Static Power Path Management (SPPM) to Provide an Effective Handheld Battery Charger Solution
Handheld electronic devices -- regardless of what they are used for or who manufactures them -- share one common denominator: a re-chargeable battery and, in most cases, a built-in charger circuit. When an external power source such as an AC adapter or a USB port is attached at the input terminal, the charger circuit will re-charge the battery. If the device is on when the external power source is attached, ideally, it should draw as much power from the external source as possible to conserve the battery life. To implement this function requires an automatic power source selection circuit, which is normally integrated in the charger control IC. Designers can select from either of two options, dynamic or static power path management, for power management.
Dynamic power path management (DPPM) is the more popular scheme in battery powered handheld applications. The DPPM block diagram is shown in Figure 1. The main pass element QMAIN is a voltage regulator that provides a regulated output voltage VOUT. A secondary pass element QCharge is the charge control element, powered by VOUT to provide constant current or constant voltage to charge the battery. During charging, if the output current exceeds the power source current limit, QCharge will become a switch, connecting the battery to the system to supply a portion of the needed output current.
There are several benefits to DPPM. First, it allows the device be powered up immediately with an external power source when the battery is completely discharged (a dead battery condition). Without power path management, the system will not be powered up until the depleted battery has been charged to a voltage level where the system can start, typically above 3V. This waiting period, depending on the trickle charge setting of the charger circuit, may be some where between a few minutes to tens of minutes. Second, when charging the battery with an external power source, DPPM splits the available current from the external power source into two paths, one path delivers the necessary power to the handheld system, and the other path delivers charging current to the charge the battery. The power path management prioritizes the current to the handheld system when the input current limit of the external source is reached. Thus if the battery is being charged when the system is active, it can be charged to full in a minimum amount of time since the current is dynamically controlled to optimize this goal. When the external power source is removed, QCharge immediately turns fully on, connecting the battery to the system to deliver all the power needed by the system.
The DPPM scheme has a couple of limitations, however. The first issue is the voltage droop at a rapid system current transient. This occurs during a battery charging period, if the system current is in a burst manner, which means the load current at the output rises rapidly from a near zero value to over 2 amps periodically. Since this current exceeds the maximum capability of the input power source, the charge FET has to reverse the current to supply a portion of the burst current. Since a finite amount of time is needed for the current to reverse, during this gap, the output voltage will droop and hence the current is distorted as shown in Figure 2. The second limitation is the cost and complexity of the IC design, since there are two pass elements that require regulation. One regulates the output voltage to the system and the other regulates the voltage or current to charge the battery.
Static Power Path Management
Static power path management (SPPM) is a simplified solution to accommodate the first goal described above without the voltage droop issue. It will not, however, provide a solution for the second objective, which is to prioritize the system and the charge current.
The concept of the SPPM scheme is illustrated in Figure 3. The main FET QMAIN is the charge controller element. There is a switch SW1 connecting the battery and the system. When the battery voltage is above the VPPM threshold (the minimum operating voltage for the system, typically 3.2V), the switch is turned on and the battery is connected to the system whether or not the external power source is present. Thus when the burst current is drawn by the system, the battery is ready to supply the needed current since the switch is already ON. This solves the voltage droop and current distortion issue, as shown in Figure 5. The IC design cost and complexity is also reduced since only one regulation element is needed in the design, to regulate the output voltage or current to charge the battery. The switch, SW1 in figure 3 can be implemented external to the IC to further reduce the IC cost if an extremely low impedance switch is needed.
If the battery voltage is below the VPPM threshold, the SW1 is open and thus VOUT will become a 4.2V constant voltage source, providing voltage supply to the system. At the same time, a 50mA constant current source, ITRKL is activated, charging the battery in trickle mode until its voltage reaches the VPPM threshold (with hysteresis), then SW1 is closed. The battery is then connected to the system. This solves the dead battery startup problem.
Since the SPPM solves the problem of starting the system at a dead battery condition, it is becoming an attractive battery charger solution for handheld applications.