Precision Current Source and Sink in Mixed-Signal ASICs

A regulated voltage rail holds its output voltage while the load determines the current. Sensor excitation, LED drive, and other current-controlled applications require a different approach: the circuit establishes the current, and the load determines the resulting voltage. A current sink performs a related function by drawing a defined current from a supply-connected load. Both arrangements require enough voltage across the regulating circuit to maintain control.

The design starts with the required current, its allowable error, and the load’s voltage range. Those requirements determine whether a dedicated regulator is sufficient or whether current regulation belongs alongside monitoring and control functions. This article compares current sources and sinks, examines dedicated IC options, and develops the implementation using the op amps, analog switches, and configurable logic available in GreenPAK.

Current Sources and Current Sinks

When used in a circuit, an ideal current source is regulated to output a constant current value regardless of the load impedance. Real current sources essentially perform this function within some allowed voltage drop limit across the load. This current output is then maintained through a feedback loop, with the exact method varying in different circuits. For example, PWM regulation is used in switching current-mode regulators, while emitter/source degeneration or transistor/diode clamping may be used in other current source circuits.

Regulation continues while the supply can provide that voltage plus the headroom required by the current-regulating circuit. Several applications will use current-mode regulation instead of voltage-mode regulation:

• RTD excitation converts sensor resistance into a measurable voltage using a defined current
• LED driving regulates current as forward voltage changes with temperature and device variation
• Ramp generation involves charging a capacitor with linear voltage ramp according to dV/dt = I/C; the capacitor voltage is then output as a ramped waveform

Real current sources require a voltage input representing the maximum voltage drop that can be given to a load. For example, a 1 mA source driving 1 kOhm drops 1V across the load; the same current through 5 kOhm drops 5V, and so on. The supply must cover this voltage dropm as well as the losses in the regulator, wiring, etc. Once that voltage is exhausted, the circuit falls out of current regulation and becomes unstable, similar to what occurs in voltage regulators attempting to draw too much current.

Current Sinks

A current sink is another option for regulating current draw. It controls the current drawn through a load with either high-side or low-side placement, and a sense resistor can be placed in the return path for regulation. The control loop adjusts transistor conduction to maintain the required sense voltage. This arrangement is common in LED drivers and electronic loads.

The ground and supply connections on the load determine where a current sink should be placed in the load:

• A grounded (earthed) load calls for a regulation on its supply side (high-side)
• A supply-connected load can permit regulation inside its return path

It is important to note that a low-side sense resistor raises the load’s ground terminal above the 0V reference by a value equal to the sensing voltage; this offset must be included when another circuit measures the load voltage or shares its return connection. Essentially, any components sharing a reference net with the current sink must also connect to the high-side terminal of the low-side sense resistor (see image below).

Current Source/Sink ICs

There are options for packaged current source and sink ICs. For example, part number LT3092 from Analog Devices provides a dedicated current-source implementation with a programmable range, where two external resistors establish the current setting. Another example of a current sink is the TLC5916 from Texas Instruments, which provides eight regulated sink outputs for LED loads.

Broader system behavior still requires suitable control circuitry. Supply qualification, startup delays, and enable interlocks can require additional components around a dedicated regulator. GreenPAK addresses that integration requirement by placing the current-regulating analog circuit alongside configurable comparators, logic, and timers. The designer can develop application-specific control behavior within the same device, subject to its available resources.

Using GreenPAK for a Current Source/Sink

GreenPAK is a configurable mixed-signal processor that can consolidate multiple functions into a current source/sink design, essentially being a fully custom ASIC. An example of a current source involving the SLG47004 is shown below. This example works by holding a reference voltage on an op-amp and comparing that against a feedback resistor using the internal op-amp in the chip.

The internal op amp drives an internal MOSFET, producing a nominal load current of Vref/Rsense. In the source example, OPAMP0 controls SWITCH0’s P-channel MOSFET in Analog Switch mode. The Vref value is supplied internally and can be configured in real time using the I2C interface. The external feedback path closes the regulation loop around the load and sense resistor. This example is shown below in Go Configure.

The sink example is similar in that it also uses an op-amp and switch. Here, OPAMP1 drives SWITCH1 which is configured as an N-channel MOSFET. The internal programmable reference connects to the noninverting input and the sense feedback connects to the inverting input. The current then returns to ground through the sense resistor.

Both examples provide selectable amplifier bandwidth and a reference derived from the internal 2.048V source or VDDA through a six-bit divider. Just like with the current source example, the I2C interface allows adjustment of the reference and therefore the current setpoint.

The advantage of using GreenPAK is not just the reconfigurability of the current setpoints in these components. It is the ability to aggregate other functions into a single device which may also be needed in sensing interfaces, power electronics, or any other application area. This includes functions such as:

• Voltage measurements with an ADC
• Thresholding of analog signals with comparators
• Logic processing of digital or quantized analog inputs with LUTs
• Timing, delay, counting, and many other fundamental digital functions

The Go Configure development suite allows engineers to configure comparator thresholds, qualification logic, delays, and output behavior. Simulation and development hardware support evaluation of the resulting sequence, including early requests and interrupted settling. Device selection depends on the analog resources, timing blocks, and pins required for each independently controlled enable output.

To learn more, take a look at the GreenPAK components and reference examples.

• Learn more about GreenPAK components
• View GreenPAK supervisor examples

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