COVER STORY
Helping designers to meet new production goals
Military systems designers face a multitude of challenging design goals because in addition to the performance gains they are expected to deliver, they also find themselves in an environment where the pace of change is accelerating. Budget cuts have led to major corporate restructuring, business diversification into parallel markets and to the globalisation of the defence industry. This in turn brings new competitors and inevitable commercial pressures to the design teams. Steve Munns of Linear Technology explains
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gainst this backdrop design groups have to adapt to new work practices such as cross- functional teams working across multiple locations while specialists in some design disciplines such as power supplies and RF are also becoming scarce and time to market is reducing.
Fortunately one thing assisting the design team is the technology improvements that come with every new semiconductor product generation, and so in that way innovation flows from one industry to another. One product family where this has been evident is Linear Technology’s µModule regulators that by a combination of silicon and packaging technology advances, have enabled significant improvements in power supply solutions.
Efficiency
The term size, weight and power (SWaP) is commonly used in the defence industry to convey the idea of technical systems
advancement. If we distil this down further, reduced SWaP implies better efficiency. Efficiency is the key because there is an immediate contradiction. For a system to be small and lightweight, it needs to run at low temperatures to be reliable. However, power supply demands are on the increase, driven by more and more complex digital processing. This of course means the processing core and the power supply components will generate more heat unless the solution can be made more efficient. Linear Technology µModule regulators
provide one solution to the quest for greater efficiency in the power supply sub- system. In demonstrating this, let us think about efficiency in its broader sense and not purely the most obvious one of electrical conversion efficiency.
Physical size and PCB footprint In 2008, Linear Technology introduced the LTM8020 µModule regulator, a complete
200mA step-down DC/DC power supply in a tiny 6.25mm x 6.25mm x 2.32mm high plastic LGA package. The product met the radiated emissions requirements of EN 55022 Class B and was quickly adopted as a standard building block across many types of systems. Move forward to 2014 and the release
of the LTM4623 Ultrathin µModule regulator, now with 3A output capability and the same radiated emissions performance. Thanks to improvements in component technology and packaging it occupies the same 6.25mm x 6.25mm footprint with just 1.82mm height profile, enabling the option of mounting on the underside of the PCB in some systems. For applications with higher power needs the LTM4625 offers 5A DC output current and again occupies the same footprint with a 5.01mm height profile dictated by the larger internal inductor and BGA package.
Another example of improved integration from 2014 is the LTM4634, a triple output 5A/5A/4A step-down DC/DC µModule regulator. It offers three independent high efficiency regulator channels in a single 15mm x 15mm x 5.01mm BGA package. Contrast this with the first family member from 2005, the LTM4600 single output 10A step-down DC/DC µModule regulator that occupied the same footprint.
Figure 1: LTM4623 ultrathin 3A Step-Down DC-DC µModule Regulator 10 July/August 2015 Components in Electronics
Electrical performance Advances in conversion efficiency of the underlying regulator ICs have been coupled with package enhancements to improve thermal performance such that newer µModule regulators can now operate at higher ambient temperatures for a given output current and provide improved design margins. For example, we can compare the temperature de-rating curves of two products using the same package size, the LTM4608A, an 8A step-down DC/DC µModule regulator and a more recent product introduction, the LTM4649, a 10A step-down DC/DC µModule regulator. Figures 2 and 3 are based on a set-up with no heat sink, 5V input and 3.3V output, and the de-rating curves are plotted with the output current starting at the maximum rated and the ambient temperature at 40°C. The junctions are maintained at 120°C maximum while lowering output current with increasing ambient temperature. The decreased output current will decrease the internal module loss as ambient temperature is increased. The monitored junction temperature of 120°C minus the ambient operating temperature specifies how much module temperature rise can be allowed. These curves demonstrate that the newer LTM4649 can operate without
Figure 3: LTM4649 thermal de-rating, 5VIN to 3.3VOUT
thermal de-rating at 90°C ambient whilst at the same temperature the LTM4608A must be de-rated by about 50%. This is particularly relevant to military systems without forced air-cooling where ambient temperatures of up to 80°C to 90°C are commonplace.
Design time and expertise As design resources become stretched by increased system complexity and shortened design cycles the focus falls on development of the key intellectual property of the system. This often means the power supply gets put to one side until late in the development cycle. With little time and perhaps limited specialist power design resource, there is pressure to come up with a high efficiency solution with the smallest possible footprint. This is where the µModule regulator
provides an ideal answer - the concept is complex on the inside, simple on the outside - the efficiency of a switching regulator and the design simplicity of a linear regulator. Careful design, PCB layout and component selection are very important in the design of a switching regulator and many experienced designers have smelt the distinctive
www.cieonline.co.uk
Figure 2: LTM4608A thermal de-rating, 5VIN to 3.3VOUT
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