TM
MP3204 – 6V, 1.3MHz WHITE LED DRIVER
Table 1—I
LED
vs. R1
I
LED
(mA)
1
5
10
15
20
R1 (Ω)
104
20.8
10.4
6.93
5.2
Figure 4 shows the startup behavior of the
MP3204. The ramped voltage that is added to
the current sense amplifier reduces the current
output as the duty cycle increases. As more
LEDs are added, the output voltage rises but
the current that can be delivered to the load is
reduced as well.
V
EN
5V/div.
V
OUT
5V/div.
I
IN
100mA/div.
MP3204_F04
Figure 4—Startup Waveforms
(V
IN
=3.6V, 3 LEDs, 20mA)
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 MP3204 during normal
operation. The first way uses a DC voltage to
control the feedback voltage, which 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 VDC from 0V to 2V,
the resistor values shown for R2 and R3 can
control the LED current from 0mA to 20mA.
V
DC
LED1
CURRENT LIMIT (mA)
400
LED2
300
200
100
0
LED3
MP3204
FB
3
0
20
40
60
80
DUTY CYCLE (%)
100
MP3204_F05
MP3204_F06
Figure 5—Current Limit vs. Duty Cycle
Setting the LED Current
The LED current is controlled by the feedback
resistor, R1, as shown in Figure 2. The current
through the LEDs is equal to 104mV/R1. Table
1 shows the selection of resistors for a given
LED current.
Figure 6—Dimming Control Using a DC
Voltage
MP3204 Rev. 1.3
9/23/2005
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