Table 1. Time-Averaged LED Current vs. PWM Frequency and Duty Cycle

PWM Frequency

D = 10%

D = 20%

D = 30%

D = 50%

D = 90%

200 Hz

2.3

3.8

5.3

8.2

13.9

1 kHz

3.7

6.0

7.4

10.0

14.4

10 kHz

2.6

5.9

9.1

13.4

14.8

20 kHz

1.0

4.7

8.6

13.6

14.8

40 kHz

OFF

1.8

5.1

12.0

14.8

50 kHz

OFF

OFF

5.7

10.3

14.8

A benefit of PWM brightness adjustment is the characteristic

that LEDs are driven with the same current level (max cur-

rent) at all brightness levels. Other methods that adjust

brightness by changing the DC forward current of the LEDs

may see a slight change in color at different brightness

levels.

With the addition of an external resistor, an analog voltage

can be used to adjust LED brightness, as shown in Figure

10. When the analog voltage is above the feedback voltage,

0.515V (typ.), the analog voltage source adds current to

feedback node. In order to keep the feedback voltage regu-

lated, the LM2707 responds by reducing the current through

the LEDs. Conversely, LED current increases when the ana-

log voltage is below the feedback voltage.

A 4-level digital brightness adjustment, shown in Figure 11,

can be implemented with a pair of external resistors and two

digital logic signals. The workings of the circuit are similar to

the analog voltage control: LED currents are set based on

the current that is added to or removed from the FB node

from the digital voltage supplies.

20099235

FIGURE 10. LM2707 LED-Drive Circuit with Analog Voltage Brightness Control

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