Building a Li-Pol battery charger using GreenPAK
The market for specialized ICs designed for Li-Pol batteries charging is mature, offering the right option for every application. Available ICs are feature rich, high performance, programmable, low power, and competitively priced. Few alternatives can compete with specialized ICs for the specific applications those IC target. Nevertheless, the feature set of specialized ICs is still limited and fixed.
The Silego GreenPAK is a programmable mixed-signal device that offers an alternative solution for applications requiring specific functions not available in specialized ICs. The circuitry in GreenPAK that is unused in the charger circuit can be utilized in those applications to implement such specific functions. Functions could be directly related to battery charging process or closely related functions like power source selection, load control and similar, but may just as well be completely independent hardware functions of the target device. This article does not cover the charging of all configurations and capacities of Li-Pol batteries, but GreenPAK ecosystem offers the right solution to cover all of them with appropriate design.
Li-Pol Battery Charging Process
A single cell lithium-ion polymer (Li-Pol) battery is charged in two stages: constant current (CC) and constant voltage (CV). During the constant current phase, the charger applies a constant current to the battery at a steadily increasing voltage, until the voltage limit per cell is reached. During the constant voltage phase, the charger applies a voltage equal to the maximum cell voltage in series to the battery, as the current gradually declines towards 0, until the current is below a set threshold of about 10% of initial constant charge current.
Li-Pol batteries are available in single cell, two-cell and multi-cell configurations with single cell as the common option in the mass market. Single cell LiPo chargers are integral part of battery powered electronic devices like cell phones and IoT products, together with the Li-Pol battery itself. Common options to power the charger are regulated +5 Volt power sources like USB power, AC wall adapter etc. Some chargers provide multiple input options and automatic power source selection.
Besides the main CC and CV charging stages, other stages include preconditioning, top off and maintenance. Preconditioning is needed when charging deeply discharged batteries. Such batteries are charged with low current (10% of full rate charge current) until battery voltage reaches 3.0V. Timed top off stage continues to charge the battery to provide optimal battery capacity following a complete charge cycle. During this cycle, charging terminates when ICHG reaches 2.5% of the full-rate charge current or when TTOPOFF times out, whichever occurs first. Once the top off stage is completed, maintenance mode monitors battery voltage and if the voltage drops below 4.0V a new charge cycle is initiated. Stages and transitions are presented in the figure 1 showing the different voltages associated with each state/state transition.
Figure 1. Li-Pol Battery Charging Process
There are three main topologies for the charging circuit: switch-mode, linear, and pulse. The major difference between these topologies is the size and cost vs. performance tradeoff they offer. Linear charger is simpler, easier to control and requires a minimum of external components but has low efficiency and may need additional board area to dissipate the heat generated by the charger's pass transistor. Switchmode charger exhibits high efficiency at small size, but also exhibits ripple, spikes, noise and EMI issues, it is more complex to control and requires a passive output LC filter. The pulse charger operates efficiently as the dissipated power is much lower than that of a linear charger; It doesn't require an output LC filter, but it does require a current-limited power source.
Depending on the application, a complete charger may include various auxiliary circuits for features such as protection, input voltage monitoring, thermal monitoring/regulation, thermal protection/shutdown, battery presence detection, battery protection, start-up, and status indication/signaling, among others.
Specs and features
In this article, we focus on a single cell Li-Pol charger powered from a regulated +5 Volt supply such as a USB port power source. Since power source is regulated and voltage dropout is relatively low, linear charger circuit is a good design choice. To enable high charging currents, we select the design approach with GreenPAK as the control circuit and external “power” circuit. In this design, the GreenPAK acts as a Li-Pol charge management controller.
The desired features for this design include:
Full rate charge current programmed through an external resistor allowing charge currents up to 2500mA (Currents greater than 2500 mA can be achieved by replacing current sense resistor.)
High Accuracy Preset Voltage Regulation: 2V ± 0.75%, Settable to: 4.25V, 4.35V, 4.38V
Built-in Multiple Safety Timers
Charge Status Indication
Continuous Over-current Protection
Near-depleted Battery Pre-conditioning Settable to: 10%, 20%, 40% ICHG or Disable
Maintenance Mode with Automatic Recharge
End-of-Charge Control Settable to 5%, 10%, 15% or 20% ICHG
Battery presence detection
Bad battery detection and indication