Tiny, Accurate Battery Gas Gauges with Easy-to-Use I2C Interface and Optional Integrated Precision Sense Resistors Christoph Schwoerer, Axel Klein and Bernhard Engl Imagine your daughter is catching her first wave on a surfboard and your video camera shuts down because the battery—reading half full when you started filming a few minutes ago—is suddenly empty. The problem is inaccurate battery gas gauging. Inaccurate fuel gauging is a common nuisance as many portable devices derive remaining battery capacity directly from battery voltage. This method is cheap but inaccurate since the relationship between battery voltage and capacity has a complex dependency on temperature, load conditions and usage history. More accurate battery gauging can be achieved by monitoring not only the battery voltage but also by tracking the charge that goes in and out of the battery. For applications requiring accurate battery gas gauging, the LTC2941 and LTC2942 coulomb counters are tiny and easy-to-use solutions. These feature-rich devices are small and integrated enough to fit easily into the latest handheld gadgets. The LTC2941 is a battery gas gauge device designed for use with single Li-Ion cells and other battery types with terminal voltages between 2.7V and 5.5V. A precision coulomb counter integrates current through a sense resistor between the battery’s positive terminal and the load or charger. The high side sense resistor avoids splitting the ground path in the application. The state of charge is continuously updated in an accumulated charge register (ACR) that can be read out via an SMBus/I2C interface. The LTC2941 also features programmable high and low thresholds for accumulated charge. If a threshold is exceeded, the device 12 | April 2010 : LT Journal of Analog Innovation ANALOG INTEGRATOR ALLOWS PRECISE COULOMB COUNTING Charge is the time integral of current. The LTC2941 and LTC2942 use a continuous time analog integrator to determine charge from the voltage drop developed across the sense resistor RSENSE as shown in Figure 2. The differential voltage between SENSE+ and SENSE– is applied to an autozeroed differential integrator to convert the measured current to charge, as shown in Figure 2. When the integrator output ramps to REFHI or REFLO levels, switches S1, S2, S3 and S4 toggle to reverse the ramp direction. By observing the condition of the switches and the ramp direction, polarity is determined. A programmable prescaler adjusts integration time to match the capacity of the battery. At each underflow or overflow of the prescaler, the ACR value is incremented or decremented one count. The value of accumulated charge is read via the I2C interface. communicates an alert using either the SMBus alert protocol or by setting a flag in the internal status register. The LTC2942 adds an ADC to the coulomb counter functionality of the LTC2941. The ADC measures battery voltage and chip temperature and provides programmable thresholds for these quantities as well. The LTC2941 and LTC2942 are pin compatible and come in tiny 6-pin 2mm × 3mm DFN packages. Each consumes only 75µA in normal operation. Figure 1 shows the LTC2942 monitoring the charge status of a single cell Li-ion battery. 500mA VIN 5V BAT VCC 1µF Figure 1. Monitoring the charge status of a single-cell Li-Ion battery with the LTC2942 The use of an analog integrator distinguishes the LTC coulomb counters from most other gas gauges available on the 2k LTC4057-4.2 (CHARGER) PROG SHDN GND 2 LOAD 0.1µF 3.3V 2k VDD µP 2k 2k SENSE+ LTC2942 AL/CC SDA SENSE– SCL GND RSENSE 100mΩ + 1-CELL Li-Ion design features Figure 2. Coulomb counter section of LTC2942 LTC2942 LOAD 1 SENSE+ S1 2 market. It is common to use an ADC to periodically sample the voltage drop over the sense resistor and digitally integrate the sampled values over time. This implementation has two major drawbacks. First, any current spikes occurring inbetween sampling instants is lost, which leads to rather poor accuracy—especially in applications with pulsed loads. Second, the