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EUP2983 Datasheet(PDF) 7 Page - Eutech Microelectronics Inc |
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EUP2983 Datasheet(HTML) 7 Page - Eutech Microelectronics Inc |
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7 / 8 page  EUP2983 DS2983 Ver1.2 Nov. 2008 7 Application Information Soft-Start The EUP2983 attains soft-start by charging CCOMP gradually with a current source. When VCOMP rises above 1.25V, the internal MOSFET begins switching at a incremental duty cycle. When VCOMP rises above 2.25V, the duty cycle is at its maximum. See the Typical Operating Characteristics for an example of soft-start operation. Shutdown The EUP2983 enters shutdown when VCTRL is less than 100mV for more than 8.2ms. In shutdown, supply current is reduced to 150µA by powering down the entire IC except for the CTRL voltage-detection circuitry and internal regulator. CCOMP is discharged during shutdown, allowing the device to reinitiate soft-start when it is enabled. Typical shutdown timing characteristics are shown in the Typical Operating Characteristics. Overvoltage Protection Overvoltage lockout (OVLO) occurs when VOUT is above 32V.The protection circuitry stops the internal MOSFET from switching and causes VCOMP to decay to 0V. The device comes out of OVLO and into soft-start when VOUT falls below 30V. In normal operation, EUP2983 VOUT calculates as follows: K LED V FB V IN V OUT V × + + = Where K----------number of LEDs in each string VIN--------input voltage VFB--------feedback voltage VLED-------a LED forward voltage Adjusting LED Current Adjusting the EUP2983’s output current changes the brightness of the LEDs. An analog input CTRL and the sense-resistor value set the output current. Output current is given by: The VCTRL voltage range for adjusting output current is 0.24V to 1.68V. To set the maximum current, calculate RSENSE when VCTRL is at its maximum as follows: Power dissipation in RSENSE is typically less than 10mW, making a standard chip resistor sufficient. PWM Dimming Control CTRL is also used as a digital input allowing LED brightness control with a logic-level PWM signal applied directly to CTRL. The frequency range is from 200Hz to 200kHz, while 0% duty cycle corresponds to zero current and 100% duty cycle corresponds to full current. The error amplifier and compensation capacitor form a lowpass filter so PWM dimming results in DC current to the LEDs without the need for any additional RC filters; see the Typical Operating Characteristics. Capacitor Selection The exact values of input and output capacitors are not critical. The typical value for the input capacitor is 10µF and the typical value for the output capacitor is 2.2µF. Larger value capacitors can be used to reduce input and output ripple, but at the expense of size and higher cost. CCOMP stabilizes the converter and controls soft-start. Connect a 0.1µF capacitor from COMP to GND. Inductor Selection Inductor values range from 10µH to 47µH. A 22µH inductor optimizes the efficiency for most applications while maintaining low 50mVP-P input ripple. To prevent core saturation, ensure that the inductor-saturation current rating exceeds the peak inductor current for the application. Calculate the peak inductor current with the following formula: Schottky Diode Selection The EUP2983’s high switching frequency demands a high-speed rectification diode (D1) for optimum efficiency. A Schottky diode is recommended due to its fast recovery time and low forward-voltage drop. Ensure that the diode’s average and peak current rating exceed the average output current and peak inductor current. In addition, the diode’s reverse breakdown voltage must exceed VOUT. The RMS diode current can be calculated from: PC Board Layout Due to fast switching waveforms and high-current paths, careful PC board layout is required. An evaluation kit is available to speed design. When laying out a board, minimize trace lengths between the IC and RSENSE, the inductor, the diode, the input capacitor, and the output capacitor. Keep traces short, direct, and wide. Keep noisy traces, such as the LX node trace, away from FB. The IN bypass capacitor (CIN) should be placed as close to the IC as possible. PGND and GND should be connected directly to the exposed paddle underneath the IC. The ground connections of CIN and COUT should be as close together as possible. The traces from IN to the inductor and from the Schottky diode to the LEDs may be longer. () MAX LED I 10 1.68 SENSE R × = SENSE R 10 CTRL V LED I × = () () () () L 2 s 0.9 MIN IN V MIN IN V 0.9 MAX LED I MAX OUT V PEAK I × µ × + × × = () PEAK I OUT I RMS DIODE I × = |
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Similar Description - EUP2983 |
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