Battery Management
195
George H. Barbehenn
Complete battery charger solution for high current portable electronics
Input multiplexer
A distinctive feature of the LTC4155/LTC4156 Power- Path implementation is a dual-input multiplexer using only N-channel MOSFETs controlled by an internal charge pump.
The input multiplexer has input priority selection and inde- pendent input current limits for each channel.
Applications include any device with a high capacity Li-Ion or LiFePO4 battery that can be charged from a high current wall adapter input or from the USB input—such as a tablet PC. Figure 195.1 shows a typical application and Figure 195.2 shows its efficiency.
This dual input multiplexer implementation allows the use of inexpensive, low RDS(ON) N-channel MOSFETs. The
Introduction
The LTC4155 and LTC4156 are dual multiplexed-input bat- tery chargers with PowerPath control, featuring I2C program- mability and USB On-The-Go for systems such as tablet PCs and other high power density applications. The LTC4155’s float voltage (VFLOAT) range is optimized for Li-Ion batteries, while the LTC4156 is optimized for lithium iron phosphate (LiFePO4) batteries, supporting system loads to 4A with up to 3.5A of battery charge current. I2C controls a broad range of functions and USB On-The-Go functionality is controlled directly from the USB connector ID pin.
3.5A charger for Li-Ion/LiFePO4 batteries multiplexes USB and wall inputs
Figure 195.1 • Typical Application Using a Simple Input Multiplexer with No
Backdrive Protection Figure 195.2 • Switching Regulator
Efficiency Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00195-2
MOSFETs also provide overvoltage protection (OVP) and if desired, backdrive blocking and reverse-voltage protection (RVP). Backdrive blocking prevents voltage on the wall input from backdriving the USB, or vice-versa. Backdrive blocking can be implemented on one or both inputs. Reverse-voltage protection prevents a negative voltage applied to the pro- tected input from reaching downstream circuits.
Dual high current input application
Figure 195.3 shows a dual 3.5A input application, featuring OVP, RVP and backdrive protection. The FDMC8030 MOS- FETs provide ±40V of OVP and RVP protection.
0V ∼ 6V input on either WALL or USB
In the circuit of Figure 195.3, when a 0V ∼ 6V input is pre- sent on either input, the corresponding gate signal rises to approximately twice VIN, enabling the two series N-channel MOSFETS and connecting the input to VBUS. The undervolt- age lockout (UVLO) is approximately 4.35V on each channel.
The LTC4155/LTC4156 have an input priority bit, which defaults to WALL. If a valid voltage is present on both inputs, only WALLGT is activated. The input priority bit can be changed via I2C to make the USB channel preferred when both inputs are present.
>6V input on either WALL or USB
When either input goes above 6V, the corresponding WALLGT or USBGT pin is pulled low, shutting off the cor- responding MOSFETs and disconnecting the input. If both inputs have 5V on them, and the input that is enabled by the input priority bit goes above 6V, the LTC4155/LTC4156
automatically and seamlessly switches to the other input—
with no disturbance on VOUT.
The diode-connected NPNs (Q3 and Q4) serve to prevent excess VGS on the MOSFET closest to the input of the cor- responding channel current from the input flows through the diode, through the B-C junction of the NPN bipolar transistor, and pulls the gate up through the 5M resistor. This prevents the gate from dropping below the source by more than 2V.
A voltage greater than 6V on one input does not prevent the other input from operating normally.
<0V input on either WALL or USB
The USBSNS and WALLSNS pins ignore any negative inputs, but clamp the pins to −VF (about −0.6V). The negative volt- age forward-biases the base-emitter junction of the NPN bipolar transistor, shorting the gate to the input and ensuring that the gate is never more than about 0.5V above the source.
A negative voltage on one input does not prevent the other input from operating normally.
OTG operation
The LTC4155/LTC4156 drive the USBGT pin high when USB On-The-Go operation is enabled, connecting VBUS to the USB input and enabling up to 500mA to be sourced.