digital integration limits the precision to the accuracy of the available time base—typically low if not provided by additional external components. In contrast, the coulomb counter of the LTC2941 and LTC2942 achieves an accuracy of better than 1% over a wide range of input signals, battery voltages and temperatures without such external components, as shown in Figures 3 and 4. SENSE– ACR + S4 REFLO GND POLARITY DETECTION – TEMPERATURE AND VOLTAGE MEASUREMENT gral nonlinearity of the ADC is typically below 0.5LSB as shown in Figure 6. The LTC2942 includes a 14-bit No Latency DS™ analog-to-digital converter with internal clock and voltage reference circuits to measure battery voltage. The integrated reference circuit has a temperature coefficient typically less than 20ppm/°C, giving an ADC gain error of less than 0.3% from –45°C to 85°C (see Figure 5). The inte- The ADC is also used to read the output of the on-chip temperature sensor. The sensor generates a voltage proportional to temperature with a slope of 2.5mV/°C, resulting in a voltage of 750mV at 27°C. The total temperature error is typically below ±2°C as shown in Figure 7. 3 1.00 0.75 2 1 0 –1 –2 INTEGRATED SENSE RESISTOR VERSIONS LTC2941-1 AND LTC2942‑1 1 10 0.50 0.25 0 –0.25 –0.50 –0.75 VSENSE+ = 2.7V VSENSE+ = 4.2V –3 0.1 100 –1.00 –50 VSENSE = –50mV VSENSE = –10mV VSENSE (mV) 25 0 50 TEMPERATURE (°C) Figure 3. Charge error vs VSENSE Figure 4. Charge error vs temperature 10 –25 75 100 1.0 8 TOTAL UNADJUSTED ERROR (mV) The accuracy of the charge monitoring depends not only on the accuracy of the chosen battery gas gauge but also on the precision of the sense resistor. The LTC2941-1 and LTC2942-1 remove the need for a high precision external resistor by including an internal, factory trimmed 50mΩ sense resistor. Proprietary internal circuitry compensates the temperature coefficient of the integrated metal resistor to a residual error of only 50ppm/°C which makes the LTC2941-1 and LTC2942-1 by far the most precise internal sense resistor battery gas gauges available today. M PRESCALER CHARGE ERROR (%) 6 + CONTROL LOGIC – + S3 CHARGE ERROR (%) BATTERY – + S2 RSENSE IBAT REFHI VCC TA = 85°C 6 0.5 TA = 85°C 4 2 0 INL (VLSB) CHARGER TA = –45°C –2 –4 –6 0 TA = –40°C TA = 25°C –0.5 TA = 25°C –8 –10 2.5 3.0 3.5 4.0 4.5 5.0 VSENSE– (V) 5.5 6.0 Figure 5. ADC total unadjusted error –1.0 2.5 3.0 3.5 4.0 4.5 5.0 VSENSE– (V) 5.5 6.0 Figure 6. ADC integrated nonlinearity April 2010 : LT Journal of Analog Innovation | 13 USB CHARGING Figure 9 shows a portable application designed to charge a Li-Ion battery from a USB connection. The LTC2942-1 monitors the charge status of a singlecell Li-Ion battery in combination with the LTC4088-1 high efficiency battery charger/USB power manager. Once a charge cycle is completed, the LTC4088-1 releases the CHRG pin. The microcontroller detects this and sets the accumulated charge register to full either by writing it via the I2C interface or by applying a pulse to the charge complete (AL/CC) pin of the LTC2942-1 (if it is configured as input). Once initialized, the LTC2942-1 accurately monitors the 3 MONITORING BATTERY STACKS The LTC2941 and LTC2942 are not restricted to single cell Li-Ion applications. They can also monitor the charge state of a battery stack as shown in Figure 11. 2 TEMPERATURE ERROR (°C) Conversion of either temperature or voltage is triggered by setting the control register via the I2C interface. The LTC2942 also features an optional automatic mode where a voltage and a temperature conversion are executed once a second. At the end of each conversion the corresponding registers are updated before the converter goes to sleep to minimize quiescent current. Figure 8 shows a block diagram of the LTC2942, with the coulomb counter and its ACR, the temperature sensor, the ADC with the corresponding data registers and the I2C interface. 