The push for low-power full-featured products

With the growing feature lists and performance levels that these systems deliver, there is also a growing need to manage power dissipation for at least three reasons:

1. As function and feature counts increase, functional density and, if unchecked, dissipation density increase as well. Improvements in semiconductor integration moderate this tendency but are not sufficient to offset the trend.

2. Despite feature expansion, pressures continue to push portable electronic products into smaller case sizes, again increasing functional and dissipation densities. Here the limits appear to be set by the mechanical requirements that the MMI determines and by company standards for mechanical and electrical robustness.

3. As case sizes shrink, so do available spaces for energy sources. Though improvements in battery technologies, particularly in Lithium-Ion energy sources, balance this trend somewhat with increasing storage density, space constraints have prevented OEMs from capturing significant increases in per-charge energy storage.

The strategies OEMs have used to address power dissipation issues have also evolved. The first-level strategy focused on the efficiency of the energy-management subsystem including minimizing losses through DC-DC converter, LDO, battery-management, and battery-protection circuits.

This power-subsystem-centric approach depended largely on the semiconductor vendors' ability to produce components and integrated devices that are less dissipative than similar structures on the market. This left the OEM engineer with what was largely a component-selection task, balancing energy efficiency against other concerns such as component cost and package size.

Although this strategy has been quite effective, the component market, for the most part, has already realized its benefits. As has been the case with most analog and analog-dominant mixed-signal ICs, components supporting this strategy have not benefitted significantly from the ongoing process shrinkages that digital and digital-dominant mixed-signal ICs have been driving.

The second-level strategy cycles off the power to sections of the system or even sections of large ASICs that are not in use at any particular time. This strategy has been particularly effective when applied to large energy users such as radio-link hardware and display backlights but also extends per-charge-cycle operating times by powering off even moderate loads such as audio subsystems, I/O ports, or nonvolatile configuration memory. Current production mobile phones, for example, operate with 20 or more power domains.

In addition to saving the power dissipated through idle current in largely dissipative circuits such as radios and display backlights, this strategy helps whittle away at quiescent dissipation whenever the system can shut down a clocked-circuit section. As IC fabrication processes have aggressively pursued previously unimaginable dimensions (the not-long-ago-impossible 90 nm node is now commonplace and semiconductor-fabrication-equipment makers are already working toward the third generation after it) this strategy has effectively replaced clock gating to reduce idle currents.

This dissipation-reducing strategy depends on engineering contributions from system architects, software and hardware implementers, and ASIC vendors. Though successful, this approach has been somewhat limited by the amount that additional features have added to the application processor's load, pushing designers to draw on larger and more power-hungry computational resources. For example, mobile telephone handsets have moved from ARM-7 to ARM-9 and ARM-11 processors as the baseband and ancillary processing resource of choice. Other portable electronic products have exhibited similar trends though to a lesser extent.

The third-level strategy focuses on reducing the energy that various features and functions use without sacrificing performance. One technique is to take advantage of distributed intelligence to manage functions and features that do not require the enormous processing power and speed of the baseband or application processor.

This strategy allows the processor to turn over entire functions to semi-autonomous peripheral controllers. The results are operating modes during which the processor can enter a sleep state between tasks that occur at human-activity rates, not the data-processing or communication rates for which the processor's full capability is necessary. A smart display-backlight driver serves as an example.

Processor controlled backlighting: the cost of micro management

End users of portable electronic products require clear readable screen presentations over a wide range of ambient-light conditions. Modern portable products often make use of a photodiode or phototransistor to assess the ambient light level as an input to the backlight driver control. The photo sensor requires signal-conditioning circuitry: excitation in the form of a DC bias, amplification, and analog-to-digital conversion or, at minimum, one or two levels of threshold detection.

Be it through external components or on-chip analog I/O pins, the host processor typically monitors the photo sensor output with regular periodic data conversions. These occur at rates on the order one to a few conversions per second. The controller then assesses the conversion result, usually ascribing it to one of three levels corresponding to full daylight, a well-lit indoor environment, or a dimly lit environment such as a living room, restaurant, or night club.

