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EUP2794 Datasheet(PDF) 8 Page - Eutech Microelectronics Inc |
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EUP2794 Datasheet(HTML) 8 Page - Eutech Microelectronics Inc |
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8 / 13 page  EUP2794 DS2794 Ver0.5 Mar. 2006 8 Application Information Circuit Description The EUP2794 has four regulated current sources connected to the device’s 1.5x charge pump output (POUT). At input voltages below 3.82V (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 3.82V (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 I SET and BRGT PINS section). Soft-Start Soft start is implemented internally by limiting the current flow through the charge pump switches. During soft start, the input current and LED current will ramp up slowly and avoid current inrush. Shutdown Mode The shutdown pin (SD) disables the part and reduces the quiescent current less than 1µA (typ.).The EUP2794 has an active-low shutdown pin (LOW = shutdown, HIGH = operating). The EUP2794 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 EUP2794. Capacitor Selection The EUP2794 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, 15m ≒ Ω. typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are generally not recommended for use with the EUP2794 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the EUP2794. These capacitors have tight capacitance tolerance (as good as ±10%), hold their value over temperature (X7R: ±15% over -55°C to 125°C; X5R: ±15% over -55°C to 85°C), and typically have little voltage coefficient. ISET and BRGT Pins An external resistor, RSET, is connected to the ISET pin to set the current to be mirrored in each of the LED outputs. The internal current mirror sets each LED output current with a 10:1 ratio to the current through RSET. The current mirror circuitry matches the current through each LED to within 0.6%. In addition to RSET, a voltage may be applied to the VBRGT in to vary the LED current. By adjusting current with the Brightness pin (BRGT), the brightness of the LEDs can be smoothly varied. Applying a voltage on BRGT between 0 to 3 volts will linearly vary the LED current. Voltages above 3V do not increase the LED current any further. The voltage on the VBRGT pin is fed into an internal resistor network with a ratio of 0.385. The resulting voltage is then summed with a measured offset voltage of 0.188V, which comes from the reference voltage being fed through a resistor network (See Functional Block Diagram). The brightness control circuitry then uses the summed voltage to control the voltage across RSET. An equation for approximating the LED current is: ( ) () Ratio Mirror * SET R 0.385 * BRGT V OFFSET V LED I + = ( ) Amps 1 10 * SET R 0.385 * BRGT V 0.188 LED I + = |
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Similar Description - EUP2794 |
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