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LM2794 Datasheet(PDF) 10 Page - Texas Instruments |
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LM2794 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 24 page VIN LM2795 SD Shutdown Control *Only on LM2795 10PA * LM2794, LM2795 SNVS168L – JANUARY 2002 – REVISED MAY 2013 www.ti.com APPLICATION INFORMATION CIRCUIT DESCRIPTION The LM2794/5 is a 1.5x/1x CMOS charge pump with four matched constant current outputs, each capable of driving up to 20mA through White LEDs. This device operates over the extended Li-Ion battery range from 2.7V to 5.5V. The LM2794/5 has four regulated current sources connected to the device's 1.5x charge pump output (POUT). At input voltages below 4.7V (typ.), the charge-pump provides the needed voltage to drive high forward voltage drop White LEDs. It does this by stepping up the POUT voltage 1.5 times the input voltage. The charge pump operates in Pass Mode, providing a voltage on POUT equal to the input voltage, when the input voltage is at or above 4.7V (typ.). The device can drive up to 80mA through any combination of LEDs connected to the constant current outputs D1-D4. To set the LED drive current, the device uses a resistor connected to the ISET pin to set a reference current. This reference current is then multiplied and mirrored to each constant current output. The LED brightness can then be controlled by analog and/or digital methods. Applying an analog voltage in the range of 0V to 3.0V to the Brightness pin (BRGT) adjusts the dimming profile of the LEDs. The digital technique uses a PWM (Pulse Width Modulation) signal applied to the Shutdown pin (SD). (see ISET AND BRGT PINS). SOFT START Soft start is implemented internally by ramping the reference voltage more slowly than the applied voltage. During soft start, the current through the LED outputs will ramp up in proportion to the rate that the reference voltage is being ramped up. SHUTDOWN MODE The shutdown pin (SD) disables the part and reduces the quiescent current to 2.3µA (typ.). The LM2795 has an active-high shutdown pin (HIGH = shutdown, LOW = operating). An internal pull-up is connected between SD and VIN of the LM2795. This allows the use of open-drain logic control of the LM2795 shutdown, as shown in Figure 22. The LM2795 SD pin can also be driven with a rail-to-rail CMOS logic signal. Figure 22. Open-Drain Logic Shutdown Control The LM2794 has an active-low shutdown pin (LOW = shutdown, HIGH = operating). The LM2794 SD pin can be driven with a low-voltage CMOS logic signal (1.5V logic, 1.8V logic, etc). There is no internal pull-up or pull-down on the SD pin of the LM2794. CAPACITOR SELECTION The LM2794/5 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, ≤15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are generally not recommended for use with the LM2794/5 due to their high ESR, as compared to ceramic capacitors. 10 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 |
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