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RT9284A15PJ5 Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT9284A15PJ5 Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 14 page  10 RT9284A/B www.richtek.com DS9284A/B-05 August 2007 Application Information Power Sequence The RT9284A/B must take notice of the power sequence. The power sequence of RT9284A/B has to VDD early than EN or else the RT9284A/B maybe fall into the unknown state to result in RT9284A/B turn off. Dimming Control a. Using a PWM Signal to EN Pin For controlling the LED brightness, the RT9284A/B can perform the dimming control by applying a PWM signal to EN pin. The internal soft start and the wide range dimming frequency from100 to100kHz can eliminate inrush current and audio noise when dimming. The average LED current is proportional to the PWM signal duty cycle. The magnitude of the PWM signal should be higher than the maximum enable voltage of EN pin, in order to let the dimming control perform correctly. Version Feedback Reference Voltage VFB (V) LED Current Setting ILED (A) RT9284A 1.235 ILED = 1.235/R1 RT9284B 0.25 ILED = 0.25/R1 Table 1 Figure 6. Dimming Control Using a DC Voltage for RT9284A LED Current Control The RT9284A/B regulates the LED current by setting the current sense resistor (R1) connecting to feedback and ground. As shown in Table 1, the RT9284A feedback voltage (VFB) is 1.235V and the RT9284B feedback voltage (VFB) is 0.25V. The LED current (ILED) can be set by a resistor R1. In order to have an accurate LED current, a precision resistor is preferred (1% is recommended). EN < 0.4V EN > 1.5V VDD > 2.7V VDD EN VOUT Figure 5 b. Using a DC Voltage Using a variable DC voltage to adjust the brightness is a popular method in some applications. The dimming control using a DC voltage circuit is shown in Figure 6. According to the Superposition Theorem, as the DC voltage increases, the voltage contributed to VFB increases and the voltage drop on R2 decreases, i.e. the LED current decreases. For example, if the VDC range is from 0V to 2.8V, the selection of resistors in Figure 6 sets dimming control of LED current from 20mA to 0mA. VDD GND LX FB OVP RT9284A VIN EN 6 4 2 3 5 1 ILED 2.7V to 5.5V C1 C2 R1 D1 SS0520 L1 4.7uH to 22uH R2 VDC Dimming 0V to 2.8V R3 110 10k 13k VEN > 1.5V VEN < 0.4V c. Using a Filtered PWM signal: Another common application is using a filtered PWM signal as an adjustable DC voltage for LED dimming control. A filtered PWM signal acts as the DC voltage to regulate the output current. The recommended application circuit is shown in the Figure 7. In this circuit, the output ripple depends on the frequency of PWM signal. For smaller output voltage ripple (<100mV), the recommended frequency of 2.8V PWM signal should be above 2kHz. To fix the frequency of PWM signal and change the duty cycle of PWM signal can get different output current. Figure 8. shows the relationship between LED current and PWM duty cycle. The LED current can be calculated by the following equation. DC FB FB LED R2 (V V ) V R3 I = R1 ×− − |
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