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Modern portable electronic devices rely on multiple sources of power to provide continuous operation and maximum end-user flexibility. Some of the possible sources include: re-chargeable and back-up batteries, AC adapter, USB, firewire and Ethernet. Proper power-path management techniques are required to ensure a seamless transition between power sources while providing dynamic control for current-limited sources and maximizing battery charge-and-life under various operating conditions. This article overviews some of the techniques and trade-offs in system power-path management.
Power-path approaches
Portable equipment designers have taken different approaches to power-path management. In general, these approaches can be classified into two categories:
Direct-battery connection
This is the simplest of the power management methods. As shown in Figure 1, the entire system is connected in parallel to the battery pack. There are several advantages to this method:
* Cost: Using this approach, there is no separate power path for the system, resulting in a lower count and cost of components.
* Lower Voltage: The system’s voltage is always equal to the battery voltage. In single-cell Li-Ion- or Li-Pol-based Applications: The maximum voltage is approximately 4.2V. This is an important consideration when downstream loads cannot handle higher voltages such as the latest mobile applications in digital baseband and RF processors.
* Load Response: The relatively low-impedance of Li-xx batteries provides a reasonable response to the load transients and short-current pulses as various subsystems turn on- and-off at normal operating temperatures.
 Figure 1. Direct-battery connection system
Although the Direct Battery Connect approach is simple, it has numerous implementation disadvantages including:
* System Start-Up: In portable devices with a deeply-discharged battery pack (typically below 3.0V for single-cell Li-xx), instantaneous system power-up, in most cases, is not possible. For safety reasons, most chargers use a low pre-charge rate to bring the battery to an approximate 3V level. The pre-charge mode, depending on the battery’s condition and age, can take from a few seconds to several minutes.
* Battery-Charging: Since the load is connected directly across the battery, the total current out of the charger is shared between the battery and load. Depending on the charger’s rating, it may not be possible to fully-charge the battery under high system load conditions.
* Power Dissipation: Because the charger provides charge and system current, its overall power dissipation will be higher when the system is operational. The designer needs to pay attention to the system’s overall thermal management.
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