Low power consumption is imperative for modern meter design. Whether you’re measuring gas, water, electricity or heat, you must limit the current draw to achieve long battery life and/or prevent inaccurate readings. Fortunately, TI provides a variety of solutions to meet these requirements. In this post, I’ll review some common configurations to see where TI products can aid your design.
Electricity meters are everywhere but their power designs are hardly constant. Driven by cost, size and efficiency, designers implement power schemes of different complexities. The differences spur from how they decide to convert the AC line voltage to a DC rail for the microcontroller (MCU) or system on chip (SoC). Figure 1 is a high-level example of a single-phase electric meter.
Figure 1: Single-phase electricity-meter block diagram
There are several common techniques employed to generate a regulated DC rail for the MCU. The most cost-conscious implementation is the capacitor drop, which involves traditional half-wave rectification via a diode and capacitor, combined with a capacitor and resistor in series and a Zener diode in parallel (see Figure 2).
Figure 2: A simplified capacitor-drop power supply
The resistor and capacitor in series limit the amount of current being sourced. The Zener diode regulates the voltage at the input of the low-dropout regulator (LDO). (The Zener diode should not be used as the point-of-load [PoL] regulator to the MCU. Because the Zener voltage can vary a considerable amount, using a Zener diode as the PoL regulator risks regulating a voltage outside the MCU’s specified supply-voltage range. A Zener diode also does not incorporate other important features common in an LDO, including current limit and load and line regulation.) For more information on these components, see the additional resources section below.
The LDO provides a stable supply-voltage rail for the MCU. However, you can’t use just any LDO in this scheme. Since the voltage at the input of the LDO is prone to transient voltage spikes, the LDO must have a wide-input voltage range to accommodate such events. The system’s leakage current must also be controlled to comply with industry standards. Therefore, you must have an LDO with small quiescent current to limit current draw. The TPS709 is an example of a regulator that has both a wide-input voltage range (up to 30V) and a low quiescent current rating of 1.3µA.
In the event that the line voltage goes down, measures should be taken to ensure continued operation. The optional diode shown in Figure 2 prevents the current from taking a reverse path should the input voltage of the LDO be lower than the output voltage. It is marked optional because some LDOs (like the TPS709) have integrated reverse-current protection, which makes this diode redundant.
Figure 3: A capacitor-drop supply with a battery connected after the LDO
Figures 3 and 4 feature a battery placed in parallel with the line voltage. This is the secondary power source when the line voltage is not present. The battery kicks in once its nominal voltage less the diode voltage drop is greater than the connected-node voltage. The battery can be connected after the LDO (Figure 3) or before (Figure 4). However, I recommend connecting the battery before the LDO when possible to ensure a stable regulated voltage within the range of the MCU.
Figure 4: A capacitor-drop supply with a battery connected before the LDO
A water meter’s power supply is much simpler given that it does not operate off of an AC line; it uses a battery for operation. In some cases, this battery must provide operation for 10 years or more. To achieve this, current draw must be kept to a minimum. An LDO like the TPS782 aids this type of design since it both regulates an output voltage and draws minimal quiescent current, as shown in Figure 5.
Figure 5: The power supply for a water meter
Although it may not be intuitive, using an LDO in conjunction with a battery can reduce the amount of power consumption. This is due to the relationship between supply voltage and supply current in low power microcontrollers like TI’s MSP430. Although the supply voltage range may be wide (the supply voltage range of MSP430 may range from 1.8V to 3.6V), supplying a higher voltage rail will correspond with a higher supply current. Therefore, it is advantageous to use a lower voltage supply rail to curb unnecessary current draw. This is discussed at length in another blog linked in the additional resources section below.
Whether your meter runs off an AC line, a battery or both, using the right LDO ensures proper operation and reduced power consumption.