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FAN5701UC20X Datasheet(PDF) 9 Page - Fairchild Semiconductor |
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FAN5701UC20X Datasheet(HTML) 9 Page - Fairchild Semiconductor |
9 / 12 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5701 • Rev. 1.0.3 9 Circuit Description The FAN5701 is a white LED driver system based upon an adaptive 1.5x/1x charge pump capable of supplying up to 180mA of total output current. The tightly matched current sinks ensure uniform brightness across the LEDs. Each LED is configured in a common anode configuration with its peak drive current set during manufacturing (see Ordering Information and ISET). Charge Pump The charge pump operates in either 1x mode, where VOUT is connected to VIN through a bypass switch, or in 1.5x mode. The circuit operates in 1x mode until the LED with the highest forward voltage (VLED(MAX) ) can no longer maintain current regulation. At that point, 1.5x mode begins. If the lowest active cathode voltage is greater than 1.8V, the charge pump switches back to 1x mode. In addition to hysteresis, a 1ms transition delay is provided for the device to ignore short-duration input voltage drops in deciding mode transitions. IC Enable When the EN pin is LOW, all circuit functions are disabled. When the EN pin HIGH, the entire chip is enabled. Both PWM inputs are now functional. PWM Dimming External PWM inputs (group A and group B) directly modulate output currents in their corresponding LED channels to vary the perceived LED brightness. Two PWM inputs are provided to control two independent groups of LEDs, such as that of a main display panel and a secondary panel. They can also be connected to a single input to simultaneously dim all six LED outputs. VOUT Short-Circuit Protection The FAN5701 has integrated protection circuitry to prevent the device from being short circuited when the output voltage falls below 2V. If this occurs FAN5701 turns off the charge pump and the LED driver outputs, but a small bypass switch is left on. The device monitors the output voltage to determine if it is still in short circuit condition and, once it has passed, FAN5701 soft-starts and returns to normal operation. VOUT Over-Voltage Protection If the output voltage goes above 6V, the FAN5701 shuts down until this condition has passed. The charge pump and LED driver outputs are turned off. Once this condition has passed, FAN5701 soft-starts into normal operation. |
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