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RT4503 Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT4503 Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 13 page  10 RT4503/A www.richtek.com DS4503/A-03 June 2015 © Copyright 2015 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Application Information Soft-Start The RT4503/A includes a soft-start function to avoid high inrush current during start-up. The soft-start function is achieved by clamping the output voltage of the error amplifier with another voltage source that is increased slowly from zero to near VIN. LED Current The loop control of the Boost converter keeps VFB equal to a reference voltage, VREF. Therefore, when RSET is connected between the FB pin and GND, the LED current will be determined by the current through RSET, which is equal to VFB / RSET. Current Limit The current flowing through the inductor during a charging period is detected by a current sensing circuit. If the value exceeds the current limit, the N-MOSFET will be turned off. The inductor will then be forced to leave charging stage and enter discharging stage. Therefore, the inductor current will not increase to reach current limit. Shutdown Delay When the EN voltage is in logic low for 20ms during PWM dimming for RT4503 and 2ms during pulse dimming for RT4503A, the system will enter shutdown. FB REF V = Duty V Where : Duty = duty cycle of the PWM signal VREF = internal reference voltage (200mV typ.) The RT4503 chops up the internal 200mV reference voltage via the duty cycle of the PWM signal. Therefore, although a PWM signal is used for brightness dimming, only the LED DC current is modulated, which is often referred as analog dimming. This eliminates the audible noise which often occurs when the LED current is pulsed in replica of the frequency and duty cycle of PWM control. For optimum performance, use the PWM dimming frequency in the range of 1kHz to 100kHz. PWM Dimming When the EN pin is constantly high, the FB voltage is regulated to 200mV typically. The RT4503 allows a PWM signal from EN pin to reduce the regulation voltage; and achieve LED brightness dimming. The relationship between the duty cycle and FB voltage is given by FB Duty1 x 200mV 1kHz < fPWM < 100kHz Duty2 Duty1 Duty2 x 200mV TSHDN > 20ms EN Period1 Period2 |
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Similar Description - RT4503 |
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