With every new generation of consumer and portable battery-powered electronics products, we see increasing levels of integration. People want smaller, easy-to-carry devices or gadgets, but they want more features including the battery life to last as long as possible.
In meeting this demand, the electronics industry has inevitably advanced both semiconductor and battery technologies. One of the trends resulting from the latter is the emergence of sophisticated, highly integrated power management ICs (PMICs) with many voltage regulators, dc-dc converters, other power management blocks plus other functions such as audio codecs and amplifiers, all integrated on the same chip.
Designers of portable electronics systems are considering the use of these highly integrated PMICs to reduce component count as well as to become more efficient in managing power consumption. This involves using some of the different advanced power management techniques built into the highly sophisticated systems on a chip that form today's integrated PMIC.
Although there are benefits to using a highly integrated PMIC in place of multiple single-function components, there are also potential drawbacks to be considered. In addition to weighing the pros and cons of the integrated approach, portable system designers must consider power dissipation and signal routing issues that arise with these chips.
The Case for Integration
Advances in battery technology combined with the need for multiple supply voltages, including those at very low values, put new demands on the design of power management systems in portable electronics equipment. In adding new functions to mobile phones, portable media players and other devices, the need to improve power management becomes essential as designers strive to maintain the levels of battery standby time to which the modern consumer has become accustomed. With abilities such as multimedia audio, video or gaming becoming more commonplace, this invariably puts a greater drain on the system battery. The onus is on the manufacturer to increase operating time so that the standby time appears equivalent to previous-generation phones.
The battery chemistry of choice for most consumer and professional mobile applications is lithium ion (Li-ion) or lithium polymer (Li-poly). These types of cells exhibit excellent capacity and also have terminal voltages in the region of 3.3 V to 4.2 V. However, IC supplies are continuing to fall in order to maximize operating times.
Traditionally, designers have been able to choose from a wide variety of off-the-shelf low-integration power management components from multiple vendors, enabling rapid product development. But with systems becoming more complex, more control is required — either to optimize efficiency by turning off unused functionality or to ensure correct and stable operation when running multiple supplies, often to the same IC.
This increased complexity adds system requirements to the basic functions of voltage generation and regulation. Although this can be met by adding devices to the collection of parts already used, it invariably increases cost and, more importantly, pc-board area in many cases.
Hence, more attention must be given to the integration of many of the essential power management functions in the same technology while not compromising technical performance. For example, a typical handheld application has a Li-ion power source — a processor controlling the main functionality, memory, external peripherals such as a display, memory expansion such as secure-digital or multimedia cards, and some analog functionality such as audio, data conversion or sensor interfacing.
Such a system requires several different power-supply voltages, as each function has different requirements. The processor typically may require two supplies: a low voltage for its core to save power consumption and a higher voltage to interface with the other devices. Analog functions tend to require higher voltages to guarantee operating headroom or provide powerful output drivers, and display drivers tend to require higher voltages as display size and complexity increases.
In addition to the supplies, a designer needs to consider other issues such as:
How the system will be switched on. Is a mechanical switch sufficient or is an electronic method needed?
What happens when the battery supply is out of range (undervoltage and overvoltage)?
How do individual supplies need to be sequenced?
Are any permanently active supplies required for data retention or sleep functionality?
How can the system be switched off without removing the battery?
Issues such as these invoke different design options. For example, if an electronic switch such as an output from a sensor circuit or an alarm signal is used to turn the system on, how does this switch interface with the controller? A classic example is a dual-function key that combines power and another function.
These issues can be addressed using standard parts such as power-on-reset (POR) circuits, battery monitor ICs and programmable logic devices (PLDs). However, this adds components to the system.
Size constraints drive us to use highly integrated power management. While this offers many benefits, we have to be aware of the design techniques required to get the best performance from an integrated solution. Integrating functionality into a single IC also presents challenges — with factors like heat dissipation and pc-board routing of power lines coming into play.
