lm2707 National Semiconductor Corporation, lm2707 Datasheet - Page 15

• Manufacturer Part Number: lm2707
• Description: Inductive-boost Series Led Driver With Programmable Oscillator Frequency
• Manufacturer: National Semiconductor Corporation

Application Information
V
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
IN
= 3.6V, 4 LEDs, R
Table 1. Time-Averaged LED Current vs. PWM Frequency and Duty Cycle
PWM Frequency
200 Hz
10 kHz
20 kHz
40 kHz
50 kHz
1 kHz
FB
= 34Ω, I
FIGURE 10. LM2707 LED-Drive Circuit with Analog Voltage Brightness Control
LED
= 14.9mA when V(EN) is HIGH.
D = 10%
OFF
OFF
2.3
3.7
2.6
1.0
(Continued)
D = 20%
OFF
3.8
6.0
5.9
4.7
1.8
15
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.
D = 30%
5.3
7.4
9.1
8.6
5.1
5.7
D = 50%
10.0
13.4
13.6
12.0
10.3
8.2
D = 90%
20099235
13.9
14.4
14.8
14.8
14.8
14.8
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