The size and cost of many devices, such as cell phones, pagers and portable digital assistants, can be reduced by taking advantage of cell libraries to integrate additional functions onto BiCMOS integrated circuit power supply devices.
A wide range of cells are now being integrated onto power supplies including dual-tone multiple-frequency (DTMF) circuits, inspection circuits, speaker amplifiers, voltage regulators, battery chargers, control cells, analog/digital and digital/analog converters, ringers, logic controllers, inspection circuits and many others.
These improvements are being driven by the higher yields of BiCMOS processes the only technology capable of handling both the high voltage demands of the power supply and the logic requirements of the integrated functions. The bottom line is reduced component count, packaging and assembly costs, smaller overall size, with reduced weight and shorter design cycle times.
The primary reason that power management has traditionally been isolated on a stand-alone device is that there are grossly disparate process requirements between the manufacturing of power supply and power management ICs and microprocessors and microcontrollers. Power supply management is mostly an analog function that must deal with higher input voltages than most conventional digital processes can handle.
Microprocessor technology involves smaller transistor feature sizes and reduced voltages because they are usually driven by the goal of faster switching. As a result, the underlying structure of the chip can no longer support working at higher voltages.
Increasing voltage regulation demands
One of the biggest factors that is motivating designers to add ancillary functionality to the power supply is the increasing voltage regulation demands. A few years ago, most devices had only one or two regulators, while today many have five or six.
More regulators are required because increased functionality and complexity makes it necessary to provide differing voltage levels. In addition, multiple regulators are often required for a common voltage because of the need to isolate different parts of the system from a particular regulator with insufficient stability for certain critical functions.
The movement towards power supply integration has been led by designers of weight-, size- and cost-sensitive products that found themselves forced to use up to a half dozen regulators, each costing somewhere in the neighborhood of 20 cents and taking up valuable space in hand-held devices.
Designers have also been motivated by the proliferation of features that require additional special purpose ICs such as speaker drivers, vibrator drivers, A/D and D/A circuits, etc. The pager manufacturers were on the cusp of this trend. Competitive pressures demanded that they add more functionality but they didn't have room.
Increasing BiCMOS yields
The increasing process yields in BiCMOS technology is the key innovation that has made it possible to begin integrating new functionality onto power supplies. By combining the bipolar technologies used in power supplies with the CMOS technology used in microprocessors, BiCMOS is obviously the technology of choice to integrate new features onto power supplies.
Yet, ASIC manufacturers that tried a few years ago to integrate new functions onto the power supply did not at first meet with much success. BiCMOS technology was relatively undeveloped at this time so difficulty was experienced in obtaining good yields with the small design rules required to integrate new functions into the power supply.
Eventually, however, their efforts paid off as they rode the learning curve to the point where previously unheard of levels of integration are starting to be achieved on a regular basis.
For example, one Japanese cell phone manufacturer has integrated 5 CMOS regulators, 12 detectors, 4 ringer drivers, a vibrator driver, 3 LED drivers, a receiver amplifier, a microphone bias circuit, an oscillator, a key scan, a Serial/Parallel controller, watch dog timer, a thermal monitoring diode and 2 digital transistors's on the power supply. Most of these functions were provided by discrete components just a few years back.
What types of functions?
There is, of course, not much point in integrating onto the power supply functions that could just as easily be put into a system on a chip. The functions that are included in the power supply are therefore typically not compatible with the types of processes used when integrating logic. Examples are the external driver used in some pagers to drive the beeper, a driver that controls the vibrator, the amplifier that drives the speaker, etc.
These functions are not suitable for a system on a chip because they draw too much power or have other special requirements so they typically end up as discrete ICs or in some cases are assembled from individual transistors.
An important factor in the trend towards power supply integration is the development of libraries of functions that have been created especially for power supply integration.
Since the functions being integrated onto the power supply are now being performed by low cost standard ICs, manufacturers of cost-sensitive devices aren't interested in spending the time and money required to design in the functionality from scratch.
ASIC manufacturers have responded by developing cell libraries of functions such as ringers, voltage detectors and battery chargers that are tuned for the new tighter BiCMOS design rules.
The development of these libraries is a far more engineering-intensive process than simply designing standard components, particularly when you consider that both real estate and performance are more of an issue than in a standard component.
The libraries need to be re-characterized at every corner of the wafer process low temperature and low voltage, low temperature and high voltage, etc. while taking geometrical tolerances into account to make sure the part will work.
Regulator performance gains
Developers of voltage regulator cells were not satisfied with simply matching the performance of discrete components but instead sought out significant improvements. For example, they have implemented bipolar regulators that provide exceptionally short recovery times in output load regulation.
At the same time, output variance at power-on is held to levels that can be matched by few discrete components. Additionally, high frequency ripple rejection is maintained at unusually tight levels.
In one specific example, three detectors compare the battery voltage to three different reference voltages. If a single AA battery is used, then the first detector might be set to detect the point at which the battery drops below 1.2 volts. A warning might be displayed to the user and certain parts of the device might be shut off.
