APPLICATION NOTE 4480

Precision Constant-Current Sink Provides Wide, Close-to-the-Rail Compliance


Abstract: This precision constant-current sink produces a current of 1mA with a compliance range of 0.7V to 50V, and a dynamic output impedance of 106 ohms. It is stable with any load type (resistive or reactive), and stable over the allowed ranges of temperature, power-supply voltage, and compliance voltage (upper and lower limits).

A similar version of this article appeared in the February 2009 issue of Portable Design magazine.

A precision current sink can be used to generate a voltage bias for sensors, amplifiers, and other analog circuits. They can also generate a reference voltage (across a resistor in series with its output) referred to voltage levels that are variable or well above the common equipotential. Such current sinks consist of a voltage reference, an error amplifier, a current-sense resistor, and an output device—usually one capable of handling a useful amount of power, and rated for voltages at the expected upper limit of the output-compliance range.

Desirable characteristics for this type of circuit block include a very large dynamic output impedance, stability with any load type (resistive or reactive), and stability over the allowed ranges of temperature, power-supply voltage, and compliance voltage (upper and lower limits).

The circuit of Figure 1a, with component values as shown, produces a current of 1mA with a compliance range of 0.7V to 50V, and a dynamic output impedance of 106 ohms. The circuit of Figure 1b can do the same, with a compliance range that extends down to 0.25V, but it requires an extra, separate reference voltage.

Figure 1. These current-sink circuits produce 1mA with an output-compliance range of 0.7V to 50V (a), and 0.25V to 50V (b).
Figure 1. These current-sink circuits produce 1mA with an output-compliance range of 0.7V to 50V (a), and 0.25V to 50V (b).

The reference/amplifier function is implemented with a shunt regulator (MAX8515), and the power output device (BSS123) is a small n-channel MOSFET with maximum VDS of 100V (VDS is the current sink's higher compliance limit). The current sample is obtained from a resistor (RSOURCE) in series with the BSS123 source. You can adjust this current by changing the resistor value according to the following equation:
Circuit of Figure 1a: ISINK = 0.6V/RSOURCE.
Circuit of Figure 1b: ISINK = 0.091V/RSOURCE.
Other performance includes the stability of sink current with output voltage (Figure 2), and the variation of sink current with temperature over the range 0°C to +50°C (±0.75%, Figure 3).

Figure 2. Sink current vs. output voltage for the circuit of Figure 1a.
Figure 2. Sink current vs. output voltage for the circuit of Figure 1a.

Figure 3. Sink current varies ±0.75% over the temperature range shown, for the circuit of Figure 1a.
Figure 3. Sink current varies ±0.75% over the temperature range shown, for the circuit of Figure 1a.

Related Parts
MAX8515 Free Sample
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APP 4480:
APPLICATION NOTE 4480,AN4480, AN 4480, APP4480, Appnote4480, Appnote 4480