Dynamic power path management in battery chargers: a highly integrated implementation

Transcription

1 from ams AG Dynamic power path management in battery chargers: a highly integrated implementation By Mark Shepherd Field Applications Engineer (US), ams AG In portable electronic devices with a rechargeable battery, an internal power charger IC normally regulates charging, with (for a lithium-ion battery) a constant-current followed by a constant-voltage input. Nowadays, these ICs generally implement optimised charging routines, including provision for dynamic power path management. Various topologies for implementing power path management have been used to date: by choosing the most effective of them, power system designers can provide users with the best experience while at the same time putting the least strain on the battery. This article describes the operation of the optimal topology for dynamic power path management, and shows how it may be implemented in a highly integrated PMU, to provide a space-saving solution to the problem of power system and charger design in devices such as e-book readers, tablets and media players. Basic functional requirements of a charger circuit Battery charging is a simple enough function: when the device is connected to a USB port or AC adapter, battery charging will be initiated. Connecting the charger typically also wakes up the system and draws power from the external power source to supply the system load as well as the charger circuit. A separate power path is used to supply the system, rather than drawing power from the battery while charging it, so that it keeps to a minimum the charge cycles completed by the battery. The charging and discharging processes both have the effect of ageing the battery; every lithium-ion battery has a known failure rate after a specified number of charge-discharge cycles. By avoiding drawing power from the battery unnecessarily, therefore, power path management has the effect of prolonging the cycle life of the battery.

2 An additional benefit of power path management is that it isolates the battery from the system supply voltage: this means that the device can turn on instantly, without requiring the user to wait until the battery s state of charge, and with it the output voltage, rise to the point at which the output voltage meets a minimum threshold. Fig. 1: the simplest topology for a power circuit with power path management, using two ORing diodes In the simplest form of power path management, two ORing diodes may be used to isolate the battery from the system load, as shown in Figure 1. This allows the system to power up when plugged in to an external power source without having to wait for the battery to reach its minimum voltage threshold. This circuit design has many drawbacks, the most obvious being the power loss in the Schottky diodes; in Figure 1, diode D2 is especially wasteful since it operates when the system is running from the battery alone, thus wasting battery power. A less obvious drawback is that the battery charger will try to charge the battery regardless of the system load that is being supplied by D1. If the device is connected via a USB port with a current limit of just 500mA, the battery charger could easily draw all of the current, leaving nothing for the system supply requirements. Worse still, it will cause the USB port to sink to the battery voltage when it reaches its current limit, thus violating the USB specifications. page 2/9

3 Fig. 2: replacing the diodes in Figure 1 with a MOSFET switch saves power when running the system from the battery A step in the right direction is to replace the diodes with a PowerPath FET (M1 in Figure 2). Here, a low-impedance connection between the battery and the output is established through the PowerPath FET, enabling both battery charging and the instant-on function when the source is removed. If the system load requires more current than the source can provide, the battery can make up the difference via the PowerPath FET. With D1 now removed, simple current limiting can be set up internally in the charger IC to prevent the USB port from crashing. A further problem, however, arises as a consequence: while current limiting will protect the USB port from dropping below its minimum voltage specification, it does not provide a means to prioritise the needs of the system over those of the battery charging circuit. Clearly the user would prefer to enjoy the system s peak performance at the expense of slower charging, rather than getting accelerated charging at the cost of starving system functions of power. The term dynamic power path management was coined to describe designs that address exactly this requirement. Dynamic power path management is a method of dynamically adjusting the charge current based on the total available power from the source and weighing it against the requirements of the system. The overall goal is to provide for a fully operational system when attached to an external power source, while minimising use of the battery during this time. page 3/9

4 Fig. 3: this topology provides for dynamic power path management The circuit in Figure 3, which contains both an external and an internal PowerPath FET, provides a way to implement dynamic power path management. The external PowerPath FET is optional: applications supporting high system loads and requiring the dissipation of large amounts of heat will benefit from its inclusion. When the charger is disconnected, all system current is supplied by the battery. Fig. 4: when the system load is low, there is headroom below the source s current limit to supply power to the battery charger page 4/9

