AN4130 datasheet(20/41 Pages) STMICROELECTRONICS

20 / 41 page

Design guidance

AN4130

20/41

Doc ID 023315 Rev 1

Design guidance

This section, like any power design, proceeds from output to input. Please refer to the

schematic on the previous page.

4.1

The load

The converter design is optimized for a string of 18 LEDs, about 54 Vdc at a current of

175 mA.

4.2

Preload resistor (R9)

While the unit's dimmed output current lies inside the NEMA limits, performance can be

improved by adding a light preload. This reduces efficiency, but the unit's operation is much

more stable at low conduction angles.

The reason is that most dimmers rely on line voltage to set the Triac firing delay after the

zero crossing. At low conduction angles the delay is strongly dependent on line voltage. The

slightest variation is visible, because the LED light output reacts very quickly to current

changes, much more quickly than do incandescent lamps, where the filament is a thermal

reservoir that slows the lamp’s response to changes in the dimmed RMS input voltage.

The preload resistor is also required for the open-load protection to work. Without it, the

output voltage climbs until the leakage current in the output filter capacitor limits it. This

causes no short-term damage, but should be avoided.

4.3

Output filter capacitor (C11)

4.3.1

LED ripple current

LEDs require more filtering than normal loads. LEDs are diodes, and their impedance is a

function of the current through them. Typically, an LED has a dynamic impedance, or slope

resistance, of about 1/10 of the ratio of DC voltage to DC current.

AN4130 Datasheet(PDF) 20 Page - STMicroelectronics