Sophisticated smart battery makes charging easy

The emergence of smart battery systems (SBS) has greatly simplified the design of independent battery systems, so its application has expanded beyond the field of notebook computers and 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 circuitry to measure, calculate and store battery data, which makes power usage more predictable. Furthermore, smart batteries have the important benefit of preventing unexpected system shutdowns.

　　Smart battery system

A basic SBS system consists of the following parts: System Management Bus (SMBus), smart battery charger and smart battery.

The modular nature of SBS makes it very easy to design a closed-loop battery charging system. Such a system allows the use of battery pack independent chargers (smart chargers), minimizing the non-repetitive engineering (NRE) cost of hardware and software. and results in 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 battery's actual capacity value, thus eliminating bias and ensuring accuracy.

　　Smart battery charging and protection

The main function of the smart battery charger is to provide a voltage source and current source for smart battery charging. The smart battery communicates with the smart charger via the SMBus interface and optionally with the host computer. To prevent overcharging due to loss of SMBus functionality, a watchdog 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. Additionally, the battery can control the charger via a forced shutdown feature, which bypasses the SMBus to provide a level of redundancy and let the charger know that the battery is actually present.

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

　　① Truly plug and play, not affected by battery chemical characteristics and battery configuration. Any smart battery pack works with any smart battery charger. Batteries with different chemistries, configurations, and even different charging algorithms can use the charger circuit without modification.

　　② Built-in safety function. The SBS standard provides watchdog timers and a special "safety signal" interface between the battery and 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-level battery charger and selector for products using dual smart batteries. It is a buck switching topology battery charger with multiple features as defined by smart battery standards and other new features such as input current limit and safety limit, 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 master to monitor the status of either battery. This servo technology enables the charger's accuracy to be within ±0.2% of the battery's internal voltage and current measurements.

Traditionally, dual-battery systems have been sequential discharge systems, allowing the batteries to be consumed sequentially (Battery 1, then Battery 2) to simply extend the total battery operating time. The LTC1760 uses proprietary analog control technology that allows 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 the charging speed by 50% and extends the battery working time by 10%. Additionally, parallel discharge not only enhances current capabilities, but also reduces I2R losses and improves voltage regulation under extremely high load conditions. Reducing I2R losses and improving voltage regulation both 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 times

　　● Main features of LTC1760

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

　　② Quick charging mode can be used to further shorten charging time.

　　③ Support battery check to achieve barometer calibration.

　　④ 3 power path FET diodes allow safe and low loss simultaneous discharge from DCIN and both batteries.

　　⑤ Two FET diodes realize safe and low-loss discharge of two batteries at the same time.

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

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

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

Figure 3 LTC1760 dual battery charger/selector system architecture

　　●Input current limit detection resistor RCL

　　

Figure 4 Input current limiting sensing resistor circuit

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

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

　　RCL＝100mV/ILIM(2)

However, AC adapters can have a current limit margin of at least +10%, 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 LTC1760's SMBus interface the maximum allowable current the charger can supply 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 of the SMBus interface.

　　● Voltage limiting resistor RVLIM

　　The value of the external resistor connected from the VLIM pin to GND can determine any of the five charger output voltage limit values ​​(see Table 3). This method of using hardware to realize the voltage limit value 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 in series with a short-circuit sense resistor RSC. The equivalent circuit of the battery power path (PowerPathTM) switch driver is shown in Figure 5.

Figure 5 LTC1760 power path circuit

　　The short circuit protection function works 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 opened and the POWER_NOT_GOOD bit is set. Similarly, if the voltage drops below 3V for more than 15ms, all power path switches will also be turned off and the POWER_NOT_GOOD bit will also be set. Removing all power will reset the POWER_NOT_GOOD bit. If the POWER_NOT_GOOD bit is set, charging is also disabled.

　　● No software required

　　LTC1760-based chargers require no software. Placing this integrated circuit in the initial hardware prototype will allow the system to charge and discharge the battery. However, in some cases, some software can be written so that the host can complete the following actions.

　　① Collect "charger status" information directly from the smart battery (that is, as a barometer);

　　② Support battery inspection.

　　in conclusion

Smart battery systems provide advanced functionality with minimal design effort. LTC1760 is a representative of a very comprehensive single-chip dual-intelligent battery system. It is simple and easy to use. It only needs to determine 4 parameters to complete a complete design, and no software code is required. The device requires minimal NRE operation to form a complete stand-alone battery charger system and function properly.