts, the output voltage is regulated at 39 V.

Short-circuit condition

When a short-circuit occurs the device works at the minimum admissible duty cycle until the

voltage on the pin of the VIPER22A-E falls below the shutdown threshold. This is due to the

supply circuit, composed of D2 and C1. When the load is connected at the output, the C1

capacitor is charged to the same voltage as the output voltage.

In a short-circuit condition the C1 capacitor is not charged and after each startup, the

voltage decrease causes the VIPER22A-E to switch off. After this, the VIPER22A-E repeats

the startup sequence. The board is also capable of protecting itself under these conditions.