Integrating Linear Power Regulation On-chip

( 1 Jan 2006 )

Power regulation and management ICs have become one of the fastest growing segments of the electronics industry, largely due to the proliferation of portable electronic devices. This has led to increasing demand for higher levels of integration in order to reduce board space requirements and lower the bill of materials.

The requirements for low cost, long battery life and small size in this market are changing the requirements and specifications for such integrated power functions. Generally, switch-mode or linear regulators are used to supply regulated voltage and power to different sections of highly integrated devices.

One of the first decisions that needs to be made is whether a particular application requires a linear or switching regulator. There are reasons to use either type of regulator in portable devices, though in general, it is widely assumed that a switchingtype regulator will exhibit higher efficiency and is therefore a better solution for portable applications. The low noise characteristics and smaller size and complexity of linear regulators, however, makes them ideal candidates for many applications, especially where the Vin—Vout difference is small.

For very low power applications, linear regulators are actually preferable, even if the efficiency is low, not only due to the lower cost and complexity, but also because their quiescent currents are lower than those of switchers.

Low Dropout Regulators
Due to their lower noise contribution, LDO linear regulators are favored for powering many sections of the typical cellular handset. The PA usually has the requirement to provide buck and boost from a single-cell Li-ion battery with an operating range of 3 to 4.2 V. A DC/DC regulator is best suited to vary the collector bias voltage of the PA in order to improve the PA efficiency when it is not operating at full power.

LDOs, however, are very suitable for powering the baseband, RF, TCXO, RTC, and audio sections of the typical handset. They are also being used to power white LEDs for backlighting in other portable applications. Each of these applications has different requirements which have traditionally been met by off-chip regulator ICs. Keeping the regulators off-chip not only increases BOM cost — it also lowers system reliability, requires valuable board space and creates more stringent requirements on the regulator due to losses on the PC board itself.

Thermal Management
Linear regulators’ inherent thermal issues preclude their use on-chip for high-power applications. When operating close to dropout, thermal dissipation is not as great an issue, but since battery-operated devices see a large drop in voltage over time, this condition cannot be guaranteed. New packages and surface-mounting can help dissipate heat further, but in general these options are not available for devices with integrated regulators.

On-chip Integration
The next question that needs to be answered is where to integrate the on-chip regulators. Generally, the regulator should be integrated onto the same chip that needs to be supplied. This requires regulators to be available in the same process technology as the application. If the discussion were limited to highly integrated radios and transceivers on 0.18 µm RFCMOS technologies, all the linear regulators could be integrated on-chip using dual-gate process 3.3V FETs. LDOs can be fully integrated, on standard processes, with the only external component being an optional 10 to 100 nF bypass capacitor, to lowpass filter the reference output for very low noise applications.

LDOs as Power Switch
One often-used technique to lower overall power consumption is to selectively turn-off sections of a circuit when they are not needed. Using an LDO with a digitally-controlled shutdown pin allows the LDO to be used as a power switch. This technique works better than using a logic-driven p-FET switch because the FET’s on-resistance must be sufficiently low to maintain the load supply above the minimum specified level. At low supply voltages, this becomes an even greater problem.

Such low-resistance FETs are also not available in standard processes necessitating the use of off-chip switches. This adds to pin count, board space and complexity. Using an on-chip LDO with shutdown provides the functionality of a low Ron switch essentially for free.

Since a number of LDOs can be integrated onchip, power can be managed at a finer level to further reduce power consumption. In addition, by using LDOs with multiple preset or programmable output voltages, voltage supply can be reduced to non-critical areas of a device as needed.

Summary
On-chip voltage regulation will play an increasingly important role in portable applications, powered by the relentless drive to lower costs and increase battery-life.

Integrated regulation and power management will become even more prevalent in offerings from semiconductor suppliers, as silicon-verified IP blocks containing the regulator blocks become more widely available from foundries and other third-party sources. Most semiconductor design teams do not have IC power-supply designers readily available since they usually do not contribute to the core competency of the company or design team. Having readily available blocks that are easy to integrate and at low cost will expand the use of on-chip regulation across many applications.

Sunvir can be reached at sunvir@onyxsemi.com; or visit www.onyxsemi.com

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