Direct Battery Connection Benefits Portable Designs

Jul 1, 2007 12:00 PM
By Warren Schroeder, Principal Applications Engineer, Semtech, Camarillo, Calif.

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In any in-circuit rechargeable battery application, such as a mobile phone, Bluetooth headset or GPS receiver, power-management circuit designers have two options. One approach is to isolate the battery from the load while charging the battery, powering the load directly from the adapter (Fig. 1). Alternatively, keep the battery connected to the load, powering the load with the battery while charging the battery from the adaptor (Fig. 2). There are many commercial battery charger ICs that implement either option. While both connection topologies have their own sets of tradeoffs, the advantages in cost and performance of the direct permanent connection of the load to the battery is the better choice in many applications.

There are cases in which the battery must be isolated from the load while charging. For example, consider the case in which the system can operate at voltages below the pre-charge threshold voltage of the charger IC. Charger ICs typically supply a small trickle-charge current until the cell voltage exceeds a fast-charge threshold. Once the voltage reaches this threshold, it is considered safe to fast-charge the Li-ion battery.

If the system requires normal operation at voltages below this threshold, the charger IC may not be capable of supplying adequate current to charge the battery and supply the load. By separating the battery charging system from the load supply, as shown in Fig. 1, the designer can supply necessary currents to the load regardless of the battery-cell voltage with no degradation in performance or reduction in battery-charging time. However, if it is possible to operate all components of the load from a low-battery voltage, there are many advantages to charging a battery and supplying the load from the same output (Fig. 2).

Non-Isolated Charging Benefits

One advantage of the non-isolated or direct battery-charging configuration is that it enables simple, compact and low-cost designs. The linear and switching regulators, and other load elements that are powered from the battery, can operate even while the battery is charging. Connecting the load directly to the battery also eliminates the need for low-resistance isolation switches. These switches consume large areas of silicon, increasing the cost of a charger IC or requiring the use of discrete components, such as the MOSFET shown in Fig. 1.

Another benefit of this configuration is that continuity of load power is ensured while connecting or disconnecting the charging adapter, or at the beginning or end of a battery-charge cycle. This is because no switching devices are used that could potentially disrupt current flow to the load.

Also, load devices do not need to be tolerant of input transients and overshoots caused by ESD, adapter hot-plug overshoots, adapter noise and charging adapters with incorrect output voltages. The charger isolates the charging adapter from the regulator stages of the load circuitry powered by the battery, providing overvoltage protection and power-supply regulation for the latter. Furthermore, the battery acts as a large capacitive source for load transients, such as inrush currents through the input capacitor of a regulator, without reliance on the transient response of the adapter to power the load.

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© 2007 Penton Media, Inc.