Dutch start-up Anagear has released a power management IC (PMIC) for wireless sensor nodes, that consumes an average of 600nA, said the firm.
Typically, wireless sensor nodes have a microcontroller that spends most of its time in sleep, waking briefly periodically to sample the associated sensor and transmit the resulting measurement.
Power can come from a local primary cell or from harvesting. In the latter case energy has to be gathered, and stored in such a way that can support transmitter peak power.
There are microcontrollers made on low-leakage processes that have in-built autonomous timers to allow the core to sleep much of the time. However, few or none have built-in power conditioning for photovoltaic harvesting.
In its chip, called ANG1010, Anagear has provided power conditioning to automatically manage the interface between a solar cell, a rechargeable battery (or super capacitor) and a back-up coin cell.
To allow it to be used with any microprocessor and any radio, instead of only ultra-low power types, the 1010 also includes a self-timed scheduler and power switches that completely de-power the associated microcontroller and radio to between samples, while on-board RAM holds critical data for the processor.
"We know from talking to industry, that if you use a microcontroller made on a 65nm or 45nm process to take care of timing, they have significant inherent leakage current: 1.5-2µA," said Guus Dhaeze, Anagear v-p of sales and marketing. "By taking this function away into a mixed-signal chip implemented in low-leakage silicon, we can offer better solution: 500-600nA average consumption."
To save the microcontroller from being woken for housekeeping, another feature of the 1010 is an ADC with programmable alarm thresholds that automatically makes scheduled measurements of battery voltage, capacitor voltage, die temperature and an external analogue input.
600nA includes the power consumed by the ADC, but not that consumed by the microprocessor and radio when they are periodically woken.
"We have not seen a microcontroller that can be used to generate this low stand-by current," said Dhaeze.
An on-chip oscillator provides 16kHz for scheduling.
"Its is a very precise temperature and voltage compensated time base with better than 5ppm accuracy across -25 to 85°C," said Dhaeze. "We have also implemented a 10-32MHz crystal oscillator for calibrating the on-chip oscillator from time to time, and to generate clocks for the radio and microcontroller."
From the energy source are supplied two independent power planes, one for the MCU and one for the radio, both programmable in 50mV steps over 0.9-2.5V. One is supplied by an 85% efficient dc-dc delivering up to 50mA, and the other from an LDO delivering up to 30mA.
Each power plane is timed independently, and has associated independently sequenced clock and reset outputs.
Reset is also qualified by the supply voltage, and there is brown-out detection.
Solar cells have tricky output characteristics and matching these to extract maximum power is difficult.
The gold-standard technique is 'maximum power point tracking' (MPPT) using a dithered servo loop. This acronym has been diluted by less optimised techniques that are being branded as 'MPPT' by some chip makers.
"We have a real dynamic MPPT," said Dhaeze. "It is not exactly the dither technique, which would require some sort of sequencer. We adjust the operating point of the front-end boost converter in a analogue technique we have patented."
Without another power source, the chip can boot itself into operation once the photovoltaic cell is delivering 1.6V. "So a three or four cell indoor panel, not a 1 cell panel," said Dhaeze.
The PV cell voltage can be boosted using the front-end inductor (see diagram) to a programmable voltage up to 5V for local super capacitor storage.
For applications where local storage is a Li-ion cell, the chip has programmable over-voltage protection - although any temperature qualification for charging has to be via the associated microcontroller.
Then there is provision to connect a non-rechargeable lithium coin cell for back-up when harvested energy runs out.
A threshold can be set in on-board e2prom to wake the processor when power is low to transmit a 'don't expect data for a while' message.
Other peripherals included to save waking up the processor are a real-time clock and a watchdog timer.
Two derivative chips, ANG1012 and ANG1014, are available without the photovoltaic interface for coin-cell-only operation.
1014 also omits the ADC and window comparators, as it is purely for time sequencing an external microcontroller and radio.
There is an evaluation kit available for the full-featured 1010 that allows settings to be programmed over USB from a PC.
Source:
http://www.electronicsweekly.com/Articles/2013/01/18/55402/chips-cuts-power-in-wireless-sensor-nodes.htm
