10.1 System Load After Sensing Resistor

One of the simpler high-efficiency topologies connects the system load directly across the battery pack, as shown in Figure 36. The input voltage has been converted to a usable system voltage with good efficiency from the input. When the input power is on, it supplies the system load and charges the battery pack at the same time. When the input power is off, the battery pack powers the system directly.

Figure 36. System Load After Sensing Resistor

The advantages:

1. When the AC adapter is disconnected, the battery pack powers the system load with minimum power dissipation. Consequently, the time that the system runs on the battery pack can be maximized.
2. It reduces the number of external path selection components and offers a low-cost solution.
3. Dynamic power management (DPM) can be achieved. The total of the charge current and the system current can be limited to a desired value by setting the charge current value. When the system current increases, the charge current drops by the same amount. As a result, no potential overcurrent or overheating issues are caused by excessive system load demand.
4. The total input current can be limited to a desired value by setting the input current limit value. USB specifications can be met easily.
5. The supply voltage variation range for the system can be minimized.
6. The input current soft-start can be achieved by the generic soft-start feature of the IC.

Design considerations and potential issues:

1. If the system always demands a high current (but lower than the regulation current), the battery charging never terminates. Thus, the battery is always charged, and its lifetime may be reduced.
2. Because the total current regulation threshold is fixed and the system always demands some current, the battery may not be charged with a full-charge rate and thus may lead to a longer charge time.
3. If the system load current is large after the charger has been terminated, the IR drop across the battery impedance may cause the battery voltage to drop below the refresh threshold and start a new charge cycle. The charger would then terminate due to low charge current. Therefore, the charger would cycle between charging and terminating. If the load is smaller, the battery has to discharge down to the refresh threshold, resulting in a much slower cycling.
4. In a charger system, the charge current is typically limited to about 30 mA, if the sensed battery voltage is below the 2-V short circuit protection threshold. This results in low power availability at the system bus. If an external supply is connected and the battery is deeply discharged below the short circuit protection threshold, the charge current is clamped to the short circuit current limit. This then is the current available to the system during the power-up phase. Most systems cannot function with such limited supply current, and the battery supplements the additional power required by the system. Note that the battery pack is already at the depleted condition, and it discharges further until the battery protector opens, resulting in a system shutdown.
5. If the battery is below the short circuit threshold and the system requires a bias current budget lower than the short circuit current limit, the end-equipment will be operational, but the charging process can be affected depending on the current left to charge the battery pack. Under extreme conditions, the system current is close to the short circuit current levels and the battery may not reach the fast-charge region in a timely manner. As a result, the safety timers flag the battery pack as defective, terminating the charging process. Because the safety timer cannot be disabled, the inserted battery pack must not be depleted to make the application possible.
6. If the battery pack voltage is too low, highly depleted, totally dead or even shorted, the system voltage is clamped by the battery and it cannot operate even if the input power is on.