1 0 In this configuration, the power consumption of the gas gauge might lead to an unacceptable imbalance between the lower and the upper Li-Ion cells in the stack. This imbalance can be eliminated by supervising every cell individually, as shown in Figure 12. –1 –2 –3 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 Figure 7. Temperature sensor error charge flowing in and out of the battery, and the microcontroller can monitor the state of charge by reading the accumulated charge register via the I2C interface. By monitoring each cell’s state of charge, the LTC2942-1 provides enough information to balance the cells while charging and discharging. ENERGY MONITORING TRACKING BATTERY CAPACITY WITH TEMPERATURE AND AGING Real time energy monitoring is increasingly used in non-portable, wall powered systems such as servers or networking equipment. The LTC2941 and LTC2942 are just as well suited to monitor energy flow in any 3.3V or 5V rail application as they are in battery powered applications. Figure 10 shows an example. Battery capacity varies with temperature and aging. There is a wide variety of approaches and algorithms to track the battery capacity tailored to specific applications and the chemistry of the battery. The LTC2942 measures all physical quantities—charge, voltage, temperature and (by differentiating charge) current—necessary to model the effects of temperature and aging on battery capacity. The measured quantities are easily accessible by reading the corresponding registers with standard With a constant supply voltage, the charge flowing through the sense resistor is proportional to the energy consumed by the load. Thus several LTC2941 devices can help determine exactly where system energy is consumed. Figure 8. Block diagram of the LTC2942 1 SENSE+ VSUPPLY COULOMB COUNTER REF TEMPERATURE SENSOR 6 2 SENSE– MUX CC CLK REFERENCE GENERATOR OSCILLATOR REF+ CLK IN ACCUMULATED CHARGE REGISTER AL I2C/ SMBus AL/CC SCL SDA ADC DATA AND CONTROL REGISTERS REF– GND LTC2942 14 | April 2010 : LT Journal of Analog Innovation 5 3 4 design features I2C commands. No special instruction language or programming is required. 3.3uH The LTC2941 and LTC2942 do not impose a particular approach or algorithm to determine battery capacity from the measured quantities, but instead allow the system designer to implement algorithms tailored to the special needs of the system via the host controller. The microcontroller can then simply adapt the charge thresholds of LTC2941 and LTC2942 based on these calculations. SW VBUS USB 10µF VOUT LOAD 10µF VOUTS GATE BAT LT4088-1 0.1µF 3.3V D0 VDD D1 SENSE+ 2k 2k 8.2k 0.1µF PROG C/X + 1 CELL LI-ION AL/CC SDA µP CHRG _ LTC2942-1 D2 CLPROG SENSE GND NTC SCL GND 2.94k 2k CONCLUSION Battery gas gauging is one feature that lags behind other technological improvements in many portable electronics. The LTC2941 and LTC2942 integrated coulomb counters solve this problem with accurate battery gas gauging that is easy to implement and fit into the latest portable applications. For high accuracy in the tightest spots, the LTC2941-1 and LTC2942-1 versions integrate factory trimmed and temperature compensated sense resistors for the ultimate small coulomb counters. This new family of accurate coulomb counters can help prevent you from ever again missing those priceless vacation moments due to inaccurate battery charge monitoring. n Figure 11. Using the LTC2942 in a battery stack CHARGER Figure 9. Battery gas gauge with USB charger TO WALL ADAPTER 3.3V OUT GND SCL _ SENSE GND SENSE+ SENSE _ 2k 2k 2k 2k 2k 2k LTC2941 VDD AL/CC SDA 0.1µF SENSE+ SENSE _ GND SCL µP AL/CC SDA GND SCL Figure 10. Monitoring system energy flow using LTC2941s at the loads + SENSE+ I2C/SMBus TO HOST 3.3V 5A LOAD LTC2941 + AL/CC SDA 0.1µF DC/DC 5V 10A LOAD 5mΩ 5V OUT 10mΩ + LTC2942 VIN LOAD 1 CELL LI-ION 1 CELL LI-ION 1 CELL LI-ION Figure 12. Individual cell monitoring of a 2-cell stack CHARGER LTC2942-1 SENSE+ I2C/SMBus TO HOST RSENSE + LOAD LEVEL– SHIFT AL/CC SDA SCL 1 CELL LI-ION SENSE GND _ + 1 CELL LI-ION LTC2942-1 SENSE+ I2C/SMBus TO HOST AL/CC SDA SCL SENSE GND _ + 1 CELL LI-ION April 2010 : LT Journal of Analog Innovation | 15