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SC440AMLTRT Datasheet(PDF) 11 Page - Semtech Corporation |
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SC440AMLTRT Datasheet(HTML) 11 Page - Semtech Corporation |
11 / 21 page SC440A 11 www.semtech.com © 2010 Semtech Corp. Applications Information SC440A Detailed Description The SC440A contains a high frequency, current-mode boost regulator and six string LED current sources. The LED current for all strings is programmed by an external resistor and the PWM controller operates to maintain the output voltage at a level which will keep the current of each string regulated. A typical application would use 3-8 backlight LEDs for each string, driven with approximately 30mA. Operation The SC440A regulates the boost converter output voltage based on instantaneous requirement of the six string current sources. Therefore, only a single inductor and power switch is needed to provide power to the entire lighting subsystem, increasing efficiency and reducing part count. A logic interface to output control circuit has high-bandwidth, and supports PWM dimming with 50Hz to 50kHz dimming frequency while the entire supply current is reduced to 4.5mA (typical) when all LED strings are off. High frequency switching provides high output power using a 1.0mm height inductor, maximizing efficiency for space-constrained and cost-sensitive applications. Additionally, the converter and output capacitor are protected from open-LED conditions by programmable over voltage protection. LED Current Programming The SC440A is a LED current programmable regulator. The LED current set point is chosen using external an resistor connected to the IOSET pin. The relationship between the programming resistor value and the LED current set point of each string can be described as follows: IOSET LED R 60 I Where, R IOSET is in k Ω. I LED is the output current of each string in mA. Start-Up During start-up, when the VIN pin voltage reaches its UVLO threshold, and both EN and PWM signals are set to high, the SS pin begins to source 6 μA to the SS capacitor and its voltage begins to rise from 0V to its end value (2.6V). The output voltage of the internal error amplifier (COMP) is increases and clamped by the SS pin voltage. When the SS pin voltage reaches its switching threshold, the SC440A starts to switch and the output voltage increases. Each internal LED current source (IO1 ~ IO6) tries to regulate the LED current to its set point. While the output voltage increases, a suitable amount of error information will be generated on the internal error amplifier as the COMP pin voltage keeps rising. Once each LED current reaches its set point, the error information is not generated by the LED current source. The COMP pin voltage stays at a level which keeps the LED current at its set point. If the EN pin voltage is pulled below 0.4V and VIN reaches to its UVLO, SC440A will stay at shutdown mode, drawing less than 1μA from the input power supply. If the PWM pin voltage is pulled below 0.4V when the EN pin is pulled high and VIN reaches its UVLO, the SC440A runs in standby mode, drawing 4.5mA (typical) from the input power supply. Under this condition, soft-start is initiated and the SS pin voltage is raised to its end value since the EN pin is pulled high. After that, when PWM signal goes high to enable SC440A, the COMP pin voltage will rise as quickly as it can since it is not being limited by the SS pin. A proper capacitance (10nF ~ 100nF) is required for the COMP pin and its external RC network in order to prevent output voltage overshoot. Shut Down When the VIN pin voltage falls below its UVLO or EN pin voltage goes low, the SC440A will run in shutdown mode. The internal switch and LED current sources will be immediately turned off. The SS capacitor is discharged by SS pin internal current source and the SS pin voltage decreases to 0V. The output voltage falls to the same level as the input voltage. |
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