Performance with LED loads

AN3256

6/23

Doc ID 17790 Rev 1

3

Performance with LED loads

The data in this section was taken with loads consisting of two parallel strings of LEDs. The

driver acts as a current source, the LEDs determine the output voltage.

3.1

Strings of 4 series-connected 1 W white LEDs

At low line input the circuit stops operating when housekeeping power fails to keep the

converter running. The result is that the LEDs flash, at an annoying rate. The line voltage at

which this occurs is noted below.

Dropout to flashing, 88 V (too close to the minimum spec line voltage, 96 V. Strings of 4

LEDs cannot be used without modifying the circuit. Strings of 5 LEDs give sufficient margin).

●

Trace colors:

–

Yellow= line voltage, 50 V/div ref 0

–

Magenta= line current, 100 mA/div ref 0

–

Blue= LED voltage, 5 V/div ref -3 div

–

Green= LED current, 100 mA/div ref -3 div

Table 1.

Performance overline voltage range with 4 LEDs per string

Parameter

20 % low line

10 % low line

Nominal line

10 % high line

Line voltage

95.5

107.7

119.7

132.3

Line current, mA

69.5

65.3

61.8

59.2

Input power, Watts

5.90

5.985

6.015

6.050

Power factor

0.888

0.851

0.815

0.769

Output voltage

12.52

12.53

12.52

12.51

Output current, Amps

0.394

0.398

0.398

0.395

Output power, Watts

4.933

4.987

4.983

4.941

Efficiency

83.6 %

83.3 %

82.8 %

81.7 %

Power loss

0.967

0.998

1.032

1.109

U1 Vcc

11.41

13.37

15.16

16.35