PMIC reduces processor hotspot on space-constrained PCBs

The AS3722 PMIC and multiple AS3728 devices power the Tegra K1 processor in Nvidia’s Jetson TK1 reference design board. The Tegra K1, Nvidia’s latest mobile SoC, features a quad-core ARM Cortex-A15 CPU and 192 CUDA GPU cores.

The AS3722 and AS3728 combine to provide a space-saving, thermally efficient power solution for advanced multi-core ARM processors, making them suitable for use in compact mobile devices, and in industrial applications such as security cameras and fanless computers, in which the system designer must ensure careful thermal management of circuit boards loaded with multiple high-power components.

The feedback interface to the AS3722 only requires two wires (one control signal, one temperature signal) instead of the four or five wires typically required by other PMICs, enabling a board layout optimised for thermal performance.

With fewer traces connecting the PMIC to the processor, the two devices can be placed far apart in the board layouts of space-constrained devices. This dramatically reduces the size and intensity of the hotspot around the processor compared to conventional power architectures in which the processor and PMIC, both handling high currents simultaneously, must be located side-by-side.

The AS3722 also integrates all the power rails required by a sophisticated processor such as the Tegra K1, helping to reduce board space requirements. The AS3722 includes four step-down DC/DC regulators operating at up to 4MHz, three step-down DC/DC controllers, 11 universal LDOs, a real-time clock with a 1μA total current requirement, and a control interface accessed via an I2C or serial peripheral interface.

The AS3728 power stage supports two phases, each of a maximum 4A, with an independent control input for each. The device, designed for high thermal efficiency, incorporates separate low and high-side N-channel MOSFETs for each phase.

The AS3722’s three DC/DC controllers may each control the two phases of an AS3728, to provide a maximum 24A power output. The 3MHz switching frequency of the DC/DC controllers provides for a fast response to load transients and keeps the size of the external components to a minimum.

The DC/DC controllers also implement automatic phase selection, ensuring that the circuit switches between single- and multiple-phase modes depending on the output current, to optimise energy efficiency while maintaining instant full-load capability when required in the event of a load transient.

“The latest ARM processors for mobile devices are packed with high-performance cores, and so require a complex power circuit that can supply high currents in a tiny space without the risk of overheating the components,” commented Don Travers, Product Line Manager, ams. “The AS3722 meets the need for sophisticated power control in the Tegra K1 and similar devices, integrating a huge number of supply rails while providing for ample physical separation between the hot processor and the PMIC.”

The AS3722 is housed in a 124-pin CTBGA package (8x8x0.5mm) or a 108-pin CSP (4.8x3.6x0.4mm), whilst the AS3728 is housed in a 2.4x1.6x0.4mm package which can be assembled on a standard 4mm PCB without the need for vias. Both the AS3722 PMIC and the AS3728 power stages are available for volume production, the AS3722 priced at $4.12 per unit for 1,000-unit quantities. The AS3728 is $0.54. An evaluation kit, the AS3722-CT-00_EK_ST AS3728, is available online from ams.