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LM2795BL Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM2795BL Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 16 page Functional Block Diagram 20028530 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 (P OUT). At input volt- ages 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 P OUT voltage 1.5 times the input voltage. The charge pump operates in Pass Mode, providing a voltage on P OUT 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 con- nected to the constant current outputs D 1-D4. To set the LED drive current, the device uses a resistor connected to the I SET 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 I SET and BRGT PINS section). 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. www.national.com 9 |
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