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MX841 Fiches technique(PDF) 4 Page - IXYS Corporation |
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MX841 Fiches technique(HTML) 4 Page - IXYS Corporation |
4 / 6 page Preliminary MX841 4 03/10/05 MX841 IXYS Functional Description The MX841 is specifically designed to be operated from a single cell battery source. The step-up voltage necessary for the constant current source to drive up to 5 white LED’s in series, or multiple strings of white LED’s, is achieved with minimal external components. (See Application Schematics on pages 1&4). Setting the LED Current The regulated output current is determined by the selection of the sense Resistor connected from the ISENSE input to ground, and the maximum user defined voltage of the CTRL input. The desired output current, (typically 15mA to 20mA for white LEDs), is derived using the following formula for the RSENSE value: RSENSE Ω = (VCTRL X 1/13) / ILED. Current Regulation / Light Intensity Control The ISENSE voltage is fed into an ErrorAmp/Comparator. The user defined Control Voltage applied at the CTRL input is scaled by a gain factor of 1/13 and compared to the ISENSE voltage. If the scaled control voltage is greater than the ISENSE voltage, the 1.0MHz oscillator signal driving the N-Channel MOSFET will be enabled. Once the ISENSE voltage reaches the scaled CTRL voltage the oscillator signal is modulated on and off by the ErrorAmp/Comparator to maintain the desired regulated output current. Light intensity is controlled by reducing the voltage at the CTRL input. As the CTRL input voltage is reduced below the user defined maximum value, the oscillator signal driving the N-Channel MOSFET is interrupted by the ErrorAmp/Comparator. The pulse skip modulating of the N-Channel MOSFET causes a reduction in the regulated current equal to the ratio of the reduced CTRL voltage to the maximum CTRL voltage. Sleep Mode The MX841 goes into sleep mode when the VCTRL input is less than 100mV. All circuits except VCTRL detection are shutdown and the supply current falls to 6uA. Once the VCTRL input rises above 260mV the MX841 resumes normal operation. Over Voltage Detection The MX841 will go into a shutdown state to avoid an over voltage condition if the current path between the SW output and the RSENSE input is interrupted. A voltage reference and comparator circuit evaluates the voltage potential at the OVD input and shuts down the oscillator and N- Channel MOSFET if an over voltage condition is detected. Normal operation is resumed once the OVD input voltage falls below the reference voltage, (20V typical). Drawing No. 17023 www.ixys.net |
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