Batteries & Fuel Cells
One-size-fits-all battery charger T
By Zachary Pantely, Analog Devices
he first step to designing a battery charger is choosing a battery charger IC from the vast field of available solutions. To make an informed decision, a design team must first clearly define the battery parameters (chemistry, cell count, etc) and the input parameters (solar, USB, etc). The team must then search for chargers that fit the input and output parameters, comparing numerous data sheets to settle on the best solution. The selection process should allow the team to pick the best solution for the application, until of course, the design parameters change, at which point: back to the data sheets.
What if this step could be skipped altogether? What if a designer could focus on an application solution, treating the battery charger IC as a black box to be filled in with a real IC when the time comes to produce a working solution? At that time, the designer simply reaches to the shelf for a generic battery charger IC, regardless of the essential design parameters. Even if application parameters change (inputs switched out, battery type changed, etc.) the off-the-shelf battery charger IC still fits. No additional data sheet search required. This problem can be illustrated by looking at two very different battery charger problems: Design team A is tasked with designing a battery charger that takes solar panel input and charges a lead-acid battery. The charger must stand alone—no microcontroller here—but should be versatile enough to support a few different solar panel models. They have one week to produce a schematic design.
Design team B has a more involved charger problem. Their design takes a 5 V USB supply and charges a 1 cell Li-ion battery with 1.3 A to a termination voltage of 4.1 V per cell. Above 47°C, they want to decrease their charge voltage to 4 V per cell at 0.5 A, and above 72°C, they want charging to stop. The microcontroller in their system needs to know the battery’s voltage, current, temperature, and health. They also have one week to produce a schematic design. It turns out that both design teams can use the same battery charger IC, and that this device is arguably the best choice available for both applications.
Good things and small packages The LTC4162 35 V, 3.2 A monolithic buck charger boasts an elegant blend of simplicity and versatility. Capable of operating standalone or with a host controller, the LTC4162 enables solutions from basic to complex. A full-featured I2C telemetry system allows a user to optionally monitor the battery and implement custom charging parameters specific to the battery model. A true maximum power point tracking (MPPT) algorithm allows the charger to operate optimally from any high impedance source, such as a solar panel. The charging algorithm is tailored to the chosen battery chemistry: Li-ion, LiFePO4, or lead-acid. These features are packed into a 4 mm × 5 mm QFN package with a typical solution size of about 1 cm × 2 cm.
Feel the power Don’t let its small size fool you. Even with integrated switching FETs, the LTC4162 can
support over 60 W of charge power. Internal thermal self-monitoring of its die temperature enables the LTC4162 to regulate the charge current such that it never overheats, even in the hottest environments and tiniest enclosures.
It’s getting hot in here The LTC4162 enables customisable temperature- dependent charging. For lithium-based chemistries (Li-ion and LiFePO4), the LTC4162 can employ JEITA temperature-controlled charging. JEITA allows the user to set custom temperature regions,
Figure 2. The application circuit for LTC4162 is as simple as it gets for a full-featured switching battery charger
Figure 3. The LTC4162’s integrated telemetry system will fulfill almost any monitoring and alert requirements
wherein a custom battery charge voltage and current are used to charge the battery. This also allows the designer to decide the hot and cold temperatures at which the battery should stop charging. The default JEITA settings work for many batteries
without the need for host processor intervention, but this capability enables the LTC4162 to work with any battery’s temperature profile requirements.
Figure 1. What charger fits? Here are two very different battery charging systems; can they use the same charger IC? The PowerPath FETs (INFET and BATFET) ensure that the system load (VOUT) is always powered by the input voltage (VIN) if it is present or by the battery if VIN is absent. The use of external N-channel FETs allows for low loss paths with no limit to the amount of current that can be passed to the load.
12 April 2019 Components in Electronics
Telemetry and control Although the LTC4162 can operate without a host controller, many aspects of charging can be monitored and controlled through the I2C port. An on- chip telemetry system reads system and battery voltages and currents in real time. Various limits and alerts can be set to notify the host controller when a measured value meets a configurable threshold or when a particular charging state is entered. For example, a common design feature may be to enter a low power mode when the battery voltage drops to a certain lower limit. Rather than have a microcontroller continually poll the battery voltage, the LTC4162
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