Multichannel power monitors help simplify battery-based systems employing multiple current sensors.
Mitch Polonsky • Microchip Technology Inc.
Designers of power systems can be forgiven for having some confusion about power monitors. Some of them may even be using a multichannel power monitor without knowing it. That’s because, regrettably, several different phrases are used to refer to this device category. Some designers know them as high-side current sensors, some as current sensors, and some know them as power monitor ICs. The devices being referenced have a digital interface; inputs can be connected directly to voltage rails above 5 V, and will measure current, voltage, and power across a sense resistor.
These power monitors provide the ability to connect to higher voltages. Some power monitors can accept voltages as high as 100 V, while other mid-range devices can only accept up to 32 V. As such, the devices help avoid external components needed for high voltage applications.
ADI (now including Maxim and Linear Technology), Texas Instruments, Renesas (now including Intersil), and Microchip all have devices in this category. Under Maxim these devices are known as current sensors with digital outputs. However, the ADI website calls these devices power monitors. The consistency of nomenclature continues with both TI and Microchip calling these devices current/voltage/power monitors.
If the system only consists of 5 V, simple operational amplifiers and resistors may be all that is required to measure the system power. Periodic polling could be implemented to lower system power associated with the monitoring. However, this simple scheme does not address the issue of critical power rails that need more active management. Active management may be necessary to measure and optimize power efficiency or to understand the remaining battery life. It might seem as though an always-on processor would be necessary to do the monitoring. But a host processor that does the monitoring could also reside in a low-power state for much of the time, only waking if an independent current sensor with limits sends an interrupt.
Many host processors need protection components if they connect to voltage rails higher than 5 V, which brings us to an advantage associated with high-side current sensors. First consider only high-side current sensors that are analog. They are offered in common-mode voltages up to 100 V. These devices can connect directly to the higher voltage rails and avoid the need for external protection components. In addition, the devices send the host controller signals representing the current and power in the system.
It should also be noted that analog current sensors come in options with multiple channels. The power that multiple-channel sensors dissipate is normally in line with that of a single device multiplied by the number of channels. For example, a single-channel analog current sensor such as the INA290 has a maximum quiescent current of 600 µA. The dual version in the same family, the INA2290, has 1,200 µA of quiescent current for the same operating conditions.
Power monitor ICs
This brings us to the topic of the power monitoring IC, which is a mixed-signal device. These ICs are improvements on systems employing an always-on host controller and analog current sensors.
Power monitors calculate power consumption on-chip independently of the host controller. The methods used are the same as that of an analog current sense amplifier. But the power monitor IC goes further by incorporating an integrated ADC and multiplier to produce a digital representation of the power consumption. This digital value can then be made available in a register over a digital interface, thus providing a digital power calculation. As a result, there is a savings in software overhead, development time, and code complexity in the host processor. The host can also spend less time in the awake state while the power sensor accumulates data.
An ancillary benefit is that the power monitor reduces pin requirements on the host via a shared communication bus. Many general-purpose sensors include shared interfaces for communication with additional power monitors, temperature sensors, memory and more. The same cannot be said for analog current sensors, which require additional pins on the host. Also, shared communication interfaces free up GPIOs for general-purpose use.
Additionally, power monitors conserve host power by allowing the system to wait for an alert rather than poll for a reading. While waiting, the host can choose to stay in a lower-power sleep or standby state while the power monitor supervises critical voltage rails for excursions.
Now consider multichannel power monitors. What sets multichannel power monitors apart from single-channel devices is the ability create a round-robin sampling and reporting architecture that consumes less system power. Most companies use similar architectures, so we will share the Microchip architecture for the PAC1954 to relay the point.
Note that the PAC1954 device has a single ADC for measuring Vsense. This functional block is multiplexed to measure and report the Vsense voltage from four sense resistors. As a result, it takes less quiescent power to run this architecture then for four separate current sensors.
For example, consider the maximum quiescent current from a competing four-channel current sensor compared to a high-quality single-channel power monitor. We can see the inherent benefit of using one ADC for a four-channel device. The competitive device consumes 450 µA max at 85°C for each of four channels of measurement and 16 bits of resolution. In contrast, the power monitor consumes 400 µA max for 16 bits of resolution and only one channel of measurement, or 1,600 µA.
The same calculation can be performed with the latest Microchip device. Consider a dual power monitor, the PAC1952, with a maximum quiescent current of 495 µA at 125°C. Compared to competing devices at 800 µA, there is a systems power savings of 1 – (495/800) = 38% with respect to the power measurement.
Thus the many reasons to use a multichannel power monitor IC include:
Saving software overhead, development time, and code complexity
Saving time in the awake state, while the sensor accumulates data
Reducing pin count on the host or freeing up host pins for more GPIOs
Saving host power by using alerts to wake the system rather than poll for a readings for excursions
All in all, there is a measurable power savings associated with the use of multichannel power monitors in lieu of single-channel monitors. An architecture based on a shared ADC will facilitate a savings of up to 38% of the power associated with the monitoring of the system voltage rails.