We are on the cusp of a major shift in battery technology for mobile handsets as Li-ion batteries reach the limits of their capabilities and new chemistries emerge to replace them. As more energy needs to be packed into smaller, thinner handsets, and app processors and 4G networks continue to drive up power demands, higher-density chemistries such as lithium-sulphur are coming to the fore.
However, these high-energy batteries come at a cost to the designer: lower end-of-life voltages, a wider variation in battery voltage from “full” to “empty,” and higher internal resistances. All these challenges have to be overcome before these batteries can be widely adopted in handsets.
For digital components, where Moore's Law is driving lower digital supply voltages — 2.5 V, 1.8 V, or even 1.2 V — the reduction in battery voltage is no big deal. Switch-mode power supplies effectively decouple the supply voltage from that of the battery. Analog components in the RF front end — in particular the RF power amplifier (PA) — that require much higher voltages (typically 3.4 V for full power transmission) do not benefit in the same way as digital components. In the analog world, it's Maxwell's equations that apply, not Moore's Law.
Lower output voltages
Whereas Li-ion batteries maintain output voltages at 3.6 V even when almost empty, new higher-energy-density batteries have a much steeper voltage degradation curve. When 10% of their battery life is remaining, the output voltage can drop to as low as 2.5 V. For the RF PA, this would mean insufficient transmit power to the antenna, resulting in call dropouts and reduced data rates.
With the adoption of 4G in more than 20 frequency bands, broadband PAs are now a “must have” for handset designers. Broadband PAs require a higher supply voltage in order to reduce impedance transformation in the RF matching network and maintain efficiency. Also, 4G waveforms require higher peak transmit power — and hence PA supply voltages — than their 3G predecessors. As a result, to maintain full RF performance at low battery, handset manufacturers and chipset vendors need new power supply techniques that enable RF components to work across a wider range of battery voltages.
Though traditional power management approaches, such as boost DC/DC converters, can be used to help ease the problem, these further reduce the energy efficiency of the RF PA. A commercially viable solution is not going to be achieved by classic power management techniques alone. Instead designers will need to draw on a more innovative combination of elements, including many that are not traditionally considered part of the power management remit.
For example, envelope tracking (ET) is primarily an energy efficiency and RF performance boosting technology. However, by fundamentally changing the operation of the RF power amplifier, it creates several other benefits for the power supply architecture in a handset.
ET completely changes the profile of the RF output stage by significantly lowering the average supply voltage needed by the RF PA. With an ET PA running at full power and a 4G waveform, the average voltage required by the PA is typically only around 1.5-2 V, with short peaks requiring up to 4.0-4.5 V. With some of today's “AC Boost” ET ICs, full RF output power can be maintained down to a battery voltage of 2.8 V or even 2.5 V.
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