Low-dropout linear regulators deliver constant currents

Linear voltage regulators offer a simple method of producing a constant current by connecting a fixed resistor between the regulator’s output and ground nodes. The regulator’s constant output voltage produces a constant current through the resistor. You can use the basic circuit as either a high-side or a low-side current source. The high-side current source uses a positive-output linear voltage regulator, IC1, a Maxim MAX1818, to provide a constant current of 25mA to the load resistance (Figure 2). The design imposes two conditions: First, the voltage between IC1’s VCC and ground terminals must not exceed 5.5V. Second, the voltage between IC1’s input and ground terminals must meet or exceed 2.5V, the minimum voltage for proper operation. To satisfy these conditions, choose an output resistance value that allows 2.5 to 5.5V between input and ground and provides a fixed output of 1.5V across the output resistance at the desired load current.

For example, if you use the circuit to drive a constant current through a 100Ω maximum load resistance while applying 5V VCC between IC1 and ground, the circuit functions properly when ROUT equals or exceeds 60Ω. This value allows a maximum programmable current of 1.5V/60Ω, or 25mA. The voltage across IC1 then equals the allowed minimum: 5V–(25mA 100Ω)= 2.5V. Available in six-pin SOT-23 packages, the MAX1818 can source as much as 500mA.

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The low-side current-source circuit draws a constant current of 2.5V divided by the output resistance through the load resistance (Figure 2). In this example, IC1, a MAX1735 linear negative-voltage regulator, provides a fixed output voltage of –2.5V. As in Figure 1, ensuring a voltage of 2.5 to 6.5V between IC1’s ground and input terminals represents the only precaution for its proper operation. To satisfy that condition, choose an output resistance value that allows 2.5 to 6.5V between ground and the input. When using the circuit to draw current through a maximum load of 100Ω with VCC at 5V, the output resistance should exceed 100Ω, which provides a maximum programmable current of 2.5V/100Ω=25mA, which in turn produces a minimum recommended voltage across the device of 5V–(25mA 100Ω)=2.5V. The MAX1735 can source as much as 200mA and occupies a five-pin SOT- 23 package.





In addition to the programmed load current, both configurations allow the regulator’s quiescent current to flow through the load and introduce a source of error that varies with the voltage you apply between the regulator’s input and ground connections. You can minimize the error by choosing a voltage regulator that draws low quiescent current or whose quiescent current remains constant through the operating range and allows you to compensate the error by adjusting the value of the output resistance. Quiescent currents for the devices in Figures 1 and 2 typically average 130μA and vary less than 40μA for a regulator input-voltage range of 2.5 to 5V.