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SS6896 데이터시트(PDF) 9 Page - Silicon Standard Corp. |
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SS6896 데이터시트(HTML) 9 Page - Silicon Standard Corp. |
9 / 12 page www.SiliconStandard.com 9 of 12 Rev.2.01 6/06/2003 SS6896 Application Information Inductor Selection A 15 µH inductor is recommended for most SS6896 applications. Although small size and high efficiency are major concerns, the inductor should have low core losses at 1.4MHz and low DCR (copper wire resistance). Capacitor Selection The small size of ceramic capacitors makes them ideal for SS6896 applications. X5R and X7R types are recommended because they retain their capacitance over wider ranges of voltage and temperature than other types, such as Y5V or Z5U. A 4.7 µF input capacitor and a 1µF output capacitor are sufficient for most SS6896 applications. Diode Selection Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for SS6896 applications. The forward voltage drop of a Schottky diode represents the conduction losses in the diode, while the diode capacitance (CT or CD) represents the switching losses. For diode selection, both forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher current ratings usually have lower forward voltage drop and larger diode capacitance, which can cause significant switching losses at the 1.4MHz switching frequency of the SS6896. A Schottky diode rated at 100mA to 200mA is sufficient for most SS6896 applications. LED Current Control LED current is controlled by a feedback resistor (R1 in Fig. 1). The feedback reference is 1.23V. The LED current is 1.23V/R1. In order to have accurate LED current, precision resistors are preferred (1% recommended). The formula for R1 selection is shown below. R1 = 1.23V/ILED (1) Open-Circuit Protection In the cases of output open-circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback voltage will be zero. The SS6896 will then switch to a high duty cycle resulting in a high output voltage, which may cause the SW pin voltage to exceed its maximum 30V rating. A zener diode can be used at the output to limit the voltage on the SW pin (Fig. 20). The zener voltage should be larger than the maximum forward voltage of the LED string. The current rating of the zener should be larger than 0.1mA. Dimming Control There are three different types of dimming control circuits as follows: 1. Using a PWM signal PWM brightness control provides the widest dimming range by pulsing the LEDs on and off using the control signal. The LEDs operate at either zero or full current, The average LED current changes with the duty cycle of the PWM signal. Typically, a 1kHz to 10kHz PWM signal is used. PWM dimming with the SS6896 can be accomplished two different ways (see Fig. 21). The SHDN pin can be driven directly or a resistor can be added to drive the FB pin. If the SHDN pin is used, increasing the duty cycle will increase the LED brightness. If the FB pin is used, increasing the duty cycle will decrease the brightness. Using this method, the LEDs are dimmed using FB and turned off completely using SHDN . 2. Using a DC Voltage For some applications, the preferred method of brightness control uses a variable DC voltage to adjust the LED current. The dimming control using a |
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