The SCEL:CCS125 is a SPLat Circuit Element Library design for driving (sourcing) an analog constant DC current of 125mA full scale into a ground (0V) referred load. It provides a non-isolated output, with the load current sourced from the positive supply. A typical application would be a hydraulic proportional valve. The circuit can be fitted with a reversing relay.

The circuit has two particular features/attributes:

• To allow the load to be grounded, there is no output current sense resistor. This leads to somewhat lower accuracy and linearity than would be achieved with a current sense resistor. The advantage is that a short to ground from either load terminal cannot produce an uncontrolled high current in the load. A current mirror design results in good accuracy despite this.
• As designed the circuit is optimized to drive 125mA into a load of up to 24Ohms, developing up to 3V across the load, from a supply voltage between 10V and 28V.

The circuit was initially designed for a proportional hydraulic valve in an automotive application. It was driven off the 3.7kHz PWM output on an MMi99 SPice connector.

The circuit appears to the SPLat programmer as an analog output. Linearity, scale factor accuracy and temperature stability are good, with some non-linearity at very low outputs.

In the following characteristic data the “typical” figures are results of prototype testing. The “worst case” figures are our estimates of worst case performance, but are not guaranteed. When negotiating an implementation of this, or any other, circuit element in your custom SPLat controller, you must identify the parameters that are most critical.