How to design voltage reference by limiting current consumption

This article is intended to explain how to design voltage reference by limiting current consumption throughout the voltage divider branch in order to reduce power loss and increase battery life cycle.

Let's say that requirement is to design one bit analog to digital converter which is nothing else than comparator circuit with two inputs and one output.

The analog to digital converter will be used for IR analog distance sensing intended for mobile robots. What do we actually need here is a comparator, able to inform control device (microcontroller/processor/SoC etc.) with external event "object detected" in case when IR sensing device detects wall or some other object in 3D space. One input of the comparator is connected to the IR analog sensor output, while second input is connected to the voltage reference point (Vref), in order to provide triggering threshold. In case when the analog signal value (analog signal from IR distance detector) becomes equal or larger than provided voltage reference (Vref), the output of the comparator is set to a logic one, where logic one value is determined by the comparator power supply positive value. For example: if the power supply voltage level, used to supply comparator with electrical power is +12V, in that case, logic one is set to ~+12V (logic one level depends on several different characteristics related to comparator/operational amplifier, used to implement one bit analog to digital converter).

For the sake of simplicity in presented schematics only voltage dividers are shown, all other parts of the schematic is presented as block diagram, labeled as Electronic circuit (analog 2 digital), figure #1. In case when only voltage reference is required (the potential difference between the ground and the Vref), it is important to reduce current consumption throughout the branch ab (R1-R2 branch), by decreasing current to the very small value by using resistors with large resistance.

Figure #1: Voltage reference - voltage divider.

Voltage reference is implemented by two resistors connected in series in order to provide voltage reference point – Vref (comparator triggering threshold). In the first case, figure #1 (left), two resistors both declared as 10 ohms resistance, are used to implement voltage divider. In the second example, figure #1 (right), two resistors both declared as 10K resistance are used to implement voltage divider.

By equations we can see that in the first case, current consumption throughout the branch ab (R1-R2 branch) is much higher in comparison with the second case were 10K resistors are used. That also means that power loss is higher in the first case than in the second case. To avoid power loss in the branch ab (R1-R2 branch), it is important to use resistors with very high values, like in the second example. The power loss is not the only issue related to the first case. If battery pack is used to supply mobile robot, including one bit analog to digital sensing device, the battery will be discharged much faster, because a lot of current, without any reason, flows through the branch ab (R1-R2 branch), since the current always flow through the smallest resistance in the circuit.

To avoid all these kind of problems, especially in case when a voltage reference point is all what is requested by requirement, nothing else, it is always important to use resistor with very high resistance. In that case, throughout the branch ab (R1-R2 branch), very small current flows with very small power loss. Also, resistors designed for smaller power consumption are much smaller in package (in dimension) and that is also an advantage, because smaller PCB surface is required in that case.

This is only an example, so pay attention that second example (figure #1 – right) has an additional capacitor between Vref voltage point and the ground. The capacitor is not important in this case, but if you need a stable Vref point, it is important to have capacitor as a low pass filter – additional stabilization.

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zilsel-invent assumes no responsibility or liability for any errors or inaccuracies that may appear in the present document.
Specification and information contained in the present schematics are subject to change at any time without notice.


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