Conclusion
The LTC4155/LTC4156 implement an overvoltage and reverse-voltage protected, prioritized input multiplexer for products that need to support multiple system power or bat- tery charging functionality. Optional backdrive blocking pre- vents the appearance of voltages at an unconnected input.
196
George H. Barbehenn
Battery conditioner extends the life of Li-Ion batteries
batteries are in fact hybrid batteries containing a combination of polymer and gel electrolytes.
The charge process involves lithium ions moving out of the negative terminal material, through the electrolyte and into the positive terminal material. Discharging is the reverse process. Both terminals either release or absorb lithium ions, depending on whether the battery is being charged or dis- charged.
The lithium ions do not bond with the terminals, but rather enter the terminals much like water enters a sponge;
this process is call ‘intercalation.’ So, as is often the case with charge-based devices such as electrolytic capacitors, the resulting charge storage is a function of both the materials used and the physical structure of the material. In the case of the electrolytic capacitor, the foil is etched to increase its sur- face area. In the case of the Li-Ion battery the terminals must have a sponge-like physical makeup to accept the lithium ions.
The choice of positive terminal material (cobalt, manga- nese or iron phosphate) determines the capacity, safety and aging properties of the battery. In particular, cobalt provides superior capacity and aging characteristics, but it is relatively unsafe compared to the other materials. Metallic lithium is flammable and the cobalt positive terminal tends to form metallic lithium during the discharge process. If several safety
Introduction
Li-Ion batteries naturally age, with an expected lifetime of about three years. But, that life can be cut very short–to under a year–, if the batteries are mishandled. It turns out that the batteries are typically abused in applications where intelligent conditioning would otherwise significantly extend the battery lifetime. The LTC4099 battery charger and power manager contains an I2C controlled battery conditioner that maximizes battery operating life, while also optimizing battery run time and charging speed (see Figure 196.1).
The underlying aging process in Li-Ion batteries
Modern Li-Ion batteries are constructed of a graphite negative terminal, cobalt, manganese or iron phosphate positive termi- nal and an electrolyte that transports the lithium ions.
The electrolyte may be a gel, a polymer (Li-Ion/Polymer batteries) or a hybrid of a gel and a polymer. In practice, no suitable polymer has been found that transports lithium ions effectively at room temperature. Most ‘pouch’ Li-Ion/Polymer
Figure 196.1 • The LTC4099 with I2C Controlled Battery Conditioner Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00196-4
measures fail or are defeated, the resulting metallic lithium can fuel a “vent with flame” event.
Consequently, most modern Li-Ion batteries use a man- ganese or iron phosphate-based positive terminal. The price for increased safety is slightly reduced capacity and increased aging.
Aging is caused by corrosion, usually oxidation, of the posi- tive terminal by the electrolyte. This reduces both the effec- tiveness of the electrolyte in lithium-ion transport and the sponge-like lithium-ion absorption capability of the positive terminal. Battery aging results an increase of the battery series resistance (BSR) and reduced capacity, as the positive termi- nal is progressively less able to absorb lithium ions.
The aging process begins from the moment the battery is manufactured and cannot be stopped. However, battery han- dling plays an important role in how quickly aging progresses.
Conditions that affect the aging process
The corrosion of the positive terminal is a chemical process and this chemical process has an activation energy probability distribution function (PDF). The activation energy can come from heat or the terminal voltage. The more activation energy available from these two sources the greater the chemical reaction rate and the faster the aging.
Li-Ion batteries that are used in the automotive environ- ment must last 10 to 15years. So, suppliers of automotive Li- Ion batteries do not recommend charging the batteries above 3.8V. This does not allow the use of the full capacity of the battery, but is low enough on the activation energy PDF to keep corrosion to a minimum. The iron phosphate positive terminal has a shallower discharge curve, thus retaining more capacity at 3.8V.
Battery manufacturers typically store batteries at 15°C (59F) and a 40% state of charge (SoC), to minimize aging.
