TM
MP1518 – FIXED FREQUENCY WHITE LED DRIVER
Startup Waveforms
V
IN
= 3.6V, 3 LEDs, 20mA
V
EN
5V/div.
V
OV
5V/div.
I
IN
100mA/div.
Figure 4—Startup Waveforms
Figure 5 shows the dependence on current limit
versus duty cycle.
500
Analog and Digital Dimming
There are three different ways to control
dimming for the MP1518 during normal
operation. The first way uses a DC voltage to
control the feedback voltage. This can be seen
in Figure 6. As the DC voltage increases,
current starts flowing down R1, R2 and R3. The
loop will continue to regulate the feedback
voltage to 104mV. Thus the current has to
decrease through the LEDs by the same
amount of current as is being injected from the
DC voltage source. With a V
DC
from 0V to 2V,
the resistor values shown for R2 and R3 can
control the LED current from 0mA to 20mA.
V
DC
LED1
LED2
LED3
MP1518
FB
CURRENT LIMIT (mA)
400
300
200
100
0
0
20
40
60
80
DUTY CYCLE (%)
100
Figure 6—Dimming Control Using a DC
Voltage
Figure 5—Current Limit vs. Duty Cycle
Setting the LED Current
The LED current is controlled by the feedback
resistor, R1, in Figure 6. The current through
the LEDs is given by the equation 104mV/R1.
Table 2 shows the selection of resistors for a
given LED current.
Table 2—I
LED
vs. R1
I
LED
(mA)
1
5
10
15
20
R1 (Ω)
104
20.8
10.4
6.93
5.2
MP1518 Rev. 1.6
7/25/2006
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