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AN2129 APPLICATION NOTE

In Figure 10 the detail of output current is depicted, where the ripple during all the measure-

ment stayed below ±5mA, (i.e. less than 2%). And as mentioned above, if a less wavy output

is needed, bigger output capacitor should be used, but then a slower on and off edges will ap-

pear.

Efficiency of the converter is processed in Figure 11 and Figure 12, where it is showed that

more difference between input and output voltage or lower load current, causes lower efficien-

cy. For six LEDs in one serial string (voltage drop around 20Volts) and input voltage 25V the

efficiency was measured above 93%.

Figure 11. Efficiency vs. input voltage (@ 350mA output current)

70.0

75.0

80.0

85.0

90.0

95.0

100.0

0

5

10

15

20

25

30

Input Voltage [V]

1 LED

2 LEDs

3 LEDs

4 LEDs

5 LEDs

6 LEDs

Figure 12. Efficiency vs. number of LEDs

@ 25V

Figure 13. Output current variation

60

65

70

75

80

85

90

95

100

01

23

4

5

6

7

LEDs

300

310

320

330

340

350

360

012

3

4

5

6

7

LEDs

Increasing the number of LEDs in series in one string (on Figure 13) a lower output current can

be observed (for six LEDs it is 341mA instead of 350mA). That means less than 3% difference,

what should be still acceptable especially considering 5% precision of the current sensing am-

plifier in L6902D.