Low-Power Double Data Rate (LPDDR) emerged as a specialized high performance, low power memory for mobile phones. Since its first release in 2006, each new generation of LPDDR has delivered the bandwidth and capacity needed for major shifts in the mobile user experience. Once again, LPDDR is at the forefront of another key shift as the next wave of generative AI applications will be built into our mobile phones and laptops.

AI on endpoints is all about efficient inference. The process of employing trained AI models to make predictions or decisions requires specialized memory technologies with greater performance that are tailored to the unique demands of endpoint devices. Memory for AI inference on endpoints requires getting the right balance between bandwidth, capacity, power and compactness of form factor.

LPDDR evolved from DDR memory technology as a power-efficient alternative; LPDDR5, and the optional extension LPDDR5X, are the most recent updates to the standard. LPDDR5X is focused on improving performance, power, and flexibility; it offers data rates up to 8.533 Gbps, significantly boosting speed and performance. Compared to DDR5 memory, LPDDR5/5X limits the data bus width to 32 bits, while increasing the data rate. The switch to a quarter-speed clock, as compared to a half-speed clock in LPDDR4, along with a new feature – Dynamic Voltage Frequency Scaling – keeps the higher data rate LPDDR5 operation within the same thermal budget as LPDDR4-based devices.

Given the space considerations of mobiles, combined with greater memory needs for advanced applications, LPDDR5X can support capacities of up to 64GB by using multiple DRAM dies in a multi-die package. Consider the example of a 7B LLaMa 2 model: the model consumes 3.5GB of memory capacity if based on INT4. A LPDDR5X package of x64, with two LPDDR5X devices per package, provides an aggregate bandwidth of 68 GB/s and, therefore, a LLaMa 2 model can run inference at 19 tokens per second.

As demand for more memory performance grows, we see LPDDR5 evolve in the market with the major vendors announcing additional extensions to LPDDR5 known as LPDDR5T, with the T standing for turbo. LPDDR5T boosts performance to 9.6 Gbps enabling an aggregate bandwidth of 76.8 GB/s in a x64 package of multiple LPDDR5T stacks. Therefore, the above example of a 7B LLaMa 2 model can run inference at 21 tokens per second.

With its low power consumption and high bandwidth capabilities, LPDDR5 is a great choice of memory not just for cutting-edge mobile devices, but also for AI inference on endpoints where power efficiency and compact form factor are crucial considerations. Rambus offers a new LPDDR5T/5X/5 Controller IP that is fully optimized for use in applications requiring high memory throughput and low latency. The Rambus LPDDR5T/5X/5 Controller enables cutting-edge LPDDR5T memory devices and supports all third-party LPDDR5 PHYs. It maximizes bus bandwidth and minimizes latency via look-ahead command processing, bank management and auto-precharge. The controller can be delivered with additional cores such as the In-line ECC or Memory Analyzer cores to improve in-field reliability, availability and serviceability (RAS).

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Power management has received increasing focus in microelectronic systems as the need for greater power density, efficiency and precision have grown apace. One of the important ongoing trends in service of these needs has been the move to localizing power delivery. To optimize system power, it’s best to deliver as high a voltage as possible to the endpoint where the power is consumed. Then at the endpoint, that incoming high voltage can be regulated into the lower voltages with higher currents required by the endpoint components.

We saw this same trend play out in the architecting of the DDR5 generation of computer main memory. In planning for DDR5, the industry laid out ambitious goals for memory bandwidth and capacity. Concurrently, the aim was to maintain power within the same envelope as DDR4 on a per module basis. In order to achieve these goals, DDR5 required a smarter DIMM architecture; one that would embed more intelligence in the DIMM and increase its power efficiency. One of the biggest architectural changes of this smarter DIMM architecture was moving power management from the motherboard to an on-module Power Management IC (PMIC) on each DDR5 RDIMM.

In previous DDR generations, the power regulator on the motherboard had to deliver a low voltage at high current across the motherboard, through a connector and then onto the DIMM. As supply voltages were reduced over time (to maintain power levels at higher data rates), it was a growing challenge to maintain the desired voltage level because of IR drop. By implementing a PMIC on the DDR5 RDIMM, the problem with IR drop was essentially eliminated.

In addition, the on-DIMM PMIC allows for very fine-grain control of the voltage levels supplied to the various components on the DIMM. As such, DIMM suppliers can really dial in the best power levels for the performance target of a particular DIMM configuration. On-DIMM PMICs also offered an economic benefit. Power management on the motherboard meant the regulator had to be designed to support a system with fully populated DIMMs. On-DIMM PMICs means only paying for the power management capacity you need to support your specific system memory configuration.

The upshot is that power management has become a major enabler of increasing memory performance. Advancing memory performance has been the mission of Rambus for nearly 35 years. We’re intimate with memory subsystem design on modules, with expertise across many critical enabling technologies, and have demonstrated the disciplines required to successfully develop chips for the challenging module environment with its increased power density, space constraints and complex thermal management challenges.

As part of the development of our DDR5 memory interface chipset, Rambus built a world-class power management team and has now introduced a new family of DDR5 server PMICs. This new server PMIC product family lays the foundation for a roadmap of future power management chips. As AI continues to expand from training to inference, increasing demands on memory performance will extend beyond servers to client systems and drive the need for new PMIC solutions tailored for emerging use cases and form factors across the computing landscape.

Resources:

• DDR5 server memory interface chipset
• DDR5 server power management ICs
• White Paper: Data Center Evolution: DDR5 DIMMs Advance Server Performance

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