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RT8511B Datasheet(PDF) 7 Page - Richtek Technology Corporation |
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RT8511B Datasheet(HTML) 7 Page - Richtek Technology Corporation |
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7 / 11 page  RT8511B 7 DS8511B-05 February 2015 www.richtek.com © Copyright 2015 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Application Information The RT8511B is a current mode boost converter which operates at a fixed frequency of 500kHz. It is capable of driving up to 10 white LEDs in series and integrates functions such as soft-start, compensation, and internal analog dimming control. The protection block also provides over-voltage, over-temperature, and current- limit protection features. LED Current Setting The loop structure of the boost converter keeps the FB pin voltage equal to the reference voltage VFB. Therefore, by connecting the resistor, RSET between the FB pin and GND, the LED current will be determined by the current through RSET. The LED current can be calculated by the following equation : FB LED SET V I = R Brightness Control For the brightness dimming control of the RT8511B, the IC provides typically 200mV reference voltage when the PWM pin is constantly pulled high. However, the PWM pin allows a PWM signal to adjust the reference voltage by changing the PWM duty cycle to achieve LED brightness dimming control. The relationship between the duty cycle and the FB voltage can be calculated according to the following equation : VFB = 200mV x Duty where 200mV is the typical internal reference voltage and Duty is the duty cycle of the PWM signal. As shown in Figure 3, the duty cycle of the PWM signal is used to modify the internal 200mV reference voltage. With an on-chip output clamping amplifier and a serial resistor, the PWM dimming signal is easily low-pass filtered to an analog dimming signal with one external capacitor, CDIMC, for noise-free PWM dimming. Dimming frequency can be sufficiently adjusted from 100Hz to 8kHz. However, the LED current cannot be 100% proportional to the duty cycle. Referring to Table 1, the minimum dimming duty can be as low as 1% for the frequency range from 100Hz to 8kHz. It should be noted that the accuracy of 1% duty is not guaranteed. Figure 3. Block Diagram of Programmable FB Voltage Table 1. Minimum Duty for Dimming Frequency Dimming Frequency Minimum Duty Cycle 100Hz to 8kHz 1% Because the voltage of DIMC and FB is small to 2mV and easily affected by LX switching noise. + - DIMC FB To Controller 200mV R CDIMC 1µF PWM EA The FB pin voltage will be decreased by lower PWM duty ratio . That will achieve LED current diming function for different brightness. But LED current is more accurate when higher PWM duty. The Table 2. shows typical variation value comparison between different PWM duty and condition is VIN = 3.7V, LED array = 6S2P, RSET = 5 Ω. Table 2. LED Current Variation vs PWM Duty PWM Duty (%) Variation (%) PWM Duty (%) Variation (%) 1 ±60 8 ±7 2 ±25 9 ±6 3 ±17 10 ±5 4 ±13 20 ±4 5 ±10 50 ±3 6 ±9 100 ±2.5 7 ±8 It also should be noted that when the input voltage is too close to the output voltage [(VOUT −VIN) < 6V] , excessive audible noise may occur. Additionally, for accurate brightness dimming control, the input voltage should be kept lower than the LEDs' turn on voltage. When operating in the light load, excessive output ripple may occur. |
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