[Design View / Design Solution]
Design A Linear Li-ion Battery Charger For Portable Systems
Lithium-ion batteries help designers meet their goals of getting greener, whether used for storage or backup power purposes, or in highly integrated solutions to develop low-power solutions.
USB-BASED LINEAR LI-ION BATTERY CHARGERS In addition to linking peripherals and computers, the USB protocol also delivers high speed at an economic cost. Connecting devices and peripherals through USB ports to a computer has become the most popular method. With a voltage range of 4.75 to 5.25 V, USB is an excellent candidate for restoring energy back to singlecell Li-ion battery cells or packs as previously discussed. There are many methods for charging single-cell Li-ion batteries.
Table 3 lists a few basic methods for designing a single-cell Li-ion battery charger from USB ports. The first method utilizes a low-power USB port for a fixed charging current. This method usually ends up below the absolute maximum current of a low-speed USB port, which is 100 mA. Due to the resistor’s tolerance, charge current, and supply current, this charge current is typically under 90 mA. This simply treats a USB port as a 5-V, 100-mA-rated power supply.
To take advantage of high-speed USB ports, an external MOSFET can be used to set two different charging currents when driving the gate low or high. A high-speed USB port allows an absolute maximum current of 500 mA, but a port should always start at low speed until verification is complete.
An integrated MOSFET for setting two different charging currents simplifies this design and offers either a preset or resistor- programmable charge current. Figure 2 shows an example that offers three different charge-current settings and can seamlessly switch between a wall wart (ac-dc adapter) and a USB port.
When a wall wart is present, the maximum charging current can easily be higher than 500 mA from a high-speed USB port. When just a USB cable is applied, the charge current will be based on the logic level high or low. Some designs require only one input-power rail, but a different input type can be set by communication between interfaces.
Typically, the preset USB charging current is below 450 mA for a high-speed USB port for the same reason as it is in a low-speed USB port. Proper design methods should also limit the amount of input current drawn from the USB port for safety, as well as to meet USB specifications.
As today’s portable devices become more feature-rich, requirements for proper battery management increase. In spaceconstrained applications, highly integrated power-rail controls advance a designer’s experience. Each power rail must be well managed for seamless switching among the input power path, system load, and battery cell.
Figure 3 demonstrates a typical application circuit of a Li-ion battery charger with system load-sharing and power-path management features that can switch between power sources. One advantage to using this design instead of a traditional method is that each power rail is managed and the battery is in support mode when the input voltage is insufficient to keep the output voltage steady. Sometimes, additional features such as low-power indicators or controls, as well as power-source selection, offer functionality beyond just restoring energy back to batteries.
ADDITIONAL BATTERY-CHARGER FEATURES Increased use of Li-ion batteries leads to a broader range of safety and functionality requirements. These requirements may come from internal organizations that promote hazard-free design guidance; local governmental regulations or policies; regional product-manufacturer preference; battery-manufacturer specifications; a designer’s level of experience; or an enduser’s habits. Common functions include timers for each charging stage, input overvoltage protection, communication protocols, multiple channels of regulated outputs, and battery authentication.
Figure 4 shows an input overvoltage protection feature of a single-cell Li-ion battery charger. The output-charge current terminates when the input voltage passes the protection threshold, and it resumes once the input voltage falls back to the designed range (Fig. 5). Since December 2006, this technique has been recommended for mobile devices as a technical requirement and test method of charger interfaces for mobile telecommunication terminal equipment.
Limiting the input voltage for a linear battery charger keeps end users from incorrectly using wall-wart or ac-dc adapters. It also prevents voltage spikes. Recall Equation 4:
Assuming the charge current is 1 A, if the input and output voltages (battery voltage) increase, power dissipation grows. Therefore, when the differences between input and battery voltages jump to 4 V, the power dissipation is 4 W.
CONCLUSION Green technology is always a hot topic. Engineers and scientists constantly work to improve existing designs and offer better solutions for society. Li-ion batteries can be designed with fuel cells, photovoltaic solar cells, hydro power, and wind power as storage, backup, or supportive power. Highly integrated linear solutions may overcome hurdles in low-power designs, such as compactness and simplicity.
When intelligence, efficiency, or power dissipation are concerns, designers should survey their solutions thoroughly and understand the tradeoffs between platforms that are available. When designing with batteries or any power systems, safety is always the first priority.
Please give me the details Can charge the NiMh-1.2V.2100mAh batteries with Constant Voltage Source.
Nagendra -May 11, 2009
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