|
 |
greenpowerZONE Products for the week of October 13, 2008
Linear Technology Says…
High Voltage Battery Stack Monitor Supports Hybrid/Electric Vehicles & Battery Backup Systems
Linear Technology announced the LTC6802, a highly integrated multicell battery monitoring IC capable of measuring up to 12 individual battery cells. The device’s proprietary design allows multiple LTC6802s to be stacked in series without optocouplers or isolators, for precision voltage monitoring of every cell in long strings of series-connected batteries. Long battery strings enable high power, rechargeable applications, such as electric and hybrid electric vehicles, scooters, motorcycles, golf carts, wheelchairs, boats, forklifts, robotics, portable medical equipment, and uninterruptible power supply (UPS) systems.
With superior energy density, Lithium-Ion batteries are poised to be the power source of choice for these applications. However, designing a large, highly reliable and long-lasting Li-Ion battery stack is a very complex problem. Li-Ion cells are sensitive to overcharging or over-discharging, requiring that each cell in a stack is carefully managed. The LTC6802 makes this possible with quick and accurate measurements of all cell voltages, even in the presence of stack voltages over 1000V.
The maximum total measurement error is guaranteed at less than 0.25% from -40°C to 85°C and all cell voltages in a battery stack can be measured within 13ms. Each cell is monitored for undervoltage and overvoltage conditions, and an associated MOSFET switch is available to discharge overcharged cells. Each LTC6802 communicates via a 1MHz serial interface, and includes temperature sensor inputs, GPIO lines and a precision voltage reference
The LTC6802 was designed for the environmental and reliability challenges of automotive and industrial applications. It is fully specified for operation from -40°C to 85°C and offers diagnostics and fault detection. The LTC6802 is a small 8mm x 12mm surface mount device. The combined robustness, exceptional precision and tiny package directly address the critical requirements of emerging and advanced battery technologies.
"The LTC6802 provides a precision analog interface for high performance battery stacks," says Mike Kultgen, design manager for Linear Technology. "By handling the data acquisition task, the LTC6802 enables designers to implement state-of-the-art battery management techniques."
Summary of Features: LTC6802
- 0.25% Maximum Total Measurement Error from -40°C to 85°C
- Stackable Architecture Enables 1000V+ Systems
- ADC with Inherent FIR Filtering
- 1MHz Serial Interface with Packet Error Checking
- Onboard FETs for Cell Discharge
- Temperature Sensor Inputs
- Built-In Precision 3V Reference & 5V Regulator
- Diagnostics & Fault Detection
- AEC-Q100 44-Lead SSOP Package
- Fully Specified for -40°C to 85°C Operation
EN-Genius Says…
Linear is to be congratulated on its courage and foresight for introducing the LTC6802 multi-cell battery monitoring IC in anticipation of the large electric vehicle market that’s likely to emerge during the next five years. When I reviewed Tesla’s electric Roadster this spring, one of the most striking things about the car (besides its stunning looks, 4.2 s 0 – 60 mph acceleration, and cramped interior) were the huge numbers of general-purpose chips lurking within its power control module and other electronics boxes. This is to be expected in a first-of-class product like the Tesla, but we can also expect that the many discrete controllers, DSPs, analog and power devices will end up rolled together in more specialized ASICs and ASSPs as production volumes justify the development costs. The LTC6802 is a preview of the kinds of innovative, highly-integrated power devices we can expect to help drive down the cost and complexity of the Tesla high-volume electric sedan and other EVs/PHEVs as the market begins to mature.
Linear says that their SoC battery monitor is the direct result of some work they have been doing for a hybrid vehicle application, a claim that’s confirmed by the device’s clever architecture and unique features. Intended to work on the high-voltage battery stacks used by today’s EVs, the LTC6802 can perform precision measurements on up to 12 cells. It is designed to float, instead of using the automotive ground as its base reference, allowing you to stack as many monitors as you need to cover the full voltage range of the battery system without exceeding its 60 V working voltage.
The 12-bit voltage measurements from each stack segment are collected via an inter-chip daisy chain data link that uses current-mode signaling to form the equivalent of a 1 MHz SPI bus. Using current-mode communications and powering each monitor directly off its portion of the battery stack provides voltage isolation between the monitor devices without the need for more costly opto-isolation. The LTC6802 can configure its data output as a normal voltage-mode SPI device, allowing the so-called bottom device to communicate the data from the rest of the stack to the host system in a conventional manner.
Although using a current-mode data link eliminates the need for costly opto-couplers it still requires galvanic isolation for the chip at the bottom of the stack. It also introduces a potential weak point where the failure of a single sensor will result in losing any battery voltage data from any point further up the stack. This is probably acceptable in many applications but if your design requires the absolute safety that opto-isolation provides, Linear offers an alternate version of the chip (LTC6802-2) that has addressable SPI registers which can be individual addressed via opto-coupler.
Linear says that they use a delta-sigma ADC inside the LTC6802 because its longer sampling time behaves like a built-in filter that helps it ignore much of the electrical noise that’s likely to occur in the harsh, high-current environment of a high-powered battery system. The delta-sigma anti-aliasing characteristics give it the noise bandwidth equivalent to an 850 Hz RC filter (without the need for costly external passives) while still delivering a conversion rate of around 1 ksample/s.
Automotive designers will appreciate the many features that Linear has included to enhance system safety while keeping overall costs down. For example, the LTC6802 has programmable threshold functions that can be set to generate an alarm if an under- and over-voltage condition is detected without the need for host processor intervention. Besides ensuring the system gets an alert of a potential failure much more quickly than it would take a processor to notice it, the hardware alarm signal provides a fail-safe alarm that works whether or not the processor does. The LTC6802 also has built-in diagnostics and a so-called heartbeat signal that lets the system know it’s alive and well.
Whether it’s used in a hybrid car, a forklift, or a submarine, the LTC6802 cell balancing capabilities will play an important role in helping maximize overall battery life. Its on-chip MOSFETs can discharge individual over-charged cells at up to 50 mA or can drive an external FET if higher discharge rates are needed. By including this important feature Linear may well save battery system designers more than the cost of the monitor chip itself if they had to implement a cell balancing circuit using discrete parts.
By developing a highly-optimized part that takes the cost and guesswork out of ensuring the health of advanced batteries, Linear has put itself in an enviable position to take advantage of the growing demand for components that will make energy storage systems more reliable and less costly to produce in high-volume applications.
The LTC6802 is sampling and is priced at $9.95 in 1000-piece lots. Production will be in Q4 2008.
Product Page
|
|
|
|
|
