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FAN5343UMPX Folha de dados(PDF) 8 Page - Fairchild Semiconductor |
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FAN5343UMPX Folha de dados(HTML) 8 Page - Fairchild Semiconductor |
8 / 11 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5343 • Rev. 1.0.4 8 Functional Description Overview The FAN5343 is an inductive current-mode boost serial LED driver that achieves LED current regulation by maintaining 0.25V across the RSET resistor. The current through the LED string (ILED) is therefore given by: SET LED R 25 . 0 I = (1) The voltage VOUT is determined by the the sum of the forward voltages across each LED, plus the voltage across RSET, which is always 250mV. UVLO and Soft-Start If EN has been LOW for more than 1ms, the IC may initiate a “cold start” soft-start cycle when EN rises, provided VIN is above the UVLO threshold. Digital Interface The FAN5343 implements a single-wire digital interface to program the LED brightness to one of thirty two (32) levels spaced in linear steps. With this single-wire solution, the FAN5343 does not require the system processor to constantly supply a signal to drive the LEDs. Digital Dimming Control The FAN5343 starts driving the LEDs at the maximum brightness level. After startup, the control logic is ready to accept programming pulses to decrease the brightness level by the positive edges applied to the EN pin. Figure 14 illustrates the digital pulse dimming control for the FAN5343. Over-Current and Short-Circuit Detection The boost regulator employs a cycle-by-cycle peak inductor current limit of ~750mA to protect the switching elements and the device itself from being damaged. Over-Voltage / Open-Circuit Protection If the LED string is open circuit, FB remains at 0V and the output voltage continues to increase in the absence of an Over-Voltage Protection (OVP) circuit. The FAN5343’s OVP circuit disables the boost regulator when VOUT exceeds 24.5V and keeps the regulator off until VOUT drops below 22.5V. Thermal Shutdown When the die temperature exceeds 150°C, a reset occurs and remains in effect until the die cools to 125°C; at which time, the circuit is allowed to begin the soft-start sequence. Figure 14. Digital Pulse-Dimming Control Diagram |
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