The processor completes the control process by issuing control signals to the backlight driver to provide one of three possible current levels to the LED string. Although it is certainly effective, this arrangement is not efficient. It is essentially a form of micro management: A central resource that is powerful and expensive to operate oversees a process that it should delegate to a part of the system with lower operating costs. It might sound like an insignificant gesture toward processor offloading but the benefits are demonstrable and open the door to a triple win of energy improvement, reduced parts inventory, and a less cluttered PCB.

Win 1: Smart drivers offload the processor

Measurements based on the ADP5520 smart backlight driver illustrate the energy savings available from an LED driver that can operate with microcontroller configuration control but, otherwise, acts autonomously to manage display illumination. The ADP5520 comprises a non-synchronous boost converter, a programmable ambient-light management section, a state machine, and a configurable port expander that can deliver further system savings.

The boost converter can power as many as six white LEDs in a single series string with string potentials as large as 24.5 V and drive currents as large as 30 mA. The ambient-light measurement section working provides all signal conditioning for an ambient light sensor and, working in concert with the on-chip state machine and boost converter, can implement 128 current levels between 0 and 30 mA.

With a processor only performing lighting-control services as the control curve, the ADP5520 provided an immediate 15 percent improvement in per-charge operating times, in tests simulating various mobile handset usage rates (Figure 1). Adding ambient-light sensing to the ADP5520's control method improves per-charge standby time by 50 percent over the baseline measurement. These curves simulate high interaction uses of the handset that do not require the RF section such as, for example, gaming, text and email message review and composition, or camera use.

Figure 1: A smart backlight controller saves energy by offloading the processor. Adding ALS (ambient-light sensing) raises the energy savings even further. Designers who want to imbue their products with an element of elegance fade between light levels instead of just switching. Processor-controlled lighting schemes demand a great deal of processor interaction to implement fades, making such gestures relatively expensive with respect to processor loading compared to simple on-off control. An intelligent LED driver, such as the ADP5520 can implement multiple fade-in and fade-out current ramp shapes including linear, square-law, and cube-law, further reducing processor load (Figure 2). The driver is configurable with 15 discrete independent fade-in and fade-out times ranging from 300 ms to 5.5 s. An onboard resettable diming timer is programmable to one of 15 intervals from 10 s to 120 s.

Figure 2: Smart backlight LED drivers such as the ADP5520 can implement multiple fade-in and fade-out curves without processor intervention.

Win 2: Smart drivers provide additional low-bandwidth functions

In addition to energy savings, such smart drivers can deliver additional value by implementing other low-bandwidth peripheral functions. For example, the ADP5520 integrates a configurable port expander that provides eight I/O pins.

Alternatively, two I/O pins can join a third dedicated pin as independent current sinks for indicator LEDs with programmable fade, on-off, and blinking control (Figure 3). The remaining pins are programmable as keypad or general-purpose I/O.

Figure 3: Two of the ADP5520's I/O pins are programmable to serve as current sinks for indicator LEDs. Along with a dedicated LED indicator current sink, each of these as much as 14 mA and can deliver simple on-off control or 64-step fades. These auxiliary LED drivers can sink 0 to 14 mA and can fade in or out through 64 steps. Like the main backlight current sink, indicators connected to the auxiliary driver pins can switch on-off states or fade through linear or nonlinear sequences.

Win 3: Smart drivers reduce copper trace counts

To allow configuration data to flow from the processor to the smart driver and for status, I/O, or keystroke data to flow back to the processor, the ADP5520 implements an I2C interface. This arrangement simplifies the PCB layout in dense portable electronic devices by reducing both parts count and trace count between peripherals and the controller.

The savings are even greater when a hinge or slider mechanism separates displays, indicators, and keypad from the processor. In such a case a smart LED driver with an on-chip port expander can significantly reduce the size and cost of the flexible circuit connecting the product's halves (Figure 4).

Figure 4: The ADP5520's port expansion allows you to reduce the number of traces to the processor to a simple, minimum interface. This arrangement yields real cost savings when signals pass through a hinge or sliding mechanism.

About the author:

Ken Marasco is a system applications manager for Analog Devices power management group. Marasco is responsible for the technical support of portable power products. Marasco graduated from NYIT with a degree in Applied Physics and has 35 years of system and component design experience.