Powering multicore processors with multiphase DC/DCs

Did you know that mobile phone processors are now evolving to 8-core and 10-core processors? These processors need multiple cores to run many applications at the same time, operate the graphics processor for games and stream high-quality videos. The new processors require very high current (sometimes over 10A) and need that current delivered as fast as possible.

Due to the ever-growing number of cores, the nature of the devices powering these processors is changing. Truly state-of-the-art power technology is required, while fitting into a small form factor. TI has several buck converters to power mobile phone processors, such as the TPS62180, LP8758 and TPS62184. What these converters have in common is that they implement a multiphase topology, which allows you to tie multiple outputs together while achieving high current density. TI’s new LP8758 is the market’s highest current-density multiphase converter that can be used to power multi-core mobile phone processors.

By splitting the output current across multiple outputs, multiphase converters have several inherent advantages over single-output buck converters. Smaller external components, fast load transients and lower ripple make them a nice fit for powering processors in personal electronics. The ability to add or shed phases allows for high efficiency over a wide range of load conditions. The LP8758 is a great example of a state-of-the-art multiphase converter that is ideal for powering mobile phone processors. It has low IQ, a small total solution size, 16A peak current capability, low ripple and fast transients.

Storing energy in multiple inductors rather than one lowers the inductor size. This allows designers to use the LP8758 with small chip inductors and get to a solution size under 60mm². Each output is run slightly out of phase as shown in Figure 1. Since the phase currents in red and blue are out of phase, they are able to combine together without causing large ripple at the output.

Figure 1: IOUT ripple for single phase converter vs. multi-phase converter

The limited amount of output ripple per phase reduces the required capacitance at the output, allowing for smaller-sized capacitors as well as faster transient performance. Figure 2 shows the transient performance of the LP8758 as it transitions from 1A to 12A in 1µs, with only about 40mV of ripple on the output voltage.

Figure 2: LP8758 transient load step response, FPWM mode

As I mentioned earlier, the LP8758 uses four output phases depending on the output current required. Devices such as the TPS62180 and TPS62184 are also able to adjust between one or two phases to maximize efficiency. In Figure 3, you can see how phases are added or shed to adjust for load current and efficiency. For example, a mobile phone processor may be operating at maximum current, at which point all four phases or outputs would be running to maintain high efficiency. If a processer were in a lighter load state with all 4 phases activated, the device would suffer from high gate-charge losses.  Therefore, three of the phases are shed, allowing for high efficiency while operating a single phase.

Figure 3: Multiphase buck converter efficiency vs. number of phases

By allowing phases to add or shed, whether the processor is running at a heavy or light load, multi-phase converters are able to maintain high efficiency, enabling longer battery life in a mobile device. Due to the high efficiency over various load conditions, multi-phase converters from TI can provide designers of mobile phones a small, efficient power device with good thermal performance to power their multi-core processors.

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