The second detector could then be set to shut off other logic when the battery drops to 1.0 volts. Finally, when the battery drops to 0.9 volts, the entire device is shut off. This type of functionality prevents malfunction of LEDs and vibrator motors at power-on.
Dc/dc converter integration
Another typical example of a cell that can be easily incorporated into a power supply is the DC/DC converter. This cell is typically used to convert battery voltage to another level required by device logic. This functionality is normally provided by a discrete IC that is sold by many merchant semiconductor manufacturers. The cell provides both pulse width modulation and variable frequency modulation to step the voltage anywhere from 0.9 to 11 volts.
An inspection circuit cell provides an example of functionality that can easily be integrated into a power supply. A typical circuit has a battery level detect as well as a reset cell that sends a pulse to the CPU when the battery is being charged. The battery detector measures the level of the battery and begins to shut down circuits and triggers a warning to the user when the voltage drops below a certain level.
Cells offered by ASIC manufacturers are capable of being triggered at varying battery voltages and provide a range of different interrupt signals.
Speaker amplifiers provide an excellent choice for integrating into power supplies because they typically draw a considerable amount of power. A typical cell developed specifically to be integrated into a power supply features dual outputs that can be used to drive a speaker and headphones. This particular design features low current consumption of only 700 uA TYP pre-final block and 2 mA TYP final power block.
Output loads include 8 ohm, 16 ohm, 32 ohm, OTL and BTL. The speaker amplifier cell features low voltage operation of 0.9V for pagers and 1.7 or 2.4 Volts for cell phones. It incorporates an electronic voltage regulator with 2 bit gain selection that can be used to drive the speakers.
Controllers that provide various glue logic functions are an excellent target for power supply integration. For example, nearly all hand-held devices have some type of keyboard that needs to be multiplexed to the I-O of the microprocessor.
Standard cells are readily available that can provide key scan functionality. In addition, general-purpose logic controllers can consolidate I-O and reduce the microprocessor pin count. This can be a significant benefit in situations where the microprocessor itself is I-O bound. Examples of functions that can be performed by these cells include a 4 by 7 key-scan circuit, interrupt control circuit and various analog circuit controls.
Battery charging is another function that is a natural for integration into the power supply. The charger cell shown in the illustration services a lithium-ion battery and includes the ability to detect when the battery is fully charged and shut down and reset the charger.
It also includes circuitry to monitor the temperature of the battery during charging and shut down the charger if proper levels are exceeded. Finally, the cell includes a built-in LED driver used for the light that monitors the charging function.
A/D and D/A conversion provides an interesting if somewhat esoteric candidate for power supply integration. This functionality could be useful in personal digital assistants, cell phones and cordless phones. A typical implementation provides 8- to 14-bit linear sampling at a frequency of 4 kHz to 16 kHz. This particular cell is optimized for a voice signal with 1-bit type. Optional features that could be built into the device include a low pass filter with 128 times over-sampling and an 8-bit real time comparator.
Cells are even being developed to support functions that most designers have never even thought of incorporating into the power supply. A good example is the DTMF signaling scheme that has become nearly universal in telephones as well as personal computer peripherals and remote signals schemes.
While DTMF functionality has become standardized, this functionality has never been integrated into a power supply. So it's significant that cells have been developed that provide these capability and there's little doubt that they will soon begin to replace discrete ICs. A typical cell provides built-in memory of 4bit, 32 digit, with current consumption of 300 uA, changeable pause time of 3.6 seconds, ripple rejection of 75 dB and is capable of direct interface with micron and 4 bit serial input.
Cellular phone manufacturers have been among the first to take advantage of the integrated power supply concept. One manufacturer includes two CMOS regulators, two bipolar regulators, two detectors, a ringer driver, a battery charger, three LED drivers and six switches.
Another cell phone manufacturer incorporates four CMOS regulators, four bipolar regulators, two detectors, six ringer drivers, two LED drivers, a battery monitor and three switches. The result is that what would normally be 17 and 20 discrete components, respectively, are integrated into the BiCMOS power supply, significantly reducing both the size and cost of the phone.
Availability of standard products
In order to compress design schedules, some merchant manufacturers have even developed standard products that integrate typical functions required by cell phone and pager manufacturers. The use of standard products can drastically shorten design cycle times while also significantly reducing device cost.
Samples based on one typical product can be supplied in one month by modifying the standard output voltage or detector voltage. By modifying the aluminum mask of the base chip to rewire existing components, samples can be supplied in three months. On the other hand, full custom samples can be supplied in 6 months.
The integration of new features and functions onto power supplies is clearly an idea whose time has come, particularly for manufacturers of hand-held devices that are running out of space and money to incorporate new features with discrete components. Integrating functionality onto the power supply obviously saves the cost of the individual components, space and assembly time.
Another important factor is that integrating these functions can reduce the overall power consumption of the device by reducing the number of voltage regulators. The development of standard but easily modifiable components that slash design cycle times for integrated power supplies will clearly provide major impetus to this trend.
Mark Dorais is Senior Engineer at Rohm Electronics, Nashville, TN.