5 Figure 4 shows that, when connected to an external power source, the charger and the system are both supplied from the pre-regulator. Both the pre-regulator s output voltage and the current limit can be configured in the pre-regulator s settings. As the system current increases, the battery charge current will decrease automatically to maintain the overall current limit set by the pre-regulator. This is dynamic power path management in operation. Fig. 5: when the battery is fully charged, the charger switch opens and no current is drawn from the battery while connected to an external power source When the battery is fully charged, the battery switch opens as shown in Figure 5. The system is now supplied by the pre-regulator, avoiding the need to discharge energy from the battery and thus prolonging its cycle life. page 5/9

6 Fig. 6: when the system load exceeds the current limit of the external source, additional current may be drawn from the battery via the external switch Figure 6 shows that, if the system current (red) exceeds the setting for the current limit at the external source, additional current from the battery (yellow) is supplied to the system through the PowerPath FETs (shown as a battery switch plus ideal diode). When the programmed current limit is reached, VSUP_CHG will drop slightly below the battery voltage enabling current to flow from the battery to the system while protecting the charging source from falling out of regulation. Integrating dynamic power path management into a PMU Portable consumer electronics devices such as tablets are highly space-constrained. For this reason, the power system in such devices typically uses a PMU (or power management IC, or PMIC) to provide the required number of buck and boost converters in a single chip. For the purpose of reducing board footprint and simplifying the power system design, it is also desirable to integrate the charger circuit into this same PMIC. But can this be done while providing the topology described above to implement dynamic power path management? As Figure 7 shows, the optimal topology for implementing dynamic power path management can be implemented by users of the AS3711, a PMIC from ams for portable electronics devices such as media players and tablets. The AS3711 features two 1A buck regulators, one 1.5A buck regulator, a 3A buck controller, eight LDOs, two boost controllers and a 1.5A switch-mode battery charger, in a 7mm x 7mm package. page 6/9

7 Fig. 7: the charger circuit in the AS3711 PMIC provides for dynamic power path management A DC-DC switch-mode charger offers a more efficient way of charging a battery than the commonly used linear charger, and therefore has a reduced input current requirement. The lower charge current leaves more of the input current from the external source to be used by the system load (supplied by VSUP). The higher efficiency of the switch-mode charger further reduces the thermal power dissipation during charging. The AS3711 also provides a 30V over-voltage protection block and a configurable current-limited pre-regulator block that can be programmed to set the output voltage for the VSUP_CHG rail, as well as 16 different current limit choices between 0.1A and 2.5A. page 7/9

8 Fig. 8: GUI interface for configuring the AS3711 PMIC By using a PMIC with integrated battery charger, rather than a discrete battery charger IC, the designer benefits by reducing the size of the solution and the cost. Additionally, all the system rails and the charging requirements can be set and monitored from one register map. The AS3711 s GUI makes it extremely easy to configure the battery charger functions as well as all the other power blocks in the PMIC (see Figure 8). All of the blocks in Figure 7 may be programmed in the AS3711 s GUI, with configurable settings for trickle charging, constant-current charging, constant-voltage charging, charger time-out, temperature supervision, selectable current limiting and external over-voltage protection, as well as the selection of either linear or switch-mode battery charging. System benefits from use of a PMIC As this article has shown, dynamic power path management provides for reduced wear on the battery and for optimal system performance when connected to an external power source. It also allows the battery to act as a supplementary power source when the system load exceeds the external source s current limit. This means that designers can specify a smaller, cheaper power adaptor with a relatively small current capability sized for the battery charging requirement alone, rather than a larger current capability to provide for both battery charging and peak system loads. page 8/9

9 These benefits are now available to users of the latest PMICs for portable devices provided they implement a topology such as that adopted by the AS3711, which offers a power-efficient means to dynamically alter the current supplied to the battery charger in response to changes in the system power requirement. By implementing dynamic power path management through an integrated PMIC, power system designers also benefit from: Additional space savings, because an external charger IC is eliminated Simple software control of all power rails, including the charger Simplified power management, using the PMIC to monitor the input source, the battery voltage, the system supply voltage and all its voltage rails, and to generate and automatically handle intelligent system interrupts. [ENDS] [1,700 words] page 9/9