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RT9285AGQW Datasheet(PDF) 4 Page - Richtek Technology Corporation |
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RT9285AGQW Datasheet(HTML) 4 Page - Richtek Technology Corporation |
4 / 12 page RT9285A/B Preliminary 4 DS9285A/B-02 August 2007 www.richtek.com PWM Dimming (RT9285B) For controlling the LED brightness, the RT9285B can perform the dimming control by applying a PWM signal to EN pin. A low pass filter is implemented inside chip to reduce the slew rate of ILED to prevent the audio noise. 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. EN I WLED I WLED, AVG = Duty of EN Figure 2 Figure 1 Feedback Voltage Time Symbol Increase 0.5 µs to 75µs TUP Decrease 180 µs to 300µs TDOWN Delay between steps > 0.5 µs TDELAY Shutdown > 1ms TSHDN Table 2 Current Limiting The current flow through the inductor as charging period is detected by a current sensing circuit. As the value over the current limiting, the NMOS will be turned-off so that the inductor will be forced to leave charging stage and enter discharging stage. Therefore, the inductor current will not increase over the current limiting. T DOWM T UP T DELAY T SHDN EN I WLED Shut-down 100% 15/16 100% Shut-down |
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