Joshua Broline and Nattorn Pongratananukul, Intersil Corporation
Engineers and designers on product deadlines need all the time they can get to focus on what matters most - the core architecture of a system design. Designing with an FPGA, DSP, or microprocessor is the most time consuming and critical part of the design. System level designers can benefit by focusing on system design and they also need resolution to issues such as time to market and achieving small form factors. Using the latest generation DC-DC non-isolated point-of-load power modules provide them major advantages. Power modules have a high level of integration and density, advanced package technology to take advantage of the high levels of power density, and very reliable overall performance - even for the toughest power management requirements. Using power modules means minimal external components are required, so designers can realize a complex power management design quickly, and focus on the core design. Power modules are useful even if power supply requirements change during the middle or at the end of the design cycle. Before detailing specific power module benefits, let's look at the design issues. There are several that a designer must consider when working with a discrete (non-module) solution. And all can slow the process and delay a product's time-to-market. For example, selecting the proper PWM controller, FET drivers, power FETs, inductor, in order to meet the specific power supply requirements represent the first stages in what is usually a long discrete power supply design cycle. After these main power components are selected, designers have to develop a compensation circuit per output voltage specifications of the various loads that will be used in a given system. This can be tedious and take up a lot time -- and often requires rework. In addition to the compensation circuit design, the power stage, driver, power FETs, and inductor, need to be generated to meet power efficiency targets. This may take several iterations of component selection over varying application requirements. After designing the discrete power supply, the layout work -- with considerations for noise and thermal requirements - adds to the complexity of the design cycle. All in all, it's a cumbersome process. But power modules such as the Intersil DC-DC POL ISL8200M power module change the process, by integrating the PWM controller, driver, power FETs, inductor, IC support discrete components, and an optimized compensation circuit. And they are all contained on a 15x5mm QFN package. The supply is scaled according to output power requirements with its current sharing architecture, and the module comes in a thermally enhanced package and low profile of 2.2mm so it can be mounted on the back side of the PCB. When top side PCB space is an issue, the ISL8200M low profile QFN package of 2.2mm package is a benefit. The low profile package will meet most PCB back side clearance requirements, especially since the QFN package does not require a heat sink or air flow to cover the full output power range over most of the industrial temperature range. With a very low theta J/C of 2C/W on the bottom of the QFN package, most of the heat is dissipated through the bottom of the package and safely through vias and down to a PCB ground plane. This is because the internal high power dissipation components such as the power MOSFETs and the inductor are directly soldered down to these large conductive pads, allowing efficient heat transfer from the module down to the PCB for optimal thermal efficiency, ultimately allowing a 360W max point-of-load power solution to be mounted on the back side of the PCB. This is empowering when a complex power supply design is required and top side PCB space is limited, because it reduces the form factor and enables greater system functionality. In addition to the thermal capabilities, the QFN package has exposed leads around the edge of the package, which allows for access to all pins for debug and solder joint verification.
Load current requirements may change during a system design cycle, but the power supply does not have to change. The ISL8200M can support load current from less than 10A, all the way up to 60A across temperature. Each individual power module can support 10A of output current independently, but by using the module's patented current sharing architecture, the modules can be paralleled for up to 60A of output current. So once the ISL8200M is designed in, the power supply can quickly be modified to meet a wide range of changing application requirements. Furthermore, if layout constraints become an issue when a high power solution is required for a given application, paralleling multiple ISL8200M modules will provide the flexibility to help overcome this challenge due to the patented current share architecture of module. The main connections required to connect modules in parallel reduces layout sensitivity concerns as the output voltage regulation is not impacted by the layout of the module connections required. Output voltage remote sensing and active current share balancing between the modules reduces the sensitivity to PCB trace layout, so flexibility can be exercised for most complex power supply design and layout challenges. When power requirements are greater than 10A, only 5 main connections are required to parallel up to 6 modules. The input and output voltage rails need to be connected and bulk capacitance is required to reduce transients affecting the power supply. A total of 220uF of capacitance it recommended on the input and a total of 330uF is recommended on the output voltage. If very tight noise specifications are required, bypass capacitance can be added in order to filter out external high frequency noise. Next, the enable pins need to be connected in order for the supply to be disabled or enabled per system requirements. The connected enable pins can be used as a critical fault protection feature, the products fault handshaking function. If there is a fault on one of the modules and the module is disabled, all connected modules will also be disabled in order to prevent overstress conditions on the load or power modules. Then the CLCKOUT and FSYNC_IN pins should be connected. The module with the CLCKOUT pin connected is consider the master power module and will set the reference switching frequency. In a two module operation, the module with the FSYNC_IN connection will have a switching frequency 180° out of phase. For more than two modules in parallel, the phase control is adjusted by adding a resistive divider per the datasheet recommendation to the PH_CNTRL or phase control pin. With multiphase operation where the respective switching frequencies are programmed out of phase, lower external noise or ripple can be realized. This reduces the amount of external capacitance or capacitors that are required to hit the critical output voltage regulation requirements for a given point of load. Finally, a connection between the ISHARE pins is required. This pin is used to balance the load current per module. A resistor, RISHARE, is place on this connection in order to set the overall output current. An additional resistor on the module's ISET pin is used to create an internal voltage that is used to compare to the ISHARE bus in order to help balance the output current per module. Compared to equivalent solution types on the market today, the ISL8200M parallel operation has a lot less complexity due to minimal connections between the modules and minimal consideration for layout sensitive during the design cycle. Once a power module like the ISL8200M is designed in, the ability to quickly parallel 6 modules for up to 60A will accelerate future designs or quickly adapt to design requirement changes during the design cycle.
Power modules deployed in non-isolated DC-DC POL power supply can save time, help reduce R&D cost, speed time to market, and allow designers to spend more effort on core system design. The power module's high level of integration, thermally enhanced low profile QFN package and patented current sharing architecture help expedite the design cycle. The power module also has an online simulation tool (iSim) and evaluation boards available. Visit www.intersil.com/powermodules for further information. www.intersil.com