MP3204 MPS, MP3204 Datasheet - Page 6

• Manufacturer Part Number: MP3204
• Description: 1.3MHz Fixed Frequency 5 White LED Driver
• Manufacturer: MPS

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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.
Figure 5 shows the dependence on current limit
versus 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.
MP3204 Rev. 1.3
9/23/2005
Figure 5—Current Limit vs. Duty Cycle
100mA/div.
Figure 4—Startup Waveforms
5V/div.
5V/div.
V
OUT
V
500
400
300
200
100
(V
I
EN
IN
0
IN
TM
0
=3.6V, 3 LEDs, 20mA)
20
DUTY CYCLE (%)
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
40
60
80
MP3204_F05
MP3204_F04
100
© 2005 MPS. All Rights Reserved.
www.MonolithicPower.com
Analog and Digital Dimming
There are three different ways to control
dimming
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.
Figure 6—Dimming Control Using a DC
MP3204 – 6V, 1.3MHz WHITE LED DRIVER
I
LED
10
15
20
1
5
(mA)
MP3204
for
Table 1—I
the
FB
Voltage
MP3204
3
V
DC
LED
vs. R1
LED1
LED2
LED3
R1 (Ω)
during
20.8
10.4
6.93
104
5.2
MP3204_F06
normal
6