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RT8577 Datasheet(PDF) 8 Page - Richtek Technology Corporation |
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RT8577 Datasheet(HTML) 8 Page - Richtek Technology Corporation |
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8 / 12 page  8 DS8577-00 November 2011 www.richtek.com RT8577 Application information The RT8577 is an 4-CH driver controller that delivers well matched LED current to each channel of LED strings. The external N-MOSFET current source will accommodate the power dissipation difference among channels resulting from the forward voltage difference between the LED strings. With high speed current source N-MOSFET drivers, the RT8577 features highly accurate current matching, while also providing very fast turn-on and turn-off times. This allows a very narrow minimum on or off pulse. The RT8577 integrates adjustable switching frequency and soft-start and provides circuitry for over temperature, over voltage, under voltage and current limit protection. Soft-Start The RT8577 employs a soft-start feature to limit the inrush current. The soft-start circuit prevents excessive inrush current and input voltage droop. The soft-start time is determined by a capacitor, CSS, connected between SS andGNDand charged with a 6 μAconstant current as shown in the following equation. tSS (max) = CSS x 4.8 x 105 (s) The value of capacitor CSS is user-defined to satisfy the designer' requirement. Compensation The regulator loop can be compensated by adjusting the external components connected to the VC pin. The VC pin is the output of the internal error amplifier. The compensation capacitor will adjust the integrator zero to maintain stability and the resistor value will adjust the frequency integrator gain for fast transient response. Typical values of the compensation components are RC = 560 Ω, CC = 0.22μF. LED Connection The RT8577 equips 4-CH LED drivers and each channel supports up to 15 LEDs. The LED strings are connected from the output of the boost converter to pin LEDx (x = 1 to 4) respectively. If one of the LED channel is not in use, the LED pin should be opened directly. Setting and Regulation of LED current The LED current can be calculated by the following equation : ≅ LED ISET 1092 I R where RISET is the resistor between the RISET pin and GND. This setting is the reference for the LED current at pin LEDx and represents the sensed LED current for each string. The DC/DC converter regulates the LED current according to the setting. Over Voltage and Under Voltage Protection The RT8577 integrates Over Voltage Protection (OVP) and Under Voltage Protection (UVP). When the voltage at the OVP/UVP pin rises above the threshold voltage of approximately 1.2V or falls below the threshold voltage of approximately 0.6V, the internal switch will be turned off and STATUS pin will be pulled high. The internal switch will be turned on again once the voltage at the OVP/UVP pin returns to normal range. The output voltage can be clamped at a certain voltage level and can be calculated by the following equations : where ROVP1 and ROVP2 are the resistors in the resistive voltage divider connected to the OVP/UVP 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 strings in normal operation. Suggested value for ROVP2 is up to 3M Ω to prevent loading effect. LED Short Circuit Protection The RT8577 integrates LED Short Circuit Protection (SCP). If one of the LED1 to LED4 pin voltages exceeds a threshold of approximately 4.3V during normal operation, the STATUS pin will be pulled high for a fault signal. STATUS After the IC is enable. STATUS will output logic high if LED Short/OVP/UVP/OTP conditions exist. STATUS will be reset after VIN or EN is re-applied. ⎛⎞ ×⎜⎟ ⎝⎠ OVP2 OUT(OVP) OVP OVP1 R V = V 1 + R ⎛⎞ ×⎜⎟ ⎝⎠ OVP2 OUT(UVP) UVP OVP1 R V = V 1 + R |
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