When using integrated PMICs, designers need to be aware that in systems with many voltage regulators, the power dissipation ratings of the package may be lower than the worst-case dissipation when all regulators are operating at their maximum currents. IC designers usually specify their systems for typical operating characteristics in a target application, where all regulators might never be operated simultaneously at maximum current.
Therefore, the system designer should always calculate the system power budget; failure to do so could result in hot spots in the design, giving lower reliability or, at worst, system failure. Maximum power dissipation affects both pc-board design and component placement, as thermal mismatch can weaken solder joints and adversely affect nearby components with poor thermal stability.
Most integrated power management devices are supplied in BGA or QFN packages, and in both cases, the central area of the package is used to create a thermal path to the pc board. Take for example Dialog Semiconductor's DA9034, which integrates more than 50 functions in a system-on-chip audio and power management IC.
The DA9034 is housed in an 8-mm × 8-mm TFBGA containing 196 balls spaced at 0.5-mm ball pitch. The central 64 balls of the package are all dedicated to VSS connections, which in turn connect directly to the central VSS plane in the pc board. These solder connections provide a low-thermal-resistance path to a large pc-board area (table). Having such thermal vias improves the thermal rating of the package, allowing it to handle higher power. QFN packages also often have a central thermal pad that can be used to take away heat in a most efficient manner.
Designers of pc boards must be aware that an integrated solution will focus the system power dissipation onto a smaller area of pc board. Therefore, it is essential that power and ground planes be designed to disperse heat away from the device and ensure that it is evenly distributed across the board.
Connecting to the Battery
With an integrated system, connections from the battery are very important, as we need to maintain isolation between supplies and also manage voltage drops both on the pc board and the IC. Dialog Semiconductor's approach is to have multiple battery connections separating the supplies to sensitive blocks. Each supply pin is shared between a minimum number of functions to ensure voltage drop is kept to an absolute minimum.
While this approach increases the pin count of the device, having multiple supply connections improves isolation and can make pc-board routing simpler and more efficient. By splitting supplies, multiple pc-board tracks can be used, which allows more flexible pc-board routing and can lower pc-board track resistance to minimize resistive losses.
In Fig. 1, multiple regulators are shown with the associated voltage drops across the pc board and IC. The system designer needs to be aware of pc-board track resistance and therefore use wider tracks for multiple regulators or high current paths.
A mistake often seen is to minimize track width to allow tracks to be easily routed. But this can seriously impact performance, causing resistive voltage drops such that the voltage at the device being supplied is significantly lower than at the pin of the PMIC where it is being regulated.
As a general rule of thumb, input and output power connections should use tracks that are as large as possible to minimize resistance. To illustrate this point, consider a pc board that uses a minimum of 152.5-g/m2 (or 0.5-oz/ft2) copper. In this case, the pc-board track thickness will be 0.017 mm. If minimum-width 0.1-mm (4-mil) traces are used, these can be reduced by etching to 0.075 mm (3 mil).
As an example, the resistance of 1 mm of track length of minimum size can be calculated by using:
where ρ is the resistivity of copper and equals 16.7 nΩm, I is the track length and equals 1 mm, and A is the cross-sectional area of the track (0.017 mm × 0.075 mm).
Such a track will have a resistance of 13 mΩ/mm. As a real-world example, consider using this track to route the output to a buck converter that delivers 800 mA. In this case, 1 mm of minimum-sized track would drop:
0.8 A × 0.013 mΩ = 10.4 mV.
To put this into context, an 800-mA buck regulator is specified as having a load regulation of typically 10 mV. So pc-board layout can very quickly affect the overall system performance if due care is not taken.
On chip, running fewer regulators from a single supply pin allows lower voltage drops across the bond wires and IC tracking, which is essential if trying to achieve low-dropout performance.
