CONSTANT_CURRENT_SOURCE_CONVERTER

Published:2009/7/13 0:03:00 Author:May | From:SeekIC

To maintain regulation, the switching voltage regulator shown includes independent loops of current and voltage feedback. If the voltage loop is disabled, the current loop can be used to implement a general-purpose current source. The first step in obtaining a current source is to apply 5 V to V+. Because the chip expects 12 V of feedback at that terminal, it assumes a loss of regulation and shifts control to the current loop. This mode of operation allows an increasing ramp of current through Q1, causing the voltage at pin 8 to increase until it reaches the internal comparator threshold (210 mV). Timing circuitry then turns off Q1 for a fixed 2.3 μs, and the cycle repeats. The result is a relatively constant inductor current, which also happens to be the load current.With a proper choice of component values, the circuit generates constant current over a wide range of input voltages. The circuit (with component values shown) is a fast charger for NiCd batteries that provides 60-mA charging currents.



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