Ideally, storage would take place at 4% or 5% SoC, but it must never reach 0%, or the battery may be damaged. Typi- cally, a battery pack protection IC prevents a battery from reaching 0% SoC. But pack protection cannot prevent self- discharge and the pack protection IC itself consumes some
current. Although Li-Ion batteries have less self-discharge than most other secondary batteries, the storage time is some- what open-ended. So, 40% SoC represents a compromise between minimizing aging and preventing damage while in storage (see Figure 196.2).
In portable applications, the reduction in capacity from such a reduced SoC strategy is viewed negatively in market- ing specifications. But it is sufficient to detect the combina- tion of high ambient heat and high battery SoC to implement an algorithm that minimizes aging while ensuring maximum capacity availability to the user.
Battery conditioner avoids conditions that accelerate aging
The LTC4099 has a built-in battery conditioner that can be enabled or disabled (default) via the I2C interface. If the bat- tery conditioner is enabled and the LTC4099 detects that the battery temperature is higher than ∼60°C, it gently discharges the battery to minimize the effects of aging. The LTC4099 NTC temperature measurement is always on and available to monitor the battery temperature. This circuit is a micro- power circuit, drawing only 50nA while still providing full functionality.
The amount of current used to discharge the battery fol- lows the curve shown in Figure 196.3, reaching zero when the battery terminal voltage is ∼3.85V. If the temperature of the battery pack drops below ∼40°C and a source of energy is available, the LTC4099 once again charges the battery. Thus, the battery is protected from the worst-case battery aging conditions.
Figure 196.3 • Battery Discharge Current vs Voltage for the LTC4099 Battery Conditioning Function
197
Jon Munson
Simple calibration circuit maximizes accuracy in Li-Ion battery
management systems
suited as a high performance calibration source, available in the small SO-8 package. Figure 197.2 illustrates this config- uration. The calibration reference is measured with an ADC channel normally intended for temperature measurement. A programmable I/O bit controls power to the reference.
Accounting for the error sources
Fundamentally, there are several key characteristics that com- prise an overall accuracy specification:
• Quantization error of the ADC
• Initial accuracy of the ADC (or calibration reference)
• Variation from channel to channel
• Variation with temperature
• Hysteresis effects, primarily that of the soldering process
• Variation with operating time (long-term drift)
The maximum specified error in the data sheet for the LTC6802IG-2 includes the first four items and is ±0.22%;
about ±7mV when measuring 3.3V, the most demanding
Introduction
In Li-Ion battery systems it is important to match the charge condition of each cell to maximize pack performance and lon- gevity. Cell life improves by avoiding both deep discharge and overcharge, so typical systems strive for operation between 20% and 80% states of charge (SOC). Detection and correc- tion of charge imbalances assures that all cells track within the desired SOC window, preventing premature aging of some cells that could compromise the entire pack capacity. Highly accurate measurements are required to determine SOC with
Li-Ion cells due to their exceptionally flat discharge charac- teristics, particularly with the lower voltage chemistries (see example in Figure 197.1).
Although the popular LTC6802 Battery Stack Moni- tor offers high accuracy analog-to-digital conversion, some applications demand accuracy that is only attainable with a dedicated voltage reference IC. The LT1461 is especially Figure 197.1 • Discharge Characteristics of 3.3V Li-Ion Cell
Figure 197.2 • LT1461 as an External Calibration Source for an LTC6802 Li-Ion Battery Monitor
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00197-6
region of the discharge curve. The spec budgets ±3.3mV (±0.1%) as the maximum variation over the −40°C to 85°C operating temperature. Since the differential non-linearity (DNL) of the ADC is about ±0.3 LSB, the quantization error contribution is about ±0.8 LSB, or ±1.2mV. Typical channel- to-channel variation is minimal, under ±1mV, leaving about
±1.5mV for trim resolution and accuracy in the IC manufac- turing process. Thermal hysteresis is specified as 100ppm, and an additional approximately ±0.1% error may develop from the shift of the printed-circuit soldering process.
