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MP3204DJ Datasheet(PDF) 6 Page - Monolithic Power Systems |
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MP3204DJ Datasheet(HTML) 6 Page - Monolithic Power Systems |
6 / 8 page MP3204 – 6V, 1.3MHz WHITE LED DRIVER MP3204 Rev. 1.3 www.MonolithicPower.com 6 9/23/2005 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2005 MPS. All Rights Reserved. TM 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. VOUT 5V/div. VEN 5V/div. IIN 100mA/div. MP3204_F04 Figure 4—Startup Waveforms (VIN=3.6V, 3 LEDs, 20mA) Figure 5 shows the dependence on current limit versus duty cycle. 500 400 300 200 100 0 0 20406080 100 DUTY CYCLE (%) MP3204_F05 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. Table 1—ILED vs. R1 ILED (mA) R1 (Ω) 1 104 5 20.8 10 10.4 15 6.93 20 5.2 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. MP3204 LED1 LED2 LED3 FB 3 VDC MP3204_F06 Figure 6—Dimming Control Using a DC Voltage |
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