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Zakładka z wyszukiwarką danych komponentów |
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LM2791 Arkusz danych(PDF) 8 Page - National Semiconductor (TI) |
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LM2791 Arkusz danych(HTML) 8 Page - National Semiconductor (TI) |
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8 / 11 page  Circuit Description The LM2791 provides two matched current sources for driv- ing high forward voltage drop LEDs from Li-Ion battery sources. The device has on-chip current regulators which are composed of current mirrors with a 25 to 1 ratio. The mirrors control the LED current without using current limiting resistors in the LED current path. The device can drive up to a total of 36mA through the LEDs. The LED brightness can be controlled by both analog and or digital methods. The digital technique uses a PWM (Pulse Width Modulation) signal applied to the shutdown input. The analog technique applies an analog voltage to the brightness (BRGT) pin (see Application Information sections). For low- est cost, the LM2791 can be used for constant brightness by grounding BRGT and enabling the shutdown pin. Application Information SOFT START LM2791 includes a soft start function to reduce the inrush currents and high peak current during power up of the de- vice. Soft start is implemented internally by ramping the bandgap more slowly than the applied voltage. This is done by holding the bandgap in shutdown for a short time. During soft start, the switch resistances limit the inrush current used to charge the flying and hold capacitors. SHUTDOWN MODE A shutdown pin (SD or /SD) is available to disable the LM2791 and reduce the quiescent current to 1µA maximum. The LM2791 is available with both senses of shutdown polarity. During normal operation mode of the ’-L’ options, an active high logic signal to the SD pin or tying the SD pin to V IN, will enable the device. Pulling SD low or connectingSD to ground will disable the device. During normal operation mode of the ’-H’ options, an active low logic signal to the SD pin or tying the SD pin to GND, will enable the device. Pulling SD high or connecting SD to V IN will disable the device. CAPACITOR SELECTION Low equivalent series resistance (ESR) capacitors such as X5R or X7R are recommended to be used for C IN, C1, C2, and C HOLD for best performance. Ceramic capacitors with less than or equal to 0.3 ohms ESR value are recommended for this application. Table 1 below lists suggested capacitor suppliers for the typical application circuit. TABLE 1. Low ESR Capacitor Manufactures Manufacturer Contact website TDK (847) 803 6100 www.component.tdk.com MuRata (800) 831 9172 www.murata.com Taiyo Yuden (800) 348 2496 www.t-yuden.com SCHOTTKY DIODE SELECTION A schottky diode (D1) must be used between V IN and POUT for proper operation. During start-up, the low voltage drop across this diode is used to charge C OUT and start the oscillator. It is necessary to protect the device from 20018304 FIGURE 1. www.national.com 8 |
Podobny numer części - LM2791 |
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Podobny opis - LM2791 |
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