National opts for familiar outline for DC/DC modules

EDN Europe, 25 Jan 2010

National Semiconductor has entered the market for highly-integrated power modules with a range of step-down dc/dc converters in a new package. Some packages that other vendors have employed for such modules have had limitations, National asserts. With multiple contact patches on their bases, they have been difficult to prototype and have been liable to soldering errors in production. National has therefore designed a package that is in the style of a TO-263; it has a single thermal pad on its underside (that you reflow-solder to a suitable area of copper on your PCB) and all other electrical connections emerge as leads on one edge of the moulding. Internally a dual-lead-frame construction connects controller IC, power FETs and passives, and offers higher reliability than simply using an over-moulded substrate, the company claims.
National will have three device families in this Simple Switcher-branded grouping, which will carry LMZ part numbers, for inputs from 5 or 3.3V, 12V, or 24V. At introduction, there is one device for each voltage rail, and National will introduce additional parts throughout 2010. First devices do not have any new silicon – that is, they use controllers and FETs from National’s existing catalogue – but subsequent releases will host entirely new components. Later devices will extend current capacity beyond the 5-A limit of the first modules in the series.
The package construction includes shielded inductors to yield low radiated electromagnetic interference (EMI), meeting the requirements of the EN55022 (CISPR22) Class B radiated emission standard. A National spokesman comments, “The great majority of our early-adopter lead customers have opted for the devices because of their noise performance.” The modules’ efficient heat dissipation and large exposed-bottom pad allows for operation in high-ambient temperatures without the need for airflow. They typically operate as much as 10 degrees cooler than comparable modules, National claims.
For operation from -40 C to 125 C , the first release of the LMZ power modules supports the common 3.3V, 5V, 12V and 24V input voltage rails and offers load currents up to 4A. Available in 7-lead, 10.16 mm by 13.77 mm by 4.57 mm packages, they have thermal resistance (theta-JA) temperature of 20 C per watt.
The LMZ10504 supports maximum load currents of 4A with an input voltage range of 2.95V to 5.5V: the LMZ12003 supports maximum load currents of 3A with an input voltage range of 4.5V to 20V: and the LMZ14203 supports maximum load currents of 3A with an input voltage range of 6V to 42V. They cost, respectively, $7.10, $7.25, and $9.50 (500)
You can design-in the modules using a new upgrade of National’s Webench software, that now handles design of multiple output DC/DC power supplies. The software explores a number of variables including topology, intermediate voltage rails, footprint, efficiency, component count and bill of materials (BOM) cost. Under Webench Power Architect you list the inputs and outputs your system requires, and set an optimisation target that ranges from best results for minimum footprint, to maximum efficiency. The package will evaluate a number of possible configurations of intermediate-rail PSU architecture, presenting you with area, efficiency and cost outcomes. You can then re-optimise your design (at this level the Flash-based client software runs in seconds) or alternatively invoke a full simulation to verify the design’s outcome. Cost information comes from a continuously-updated database drawn from the data held by National’s network of international, broadline distributors. It covers 21,000 components from 110 manufacturers.
The software will handle system input sources (VIN) from 1V – 100V, up-to-20 system loads (VOUT) from 0.6V – 300V, power per load up to 300W, system efficiency greater than 90%, and per-supply footprints from 14 x 14 mm. When the design is complete, the tool generates a system summary report including schematics, BOMs and electrical operating values, and you can invoke a “Build It!” feature to order a prototype kit