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The LiPoly Board

This board provides a complete power management solution for LiPoly battery technologies to enable the development of low-power projects that utilize single-cell LiPoly batteries.

It is a significant improvement over existing solutions. The design of this board enable it to perform across a wide variety of load conditions, and has been fully tested in a labratory setting such that its behavior is well understood.

Features

  • 5V Boost-Regulated Output - Easily interface with 5V-based systems, such as popular microcontroller development environments
  • Simultaneous charging and system output - The intelligent charge controller prioritizes the system load over battery charging, and allows for seemless transistion between wall supply and battery supply
  • Supports all modern LiPoly batteries - All single-cell batteries are supported
  • Fuel-gauge battery monitor - Arduino libraries developed to demonstrate calibration techniques.
  • I2C interface for digital communications - Supports charge status, voltage, current, and fuel-gauge measurements
  • Charging input voltages from 5V-8V - Linear regulator and low-pass filter allow for standard, noisy wall supplies.

All of the design files are open-source and documented to support quick drop-in design scenarios.

Motivations Behind Design

Currently existing solutions, such as Sparkfun's Power Cell, are poorly implemented, cannot handle advertised load, and have completely unacceptable quintessential current consumption. These designs have been rushed to market and are not suitable for anything more advanced than the most rudimentary proof of concept demonstrations.

Significant design errors were found in the Sparkfun Power Cell, and other designs. Additionally no existing design integrates current sensing capabilities, voltage measurement, wide-range input voltages suitable for standard poorly regulated power adapters, or simultaneous load-sharing between charging and system supplies, allowing for simultaneous charging and powering of the host device.

All of these challenges needed to be solved to create a solution that would be suitable for actual usage in a commercial product, or even moderately complex hobbyest projects that require long battery life, and reliable operation.

Shortcomings In Existing Designs

Evaluated Designs

The design space currently includes products produced by Sparkfun and other small scale device manufacturers. These devices are based around less sophisticated charge controller chips such as the MAX1555. While some of the boost-up solutions use the TPS61200 as we have used, the implementation was lacking. The biggest limitations are listed below.

  • No support for simultaneous charging and system output
    • Current designs do not support both charging the LiPoly battery and providing system output voltage simultaneously. While some do accomplish this feat, it is not a part of the design. The charge controller chips aren't designed for this mode of operation and this leads to scenarios where the battery is still being discharged, due to a limited amount of charge current being available to the device.
    • Our design uses the MCP73156 chip, which intelligently manages load sharing between the battery and the system. This provides a much higher amount of available current to the system load, while maintaining a proper charge control profile on the LiPoly battery.
  • Poor standby current consumption
    • The designs that use the TPS61200 often implement the power-save functionality incorrectly, resulting in a completely unacceptable sleep power consumption of approximately 10mA.
    • Our design implements power-save mode correctly, and has ample capacitance on the output voltage to ensure proper operation and a low duty cycle.
  • Poor transient current response
    • Additionally designs using the TPS61200 exhibit a terribly current response, often sending the device into a state of oscillations or even causing damage to the part itself when a current step pulse is applied.
    • Our design buffers the output with ample output capacitance, and adds a feed-forward capacitor to the feedback path of the step-up regulator to negate this problem.
  • Poor low battery detection thresholds
    • Many designs, in order to squeeze out longer battery runtimes, employ incorrect low-battery threshold voltages. Setting the correct voltage is critical to extending the life of LiPoly batteries, and additionally the change in capacity when the battery voltage starts significantly dropping is very minimal, very little benefit is gained by this lower voltage.
    • Our design employs a higher cutoff voltage. Additionally voltage measurements are available to the host device over I2C.
  • Lack of feedback of remaining battery capacity
    • No existing designs use a fuel-gauge circuit. LiPoly technology is notoriously hard to estimate remaining life with by voltage alone, so a charge accumulating circuit is necessary to keep track of remaining battery life.
    • Our design integrates the {P/N HERE} chip, which provides current and voltage measurements in real-time, as well as an accumulated charge value. This chip has an I2C interface for use with host systems.
  • Limited input voltage range
    • Most solutions require exactly 5V to charge correctly. This proves to be problematic when one attempts to use the device with a wall adapter, which is more often than not a poorly regulated, noisy voltage source. Depending on the quality of the adapter behavior can be anywhere from nonfunctional to explosive.
    • Our design employed a linear regulator on the front-end of the input voltage, which will sufficiently regulate any input voltage to an appropriate 5V.
  • Limited charge currents
    • Most designs are limited by the 500mA specification of the USB bus during charging, which is limited further when the system is simultaneously powered and drawing some of the power budget.
    • Our design uses an intelligent IC that determines automatically if the input voltage source is coming from the USB port, or from a power supply, and adjusts the current draw accordingly. It will charge a LiPoly battery with over twice the current of the traditional solution, resulting in dramatically shorter charge times.
  • Limited USB support
    • Current designs employ the USB port, but do not pass the data lines back out for consumption by additional devices.
    • Our design provides headers for advanced users to allow them to attach USB peripherals to the D+ and D- USB lines.

Caveats While Designing

While designing this circuit, a number of challenges were encountered. Creating a proper design for the TPS61200 proved to be challenging, as non of the reference designs performed adequitly.

The big design goal of the TPS61200 part was to handle a high constant current, and to provide capacity for large transient current spikes. While the part's datasheet specified it should be perfectly capable of handling both these goals, in practice it proved extremely difficult.

Problems:

  • High current draws from the LiPoly battery increase the internal resistance of the battery, especially when the battery capacity is close to consumed. When the battery was close to being drained, and when a current spike was created, the low voltage detection circuitry would engage prematurely.
    • To solve this issue the output of the battery was supplemented with plenty of capacitance.
  • High current draws also causes the output voltage to dip quicker than the TPS61200 could respond, even after the voltage drop from the battery was mostly taken care of.
    • An additional feedforward capacitor was added after examining the frequency of the oscillations in the voltage output when the current spikes occurred, as recommended in an auxiliary design application note from TI, see here.
  • The output voltage was initially set to 5.5V to enhance stability of the device when operated with a certain brand of sensors. There is a direct relationship between the output current capabilities and the output voltage. A higher output voltage significantly decreases the current capabilities of the device.
    • To solve this issue the output voltage was lowered to 5V. Testing was done at a lower 3.3V and better current handling capacity was observed.

Lessons Learned

  • Datasheets exist in this world to trick you, the devil is often in the details and application specific information can hide in that single obscure graph.
  • It's often better to over specify a part than try to work right up against it's operating limits. The TPS61200 barely meets the requirements of the application, and as such the design requires a great deal of care to get correctly.
proj/lipoly/home.txt · Last modified: 2012/11/26 18:47 (external edit)