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Current sources can be considered the outcast of the voltage source
Current sources can be considered the outcast of the voltage source. They are a part which can be tremendously useful, but is not particularly well understood.
A constant current source is a device driving a set current out of its pin. Physics dictates this being done by varying the output voltage – just like a voltage source varies its output current to keep U constant. Current sources can be used for a variety of things; when used to drive light emitting diodes, they ensure constant brightness. When used with a zener reference, a reduction in zener voltage changes can be achieved.
Designing current sources out of readily available transistors is not particularly difficult, but can become space intensive. NXP – the company recently changed its name and now calls itself Nexperia – offers a constant current source IC named PSSI2021SAY. It provides fixed currents ranging from 15yA to 50mA, and is able to handle supply voltages of up to 75V.
PSSI2021SAY is nifty due to its ability to be programmed. The part is not a classic constant current source: designers can set the desired working point by connecting a resistor as shown in the example in figure 1.
The resistor governs the amount of current produced…
This programming design, sadly, has its limitations. Achieving large output currents is only possible when the supply voltage is high – the datasheet provides the table shown in figure 2, which can act as a guideline for selecting voltage supplies.
…Maximum output current is limited by supply voltage
If the voltage demands can be met, the part can also have a on-off capability. NXP’s PDTC124XU allows the current source to be switched off by disconnecting its ground pin. An example design is shown in figure 3.
One good part rarely comes alone…
OEMSecret’s current going price for one piece hovers around 30 US cents. The main problem is that the part is available only in the SOT 353 housing, which is not particularly easy to prototype due to a pin pitch of 0.65 mm.