PROGRAMMABLE_CURRENT_SOURCE

Published:2009/7/13 0:04:00 Author:May | From:SeekIC

Constant-current Sources are required in many applications,particularly when it comes to battery charging in such applications,it is desirable that the output current be accurate,temperaturestable,and adjustable The controller also can be successfully employed as the control element in a low-cost linear current source.Output current is sensed by resistor R1,with the value selected so that 100 mV is full-scale The voltage across R1 is amplified by a factor of 10 and averaged across capacitor C4.AN internal transconductance error amplifier compares the voltage on pin 8 against the programming voltage at pin 7.The error-amplifier outputis present on pin 2 (Iout),and is level-shiftedby Q2 to control the PNP pass transistor Output current is programmed by adjusting the voltage across R5 (1 V full-scale).The LT1121 LDO regulator provides a 5-V,±1.5 percent reference voltage so that current can be accurately programmed by simply connecting different values for Rprog. The input voltage can range from +6 to +28 V, with output current changing less than 0.3 percent Properheat sinking must be provided for Q1, especially when operating with large input-to-output voltage differentials.Transistor Q3 and R4 limit the magnitude of Q1's base drive during dropout,preventing excessive dissipation in driver transistor Q2 using voltage regulator IC2, the constant-currentSource operates directly from the unregulated input voltage. Pulling IC2's shutdown ptn lowturns off VCC to the entire circuit,and limits the reverse current drawn from the output to less than 25μA.



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