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EE Times Europe
EE Times Europe asked John Perry, system engineer at Nexperia, how the NBM7100/NBM5100 series differed from traditional DC/DC converters.
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Selecting a battery for a compact wireless-connected device like an IoT sensor presents several challenges. Do you opt for a non-rechargeable alkaline primary battery or a rechargeable lithium pouch battery? Or would a lithium coin cell be suitable? Each option has its pros and cons, with the cell’s internal resistance, energy density and the device’s power requirements key considerations. Then you have the available space constraints, and if opting for a rechargeable battery, you have to allocate board space and BOM for the charging circuitry. Selecting a non-rechargeable battery adds the operational inconvenience and high costs of scheduled “truck rolls” to replace the battery, whether it needs replacing or not. With the increasing product and corporate emphasis on sustainability, along with the emerging European Commission legislation on battery waste, engineering teams have a lot to consider.
Low-cost coin cells have always been popular for many simple embedded systems. With a relatively high energy density and a compact form factor, they are an ideal choice for many low-power embedded designs. However, their high internal resistance doesn’t suit pulsed higher-current loads, such as those encountered when establishing a radio link. The increased current draw above the ambient level results in a reduced battery voltage and a chemical reaction that further reduces the output voltage and impacts its serviceable life (Figure 1).
The higher the load, the more significant the voltage drop. Coin-cell datasheets usually indicate these characteristics, with a load as small as 10 mA resulting in a 0.2-V to 0.3-V drop from a nominal 3.3-VDC output. Pulsed loads as high as 50 mA may result in a fall in battery voltage to 2.5 VDC. Pulsed loads like this are typical for many wireless IoT devices but are very challenging for coin cells, reducing battery lifetime.
Nexperia claimed its recently announced NBM5100x/7100x series of coin-cell battery life booster ICs offers a viable solution to the pulsed load challenge. The ICs are designed to overcome the peak load and battery life challenges of wearable and wireless IoT applications. The ICs provide a complementary alternative to an energy-harvesting approach for applications in which space and BOM costs are at a premium or the ability to harvest energy is limited. Requiring only a handful of additional passive components, the ICs utilize an external capacitor to deliver a peak load capability, such as when the wireless transceiver is transmitting. An integrated DC/DC converter charges the capacitor for the next cycle between the peak loads. Figure 2 illustrates the internal architecture of the NBM7100A/B IC. Communication with the host is via the I2C serial bus, and the storage capacitor (Cstore) can be charged up to 11 VDC. The maximum load capability is 200 mA.
EE Times Europe asked John Perry, system engineer at Nexperia (Nijmegen, The Netherlands), how the NBM7100/NBM5100 series differed from traditional DC/DC converters. “Yes, this isn’t a classical DC/DC converter—it has a two-stage process,” he said. “We use the boost converter to take the battery’s low voltage and slowly store it in one of the two possible capacitor configurations at a higher voltage. When you consider classical DC/DC converters, I would say that 99% of them don’t manage the input current. This is fundamental to obtaining a longer battery life; you don’t want to stress the battery.”
Perry explained an IoT device’s typical power consumption profile (Figure 3) and noted they saw the most benefits with this type of duty-cycle scenario, switching between charging the capacitor and supplying the load.
The ICs feature a state machine optimizer with 63 settings for different duty cycles and load conditions. “If your load requirements are similar and repetitive, the optimizer will change the level at which we charge the capacitor, so we don’t leave too much residual energy in the capacitor after it has supplied the load,” Perry said. “The idea is to find an optimal level where we can supply the anticipated load and do it over and over again without more residual charge being left than necessary.”
Perry said in cases where the duty-cycle duration and load are more unpredictable, “we have a fixed mode where we just charge the capacitor fully and allow all the energy to be available for whatever load is required.”
Perry explained how they had profiled three different application scenarios based on using a coin-cell battery buffered with an 11-mF capacitor, an industry DC/DC converter with a 1-mF buffer capacitor and an NBM7100 battery booster IC with a 1-mF storage capacitor. The continuous current, peak transmit pulse load and duty cycle were the same for all three tests. The battery’s end of life was determined based on its voltage falling below 2.4 V. The results highlighted in Figure 4 illustrate the battery life in terms of transmit cycles, with the battery boost IC delivering a >7× increase in battery lifetime.
Read also:
Energy Harvesting: Powering IoT Devices Reliably and Sustainably
Robert Huntley is a contributor for EE Times Europe.
“using a coin-cell battery buffered with an 11-mF capacitor”
Does this really mean 11 milli Farads?
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