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RT8560 Datasheet(PDF) 13 Page - Richtek Technology Corporation |
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RT8560 Datasheet(HTML) 13 Page - Richtek Technology Corporation |
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13 / 15 page  RT8560 13 DS8560-01 April 2011 www.richtek.com Where, VRISET is the voltage of the RISET pin (1.21V typ.) and the RISET is the resister between RISET pin and GND. This setting is the reference for the LED current at LED1- 4 and represents the sensed LED current for each string. The DC/DC converter regulates the LED current according to the setting. Brightness Control The RT8560 features both analog and digital dimming control. Analog dimming is linearly controlled by an external voltage (0.3V < VACTL < 1.2V). With an on-chip output clamping amplifier and a 200k Ω resistor, PWM dimming signal is easily low-pass filtered to an analog dimming signal with one external capacitor for noise-free PWM dimming. A very high contrast ratio true digital PWM dimming can be achieved by driving ACTL pin with a PWM signal and suggest PWM frequency is from 200Hz to 10kHz. Refer to Figure 8, the minimum dimming duty can be as low as 1% for the frequency range from 100Hz to 300Hz. For the dimming frequency from 300Hz to 1kHz, the duty is about 5%. If the frequency is increased to 1kHz to 30kHz, the duty will be about 10%. LED Current vs. ACTL PWM Duty Cycle 0 2 4 6 8 10 12 14 16 18 20 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100 % Duty Cycle (%) ACTL = 200Hz ACTL = 1kHz ACTL = 10kHz ACTL = 20kHz ACTL = 30kHz ACTL = 0V to 3V, VIN = 12V 0 10 20 30 40 50 60 70 80 90 100 Figure 8. LED Current vs. PWM Dimming Duty Cycle Over Voltage Protection The RT8560 equips over voltage protection (OVP) function. When the voltage at OVP pin reaches a threshold of approximately 2.5V, the MOSFET drive output (SW) will turn “OFF”. The MOSFET drive output (SW) will turn “ON” again once the voltage at OVP drops below the threshold of approximately 2.5V. So, the output voltage can be clamped at a certain voltage level and it can be calculated by the following equation : OUT, OVP R1 V = 2.5 1+ R2 ⎛⎞ ×⎜⎟ ⎝⎠ Where, R1 and R2 are the voltage divider connected to OVP pin. If at least one string is in normal operation, the controller will automatically ignore the open strings and continue to regulate the current for the string(s) in normal operation. Current-limit Protection The RT8560 can limit the peak current to achieve over current protection. The RT8560 senses the inductor current of on period that flows through SW pin. The duty cycle depend on current signal and internal slope compensation compared with error signal. The internal N-MOSFET will be turned off when the current signal is large than internal slope compensation. In the off period, the inductor current will be descended until the internal N-MOSFET is on by the oscillator. Over Temperature Protection The RT8560 has over temperature protection (OTP) function to prevent the excessive power dissipation from overheating. OTP will shut down the IC while junction temperature exceeds 150 °C. Main converter starts switching once the junction temperature is cooled approximately 20 °C. Inductor Selection The value of the output inductor (L), where the transition from discontinuous to continuous mode occurs is approximated by the following equation : ) 2 2 OUT IN IN OUT OUT (V V V L = 2I f V −× ×× × |
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