The battery supplies should be decoupled close to the IC for best performance. In practice, the placement of a 4.7-µF (or higher value) multilayer ceramic capacitor close to each edge of the IC package has been found to be an effective method of decoupling. This capacitor should connect to the IC power input pins through wide pc-board tracks.
Sharing the supply pin with more than one regulator can compromise decoupling, because the decoupling component may not be matched to both supplies. This can be a particular issue for switching regulators, because they introduce significant current spikes onto their supplies.
By the nature of their operation, voltage spikes are generated on the input as the supply is chopped. These spikes then appear on the input of other regulators and need to be suppressed, otherwise they affect the quality of these supplies.
A system-level view might help to integrate a higher level of functionality into the IC. Having a highly integrated power management solution gives numerous benefits, including smaller form factor, high reliability, faster manufacturing as fewer components need to be assembled and cost savings. There are also the benefits of lower power consumption, less complex design and less noise in the system.
However, the fully integrated solution may not always be appropriate. Where form factor is not an issue, the benefits become less clear. Having all functionality in a single place on the pc board may require longer than optimum pc-board tracks, which could increase the voltage drops and pick up noise. As a consequence, the voltage at the pin of the IC being supplied may not be the same as that regulated at the pin of the PMIC. This can be overcome by careful attention to pc-board track widths.
In some cases, the top-level metal on the pc board is thicker than for buried layers and so routing on this layer can allow lower impedance tracks. Alternatively, a buried layer in the pc board can be assigned as a power plane. By restricting the number of tracks on this layer and flood filling any unused area to widen tracks, it is also possible to optimize the pc-board traces for lowest resistance.
A Typical Application
As more systems require multiple supply rails, the task for the designer is made more complicated, and finding a simple or easy-to-use power management solution becomes more critical to meet the faster time-to-market challenge that they face.
We can illustrate the use of a simple integrated PMIC in a portable media player (Fig. 2). Here many functions can be integrated into a single PMIC. This chip integrates a buck converter and four linear regulators in a 4-mm × 4-mm package. The package chosen is a QFN, which gives a good balance between the number of available pins and pc-board area.
The QFN's central pad can be soldered directly to the pc board, giving it a junction-to-ambient rating of 38°C/W. This level of thermal resistance allows the package to dissipate more than 2 W under typical operating conditions and, hence, control high currents. Its design allows short bond wires between the die and package, minimizing voltage drops. And with a fine lead pitch of 0.5 mm, 24 leads can be used.
In this example, the PMIC contains regulators optimized for analog or digital supplies. The analog-optimized supplies are used to supply the audio IC as they exhibit high power-supply rejection ratios (PSRR) and low noise, and so do not degrade the audio performance. A second analog regulator is used to supply the display. Here, good PSRR is important as ripple on the supplies can produce unwanted optical effects on the display.
The remaining linear regulators support the processor digital I/O domain and external memory where noise performance is less important. These digital supplies are optimized for transient performance as memory and bus access tends to be more “peaky” in nature. The buck regulator supports the main processor core and is capable of supplying very low voltages for saving power.
The buck converter has a programmable output voltage, but does not support dynamic voltage management, an additional power-saving feature on some PMICs. It is still possible to use the DA9025 to support variable core voltages. However, the designer must be aware that the voltage change is not closely controlled in terms of switching time and so will require such timings to be quite relaxed.
The DA9025 has separate supply pins for the buck converter and each pair of regulators. The supplies to analog and digital regulators are brought out on different pins to minimize interaction. Each major supply pin is placed on separate sides of the package to ease pc-board routing and decoupling. The chip also incorporates a state machine to sequence the power-up of the device, separating the turn-on of each regulator by 200 µs. This minimizes inrush currents, which can produce excessive voltage drops if pc-board track widths are reduced.
Integration of many different power management functions does require the designer to pay careful attention to the signal integrity and thermal management issues, but the benefits in technical performance, reduced form factor and cost make the choice of whether to use integrated components a much easier one.