Review: LTM4631 power module offers cutting edge packaging for underside PCB placement

The LTM4631 is a dual 10 A or single 20 A µModule (power module) step-down regulator in a 1.91 mm-high LGA package with a 16 x 16-mm footprint. The specs show that it operates from 4.5 to 15-V input supplies and regulates an output voltage from 0.6 to 1.8 V with ±1.5% maximum total DC output voltage error from –40°C to 125°C. Its two outputs operate 180º out-of-phase, each capable of delivering 10 A or 20 A when the outputs current share. Two devices can current share, delivering up to 40 A while minimizing input and output ripple current. The device also features output overcurrent foldback and overvoltage protection.


The packaging is what makes this module significant and sets it apart from the competition. Why? Because the device provides a regulator, including the inductor, in one package, while others like Intersil and Altera/Enpirion need two chips for the solution. That means that the Linear product needs 400 mm2 compared to the 750 mm2 for the Intersil and about 600 mm2 for the Altera/Enpirion solutions. At 1.91 mm, the height of the package is also very significant because it means it’s less than 2.00 mm, which is a barrier to designs that aim to provide solutions for the underside of the PCB. Presently, Altera/Enpirion at 1.85 mm, is the only other company that can offer a solution profile less than 2.00-mm, but as I said, it is a two-chip solution.


The LTM4631 regulator, although a very significant achievement in packaging, is not a solution for every design, because not everyone is looking for a cutting edge solution. However, for the targeted markets designers could find this device to be aspirin for their design pains. It is a solution you can’t find anywhere else. The micro-module can be placed on a PCB very close to the load such as an FPGA, and can share one heat sink covering both of the low profile packages. It frees space on the topside for components such as DDR-QDR memory and transceiver ICs. Examples of applications include plug-in and mezzanine cards in embedded systems, data storage systems, gateway controllers and 40 Gbps to 100 Gbps networks. These applications are very competitive and gaining space as shown in the figure is a significant advantage, to system designers.

When I first looked at this product I was very impressed with the specs, especially the packaging, but the price could give you heartburn. Some companies with the technical chops could design a discrete solution for much less cost, but then there’s the obvious problem of excessive footprint caused by all those components. And then there is also a potential of reduced reliability with discretes. At $24.88 ea/1,000, deciding to use this product isn’t quite a no-brainer, but if you do, it means that you want to spend your engineering time on what you design best, such as embedded systems or gateway controllers and getting the extra board space for your latest product.


The LTM4631 wasn’t just a simple redesign of what Linear already offered, although according to Afshin Odabaee, Business Unit Manger of power modules for Linear Technology, at the start of the regulator design, the company thought it would take about 6 months to finish. It took much longer – almost 2 years to finish. But they learned a lot along the way, such as how to get the inductor smaller, what materials to use in the inductor, and even how to get the accuracy down to 1.5% for the total dc output error over line, load and temperature. Other companies will probably try to duplicate this design but, like Linear found out, it will take serious time to solve the packaging challenge. For more information, visit www.linear.com/product/LTM4631.

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