Projected typical long-term drift is under 60ppm/√ khr.
If the practical vehicle battery system active life cycle is targeted at 5khr (about 15 years or 150,000 miles), an uncertainty of around ±0.5mV could develop. This is a relatively small contribution to total error.
The LT1461AIS8-3.3 voltage reference IC has an out- put tolerance of ±0.04% and less than ±1.2mV of change over temperature with its exemplary 3ppm/°C worst-case stability. The LT1461 exhibits a long-term drift of under 60ppm/√
kHr and thermal hysteresis of 75ppm. Solder reflow shift is expected to be under 250ppm (±0.8mV).
Since a significant portion of the LTC6802 ADC error accumulates after the initial delivery of the IC, an external calibration technique improves accuracy in a finished product.
Examining calibration strategies
There are a number of options to improve system accuracy, at the expense of additional complexity. With the simple cir- cuit of Figure 197.2, several options are available that take advantage of the external calibration reference. Accuracy pro- jections of several methods are tabulated in Table 197.1 and described below.
The simplest scheme (method 1) involves no local mem- ory or measurements at production. This method takes read- ings of the nominal 3.300V calibration voltage periodically
and normalizes all ADC readings with the same computed correction factor. The tolerance and drift of the reference and channel-to-channel variations are left uncorrected but the net uncertainty would be improved by almost a factor of 2, to
±6.2mV.
A slightly more complex technique (method 2) involves storage of a single correction factor that accounts for the true reference voltage as measured with high accuracy test-fixture instrumentation. This then eliminates the initial error of the LT1461, improving the overall accuracy to ±4.1mV, nearly a 3× total improvement.
While small, there is still some channel-to-channel varia- tion that can be calibrated out with a method that uses more initial test-fixture measurements (method 3). This is similar to method 2, but with high accuracy measurements of every channel taken (including the reference) and the saving of individual correction factors for each. This further reduces the error to ±3.1mV (almost a 4× total improvement).
Conclusion
A precision voltage reference, such as the LT1461, can improve the accuracy of an LTC6802-based battery manage- ment system to about ±3mV worst-case. The reference is a simple addition to the highly integrated LTC6802 Li-Ion monitoring solution, thanks to the spare general-purpose ADC channels available. The low operating current of the LT1461 voltage reference also makes it ideal for this and other battery-powered applications.
Reference
“Battery Stack Monitor Extends Life of Li-Ion Batteries in Hybrid Electric Vehicles,” Linear Technology Magazine, Vol. 19, No. 1, 2009, page 1.
Table 197.1 Accuracy of Calibration Methods Described for 3.3V Measurements EXTERNAL CALIBRATION METHOD
(ALL TOLERANCES SHOWN IN ±MV)
QUANTIZATION FACTORY TRIM
SOLDERING SHIFT
CHANNEL MATCH
THERMAL VARIATION
THERMAL HYSTERESIS
LONG- TERM DRIFT
TOTAL ERROR
198
Brian Shaffer
USB power solution includes
switching power manager, battery charger, three synchronous buck regulators and LDO
tracks the battery voltage (described below). An I2C serial interface affords the system designer complete control over the charger and the DC/DC bucks for ultimate adaptability to changing operating modes in a wide range of applications.
Switching PowerPath controller maximizes available power to the system load
The LTC3555 improves over earlier generations of USB bat- tery chargers with the addition of several new features. It uses a proprietary switching power manager to extract power from a current-limited USB port with the highest possible effi- ciency, while maintaining average input current compliance. It minimizes power lost in the linear charger with its Bat-Track feature.
First generation USB applications implemented a current- limited battery charger directly between the USB port and the battery, where the battery voltage powers the system.
