Sophisticated smart battery simplifies charging

        The advent of Smart Battery Systems (SBS) has greatly simplified the design of stand-alone battery systems, so its application has gone beyond the notebook computer field and has appeared in various other applications, such as backup power systems, high-reliability military and aerospace applications. Other key applications include automotive, security/surveillance/anti-counterfeiting systems, medical equipment, blade servers, telecommunications and portable electronics.
 

　　Smart batteries use internal electronic circuits to measure, calculate and store battery data, which makes power usage more predictable. In addition, another important benefit of smart batteries is that they can prevent unexpected system downtime.
 

　　Intelligent battery system

　　A basic SBS system consists of the following components: System Management Bus (SMBus), Smart Battery Charger and Smart Battery.
 

　　The modular nature of the SBS makes it easy to design a closed-loop battery charging system that allows the use of independent chargers (smart chargers) for the battery pack, minimizes non-recurring engineering (NRE) costs for hardware and software, and enables a rugged system, which is especially important for high-reliability battery backup applications. A high-accuracy barometer integrated into the battery pack accurately monitors the battery at all times, even when the battery is not in the system. The barometer is calibrated to the actual capacity value of the battery, thus eliminating bias and ensuring accuracy.
 

　　Smart battery charging and protection

　　The main function of the smart battery charger is to
 

　　Charging provides voltage source and current source. The smart battery communicates with the smart charger through the SMBus interface and can optionally communicate with the host. To prevent overcharging due to loss of SMBus function, the monitoring timer runs continuously to monitor the frequency of calls between the smart battery and the charger. If the battery is inactive for more than 3 minutes, the charger will pause and wait for the battery to request charging again. In addition, the battery can also control the charger through a forced shutdown function, which bypasses the SMBus to provide a level of redundancy and let the charger know that the battery is indeed present.
 

　　In summary, compared with fixed independent chargers, smart battery chargers have the following advantages.

　　① True plug-and-play, unaffected by battery chemistry and battery configuration. Any smart battery pack can be used with any smart battery charger. Batteries with different chemistry, configuration, and even different charging algorithms can be swapped with the charger circuit without modification.

　　② Built-in safety function. The SBS standard provides a monitoring timer and a special "safety signal" interface between the battery and the charger.

　　③ Reliable battery detection system.

　　④ Automatic charging management, no host required.

　　⑤ Closed-loop charging system without host intervention. The host can collect power measurement information as needed.
 

　　LTC1760 Dual Smart Battery System Manager

　　The LTC1760 is a highly integrated three-stage battery charger and selector for products using dual smart batteries. It is a step-down switching topology battery charger with multiple functions defined in the smart battery standard and other added features such as input current limiting and safety limiting, etc. Three SMBus interfaces enable the LTC1760 to implement servo functions such as tracking the internal voltage and current of two batteries and allow an SMBus host to monitor the status of either battery. This servo technology enables the charger to have an accuracy of only ±0.2% error with the internal voltage and current measurements of the battery.
 

　　Traditionally, dual-battery systems are sequential discharge systems that allow the batteries to be consumed sequentially (battery 1 first, then battery 2) to simply extend the total battery operating time. The LTC1760 uses a proprietary analog control technique that allows the two batteries to be safely charged or discharged in parallel. Figure 1 is a simplified schematic of a dual-battery system using the LTC1760. This structure increases charging speed by 50% and extends battery operating time by 10%. In addition, parallel discharge not only enhances current capability, but also reduces I2R losses and improves voltage regulation under extremely high load conditions. Both reduced I2R losses and improved voltage regulation extend the total discharge time of the timing solution (see Figure 2).

　　Figure 1 LTC1760 used in dual battery system

　　Figure 2 Comparison of dual-battery sequential battery charging time
 

　　● Main features of LTC1760

　　① An independent 3-level charger polls the battery for charging requirements and monitors the actual current and voltage (with an error of ±0.2%) as determined by the battery's internal charge measurement, enabling fast, safe and complete charging.

　　② Fast charging mode can be used to further shorten the charging time.

　　③ Support battery check to achieve barometer calibration.

　　④ Three power path FET diodes allow safe and low-loss discharge from DCIN and two batteries simultaneously.

　　⑤ Two FET diodes enable safe and low-loss discharge of two batteries simultaneously.

　　⑥ Hardware programmable current and voltage safety limits and many other safety features complement the battery’s internal protection circuitry.

　　Although the LTC1760 is sophisticated, it is very easy to use. Only four key parameters need to be determined in any given design: input current limit sense resistor RICL, current limit resistor RILIM and matching charge current sense resistor RSENSE, voltage limit resistor RVLIM, short circuit protection resistor RSC.

　　The LTC1760, some smart batteries and an AC adapter can form a simple system. The system structure is shown in Figure 3.

        Figure 3 LTC1760 dual battery charger/selector system architecture

　　● Input current limiting detection resistor RCL

　　Figure 4 Input current limiting sensing resistor circuit

　　As shown in Figure 4, this circuit limits the charging current to prevent the AC adapter from overloading when the system power is increased. To set the input current limit, the most important thing is to minimize the current rating of the wall adapter. The current limiting resistor can be calculated by the following two equations.

　　ILIM = minimum adapter current value - (minimum adapter current value × 5%) (1)

　　RCL＝100mV/ILIM（2）

　　However, AC adapters can have at least +10% current limit margin, so it is often possible to simply set the adapter current limit to the actual adapter rating.

　　● Current limiting resistor RILIM

　　The RILIM resistor has two functions. First, it tells the SMBus interface of the LTC1760 that the charger can supply the maximum allowable current to the battery, and any value above this limit will be replaced by the limit value. The second function is to synchronize the full-scale current of the PWM charger with the full-scale current limit value of the SMBus interface.

　　● Voltage limiting resistor RVLIM

　　The value of the external resistor connected between the VLIM pin and GND can determine any of the five charger output voltage limits (see Table 3). This method of implementing the voltage limit value with hardware is a relatively safe measure and cannot be replaced by software.

　　● Short circuit protection resistor RSC

　　Each power path consists of two back-to-back PFETs connected in series with a short-circuit detection resistor RSC. The equivalent circuit of the battery power path (PowerPathTM) switch driver is shown in Figure 5.

　　Figure 5 LTC1760 power path circuit

　　Short circuit protection

　　The function operates in current mode and voltage mode. If the output current exceeds the short-circuit comparator threshold for more than 15ms, all power path PFET switches are disconnected and the POWER_NOT_GOOD bit is set. Similarly, if the voltage drops below 3V for more than 15ms, all power path switches are disconnected and the POWER_NOT_GOOD bit is also set. The POWER_NOT_GOOD bit can be reset by removing all power. If the POWER_NOT_GOOD bit is set, charging is also disabled.

　　● No software required

　　LTC1760-based chargers do not require software. Placing the IC in an initial hardware prototype will allow the system to obtain battery charge and discharge information. However, in some cases, some software can be written so that the host can complete the following actions.

　　① Collecting “charger status” information directly from the smart battery (i.e. as a barometer);

　　② Support battery inspection.

　　in conclusion

　　Smart battery systems offer advanced functionality with minimal design effort. The LTC1760 is a representative of a very comprehensive single-chip dual smart battery system that is easy to use, requiring only four parameters to complete a complete design, and no software code is required. The device requires minimal NRE work to form a complete standalone battery charger system and function properly.