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FAN5340 Arkusz danych(PDF) 9 Page - Fairchild Semiconductor |
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FAN5340 Arkusz danych(HTML) 9 Page - Fairchild Semiconductor |
9 / 12 page © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5340 • Rev. 1.0.1 9 Circuit Description Overview The FAN5340 is an inductive current-mode boost serial LED driver that achieves LED current regulation by maintaining 0.5V across RSET. The current through the LED string (ILED) is therefore: SET LED R I 5 . 0 = (1) While the forward-voltage across the LEDs determines VOUT, the FAN5340’s boost regulator output can also support additional loads on VOUT (see Figure 21) provided its input current limit is not exceeded. Figure 21. Maximum Output Current vs. Input Voltage UVLO and Soft-Start If EN has been LOW for more than 20ms, the IC initiates a “cold start” soft-start cycle when EN rises, provided VIN is above the UVLO threshold. The soft-start circuit ramps the voltage reference to the error amplifier to control inrush current. PWM Dimming When EN goes LOW, the IC turns off a MOSFET (Q3 in Figure 2), which disconnects the LED load, preventing COUT from being discharged when EN is LOW. As long as EN is low for less than 20ms, the regulator’s main regulation loop quickly regains control when EN returns to a HIGH state. Short-Circuit Detection If VOUT falls below VIN – 1.5V, Q3 turns off and remains off until VOUT recovers to at least VIN – 1.3V. Over-Voltage 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 FAN5340’s OVP circuit disables the boost regulator when VOUT exceeds 19.0V and continues to keep the regulator off until VOUT drops below 18.2V. Thermal Shutdown If 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. Applications Using VOUT to Drive Additional LED Strings The VOUT pin can be used as a supply for simple current sources (shown in Figure 22 using the FAN5640) or discrete current sinks. To avoid dragging VOUT down when the EN pin is LOW, the auxiliary strings should not be enabled unless the EN pin is HIGH. The auxiliary strings can therefore be PWM dimmed using either the same line as the EN line as shown below or enabled separately, but within the on-time of the FAN5340. GND RSET FAN5640 1 2 3 6 5 4 IOUT1 EN VIN IOUT2 R1 FAN5340 FB 4 3 2 1 5 6 7 8 VLED EN GND PGND SW VOUT VIN C OUT C IN R2 Figure 22. Driving Additional LED Strings If using VOUT to drive additional loads, care should be taken not to exceed the input current limit. This limitation is shown in Figure 21 for a typical IC. The total load (IOUT1 + IOUT2 + ILED) should always remain below 70% of the value in Figure 21. |
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Podobny opis - FAN5340 |
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