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FAN5346S30X Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FAN5346S30X Datasheet(HTML) 10 Page - Fairchild Semiconductor |
10 / 14 page © 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5346 • Rev. 1.0.1 10 Circuit Description Overview The FAN5346 is an inductive current-mode boost serial LED driver that achieves LED current regulation by maintaining 0.250V across the RSET resistor. The current through the LED string (ILED) is given by: SET LED R I 250 . 0 = (1) The voltage VOUT is determined by the sum of the forward voltages across each LED, plus the voltage across RSET, which is always 250mV. Driving Eight LEDs in Series FAN5346S30X can drive 8 LEDs in series, but the minimum input voltage (VIN) must be greater than or equal to 2.9V, while the forward voltage of the white LED should be less than or equal to 3.2V, and the maximum LED current cannot exceed 20mA to maintain stable operation. 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. PWM Dimming The FAN5346 uses a PWM signal to directly modulate output current in the LED string to vary the perceived LED brightness. When the EN pin is held HIGH, the FB voltage is 250mV. This voltage is reduced when a PWM signal is applied to the EN pin, thereby enabling the LEDs to be dimmed. The FB voltage is given by the equation: mV DutyCycle V FB 250 × = (2) where DutyCycle = the duty cycle of the PWM signal and 250mV is the internal reference voltage. Figure 28 illustrates how the FAN5346 divides the internal 250mV reference voltage at the duty cycle of the PWM signal. A low-pass filter filters the PWM signal, which then is input into the error amplifier as the reference voltage for the FB pin. Figure 28. Block Diagram of FB and EN Circuit for PWM Dimming Over-Current and Short-Circuit Detection The boost regulator employs a cycle-by-cycle peak inductor current limit of 300mA (typical) and 750mA (typical) for FAN5346S20X and FAN5346S30X, respectively. Over-Voltage / Open-Circuit Protection If the LED string is an open circuit, FB remains at 0V and the output voltage continues to increase in the absence of an over- voltage protection (OVP) circuit. The FAN5346S20X OVP circuit disables the boost regulator when VOUT exceeds 20V and continues to keep the regulator off until VOUT drops below 19V. For FAN5346S30X, the OVP is 30V and it turns back on when VOUT is below 29V Thermal Shutdown When the die temperature exceeds 150°C, a reset occurs and remains in effect until the die cools to 115°C; at which time, the circuit is allowed to begin the soft-start sequence. |
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