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ANALOG & MIXED SIGNAL Fast 8 hour service for PCBs 4 day service for assembly Reliable Express services: On time or FREE Unique Online assembly from 1 component sales@pcb-pool.com Phone: +353 (0)61 701170 www.pcb-pool.com Figure 3. The LTC2944 Measures Charge with up to 99% Accuracy. battery gas gauges perform an analog-to-digital conversion of the voltage across the sense resistor and accumulate the conversion results to infer charge. In such a scheme, the differential offset voltage can be the main source of error, especially during small signal readings. For example, consider a battery gas gauge with an ADCbased coulomb counter and a maximum specified differential voltage offset of 20μV that digitally integrates a 1mV input signal – the charge error due to offset would be 2%. By comparison, the charge error due to offset using the LTC2944’s analog integrator would only be 0.04%, 50 times smaller! Back to basics – voltage, current & temperature If coulomb counting is responsible for reinforcing voltage readings and differentiating flat regions of a charge or discharge curve, then current and temperature are parameters responsible for fetching the most relevant curve to begin with. The challenge is that a battery’s terminal voltage (voltage while connected to a load) is significantly affected by the battery current and temperature. Therefore, voltage readings must be compensated with correction terms proportional to the battery current and opencircuit voltage (voltage while disconnected from load) versus temperature. Because it isn’t practical to disconnect a battery from a load during operation for the sole purpose of measuring the open-circuit voltage, it is good practice to at least adjust terminal voltage readings per current and temperature profiles. Since high SOC accuracy is the ultimate design goal, the LTC2944 uses a 14-bit No Latency ΔΣ™ ADC to measure voltage, current and temperature with up to 1.3% and ±3°C guaranteed accuracy respectively. In reality, the typical performance of the LTC2944 is much better. The graphs in Figure 4 show how certain accuracy figures of merit in the LTC2944 vary with temperature and voltage. Figure 4a shows that the ADC total unadjusted error when measuring voltage is typically less than ±0.5% and fairly constant over sense voltage. Similarly, Figure 4b shows that the ADC gain error when measuring current is typically less than ±0.5% over temperature. Lastly, Figure 4c shows that the temperature error only varies by about ±1°C over temperature for any given sense voltage. All of these accuracy figures add up and can easily compromise SOC accuracy, which is why it is important to note how accurate a particular battery gas gauge measures voltage, current and temperature amongst many specifications. The LTC2944 provides four ADC modes of operation when measuring voltage, current and temperature. In automatic mode, the device continuously performs ADC conversions every couple of milliseconds versus scan mode which converts every 10s and then goes to sleep. In manual mode, the device performs a single conversion on command and then goes to sleep. Anytime the device is in sleep mode, quiescent current is minimized to 80μA. The entire analog section of the LTC2944 can also be completely shut down to reduce quiescent current even further to 15μA, since the last thing users want is a battery gas gauge that ironically consumes a lot of battery power. Convenient interfacing Users can read out battery charge, voltage, current and temperature from the LTC2944 using a digital I2C interface. Users can also read out status, control on/off, and set alertable high and low thresholds for each parameter by configuring a few 16-bit registers via I2C. The alert system eliminates the need for continuous software polling and frees the I2C bus and host to perform other tasks. Additionally, an ALCC pin serves as both a SMBus alert output or a charge complete input that can be www.electronics-eetimes.com Electronic Engineering Times Europe March 2017 29


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