Introduction
Linear Technology offers a variety of parts to simplify the task of extracting power from a battery or a USB cable. These devices seamlessly manage the power flow between an AC adapter, USB cable and Li-Ion battery, all while maintaining USB power specification compliance. As battery capacities rise, battery chargers must keep pace by steadily improving efficiency to minimize thermal concerns and charge times.
A USB-based battery charger must squeeze as much power from the USB as possible, and do it efficiently to meet the stringent space and thermal constraints of today’s power- intensive applications.
The LTC3555 combines a USB switching power manager and battery charger with three synchronous buck regulators and an LDO to provide a complete power supply solution in one small (4mm×5mm) package (Figure 198.1). The constant- current, constant-voltage Lithium-Ion/Polymer charger utilizes a Bat-Track feature to maximize the efficiency of the bat- tery charger by generating an input voltage that automatically
Figure 198.1 • All-in-One USB Power Solution Includes Switching Power Manager, Battery Charger, Three Synchronous Buck Regulators and LDO Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00198-8
This is referred to as a battery-fed system. In a battery-fed system, the available system power is IUSBãVBAT because VBAT is the only voltage available to the system load. When the bat- tery is low, nearly half of the available power is lost to heat within the linear battery charger element.
Second generation USB chargers developed an intermedi- ate voltage between the USB port and the battery. This inter- mediate bus voltage topology is referred to as a PowerPath system. In PowerPath ICs, a current-limited switch is placed between the USB port and the intermediate voltage. The intermediate voltage, VOUT, powers the linear battery charger and the system load. By using the intermediate bus voltage topology, the battery is decoupled from the system load and charging may be carried out opportunistically. PowerPath sys- tems have the added benefit of being “instant-on” because the intermediate voltage is available for system loads as soon as power is applied to the circuit, independent of the state of the battery. In a PowerPath system, more of the 2.5W avail- able from the USB port is made available to the system load as long as the input current limit has not been exceeded. Pow- erPath systems offer improvements over battery-fed systems, but significant power may still be lost in the linear battery charger element if the battery voltage is low.
The LTC3555 is the first IC in the third generation of USB PowerPath chargers. These PowerPath devices produce an intermediate bus voltage from a USB-compliant step-down regulator that is regulated to a fixed amount over the battery voltage (a Bat-Track feature). The regulated intermediate volt- age is just high enough to allow proper charging through the linear charger. By tracking the battery voltage in this manner, power lost in the linear battery charger is minimized, effi- ciency increases and power available to the load is maximized.
Figure 198.2 provides an efficiency comparison and power savings between chargers with switching vs linear PowerPath systems. The amount of power saved while charging large bat- teries can make the difference between a device that runs in thermal limit and one that runs cool.
Complete power solution in a single IC
The LTC3555 also contains three user configurable step- down DC/DC converters capable of delivering 0.4A, 0.4A and 1A. Regulator 1 has a fixed reference voltage of 0.8V while regulators 2 and 3 may have their reference voltage changed via the I2C interface between 0.8V and 0.425V.
All of the converters operate at a switching frequency of 2.25MHz, allowing the use of small passive components while maintaining efficiencies up to 92% for output volt- ages greater than 1.8V (see Figure 198.3). All three regula- tors may be programmed to operate in pulse-skipping mode, Burst Mode operation or LDO mode via the I2C port or through I/O pins. In Burst Mode operation the output ripple amplitude is slightly increased and the switching frequency varies with the load current to improve efficiency at light loads. If noise is a concern, all of the regulators may be set to operate in LDO mode or pulse-skipping mode. The device also provides an always-on 3.3V output capable of delivering 25mA for system needs such as a real-time clock or pushbut- ton monitor.
Conclusion
The LTC3555 is an advanced and complete power solution in a single chip. The third generation PowerPath manage- ment technology with its reduction in both heat generation and battery charge time is ideally suited for tomorrow’s high density, feature rich battery-powered products. By integrat- ing three I2C-controlled, highly efficient step-down DC/DC converters, the LTC3555 allows the system designer com- plete flexibility to adapt to changing demands and operating modes.