Switching power supply improves data/talk-time

A new generation of web-surfing handheld devices is now hitting retail shelves. These cellphones, smartphones, and palm/pocket PCs are not only smaller and more colorful than previous generations, but they are capable of delivering email, html, and graphics many times faster than conventional modems.

However, system designers must maintain acceptably long battery life without increasing the size and weight of the battery pack. There are three significant blocks where high-efficiency power supplies can shave countless milliamp-hours: the core processor, the color display backlight, and the 2.5G/3G cellular radio. One solution, specific to the latter, uses a step-down switching regulator to dynamically lower the supply voltage of the power amplifier (PA) when transmitting at less than full power.

Basically, the key to improving battery life in any system is to understand the losses. In typical handheld cellular radios, the PA is usually powered directly from the Li Ion battery, and it is optimized for highest efficiency at peak output power. However, because the radio spends much of the time transmitting at less than full power, the radio frequency (RF) signal in the PA is often of small amplitude and would not require the full headroom afforded by the battery voltage.

Therefore, the PA's supply voltage rail (Vcc) may be lowered via an efficient step-down dc-dc converter in order to save the previously wasted power. A narrow-band CDMA radio by using step-down regulator — such as Maxim's MAX8503 — in conjunction with an InGaP heterojunction bipolar transistor (HBT) PA (Dynalinear Technologies' DT2032) benefits from reduced battery current and added efficiency.

The Dynalinear DT2030 family of PAs is particularly well suited for use with the MAX8500 family due to idle current of only 15mA and linear operation down to 0.6V Vcc; however, other PAs may yield similar improvements. To obtain the highest efficiency, it is important that the PA maintain good linearity and high enough adjacent-channel power rejection (ACPR) at low Vcc voltages.

Although other switching regulators may work, the MAX8500 family is specifically optimized for powering PAs in handheld wireless radios. This regulator integrates a P-channel bypass MOSFET, which is used to lower the dropout voltage between Vbatt and Vcc when transmitting at full power. This is especially important when using a miniature inductor with fairly high series resistance. Other key features are low 5mVpp output ripple, fixed 1MHz PWM switching frequency, and high efficiency, especially at low output voltages where the PA spends most of its time.

Because pc board area is at a premium, it may be desirable to trade some of the efficiency gains for a smaller solution. This is usually accomplished by adopting a two-step solution where Vcc is either set to 1.25V (low power mode) or connected directly to the battery (high power mode). Because the switching regulator is only used to generate 1.25V, a small 100mA-rated inductor will suffice. This inductor may be much smaller than an inductor used when adjusting the regulator continuously all the way up to 3.4V and 500mA. In two-step solutions, Vcc for low power mode is usually chosen to maximize the talk-time at 10dBm antenna output power. Conveniently, 1.25V is also the MAX8504's preset output voltage.

Another possible approach is a four-step solution where Vcc is stepped from Vbatt to 1.5V, 1.0V, and 0.6V. In such a system, the overall efficiency is nearly as good as in a continuous system, while the inductor only needs to support peak currents of 150mA.

To compare the overall performance of continuous Vcc adjustment, two-step adjustment, and four-step adjustment, the battery current can be multiplied by the probability density function (PDF) for Urban Voice, Suburban Voice, and Suburban Data modes. The resulting PDFs and average battery currents are a measure of how the actual transmit output power depends upon the distance to the base-station and the amount of information (voice or data) being transmitted. PDFs for Urban and Suburban Voice were measured in typical environments and are industry standards, while the Suburban Data PDF is an estimate.

With this criteria, it is apparent that the two-step solution saves about 40mA average in every mode when compared to not using a step-down regulator. Similarly, the four-step approach saves 52mA, while continuous Vcc adjustment saves a bit more, up to 62mA in data mode.

Despite the power savings, the switching regulator's pc board area (4x4mm) appears to be quite large. However, it is important to realize that it integrates not only a step-down dc-dc converter, but also a low-Rds(on) bypass MOSFET, level translation circuits to drive the MOSFET gate, and the feedback network for the step-down converter. A discrete implementation would certainly take more area.

Minor 'ideal' concern

The regulator's only weakness is in high-power bypass mode, where the dropout (Vbatt - Vcc) of 100mV (typ.) at 500mA is slightly higher than ideal. This specification affects the end-of-life (EOL) battery voltage, but only when full power is required, which has a fairly low probability. Nevertheless, this specification is not such a concern in systems where the PA's peak current draw is less, such as Wide-Band CDMA. Also, when adopting continuous Vcc adjustment, the bypass mode is not used very often because the step-down converter provides 3.5V Vcc when the LiIon battery is between 4.2V and 3.6V. In this case, the battery current is reduced for most of the battery life, and the data/talk-time at maximum power is extended despite setting the EOL voltage 100mV higher than ideal.

The second pitfall to avoid is wasting battery current in other areas. It is surprisingly common for the RF designer to implement a step-down regulator to save 40-60mA, while the baseband designer throws away 40-60mA by using an inefficient white-LED backlight driver. Some of the smallest and highest efficiency white-LED drivers available are separate charge pumps, all of which operate at fixed frequency and low ripple to avoid interfering with sensitive RF circuits.

Is the longer data/talk-time worth the added size and cost of a product such as the MAX8500? That's a tricky question and depends upon the consumer's expectations. Although European and American manufacturers are expressing interest, it is still unknown when and if they will implement this approach or choose to sacrifice some talk-time for a slightly cheaper and smaller end product.

However, in the high-tech Japanese consumer market where color displays and cellular Internet access are old news, the switching regulator solution has already been designed into most handsets. Furthermore, Korea seems likely to adopt it for both domestic and export models, so it seems likely to spread worldwide as feature-laden wireless handhelds proliferate.