Multichip Modules Enable Megahertz Switching
Fairchild Semiconductor will soon be releasing a suite of products, the FET plus Driver Multichip Module family, designed specifically to support Intel’s Vcore standard for converters. According to Guy Moxey, director of the low-voltage market segment at Fairchild, the layout and size of the switches and driver die are thermally and electrically optimized to enable switching frequencies in the megahertz range.
The modules are housed in Fairchild’s micro leadframe package, the 40-pin version sized at 6 mm x 6 mm, or the 56-pin version sized at 8 mm x 8 mm. Within the device, three N-channel MOSFETs and one driver chip contain the power stage for a single phase. The high-side MOSFET is optimized for fast switching, and two low-side MOSFETS are connected in parallel for the low side.
This arrangement is well suited for the low-duty-cycle switching needed to convert the 12 V from the supply bus to supply up to 30 A to a processor core at 1.1 V to 1.2 V. The upper device is optimized for switching speed, and the lower devices are optimized for low RDS(ON). To support current-mode control, current can be sensed either internally through connections to the low-side devices, or externally through a sense resistor.
Lacking passive components, the devices are not true modules because they were developed with a focus on semiconductor device optimization. This allows flexibility by allowing designers to select the optimal output inductor and capacitor for the application. Further flexibility will be provided by the offering range within the family, which is grouped according to target efficiencies of 85%, 82% and 80% to cover the full range of value and performance in computing platforms.
Extending the concept of enhanced packaging for discrete devices, the family provides an additional efficiency gain of 1.5% to 2% for peak- and steady-power levels over discrete solutions using D-Pak packaging. Normally, parasitic elements along circuit pathways in these discrete solutions would significantly reduce efficiency at frequencies beyond 500 kHz. The layout within the integrated packaging minimizes these parasitics and allows operation in the MHz range.
The portfolio includes devices that can support switching frequencies up to 7 MHz. For these higher frequencies, the MOSFETs are optimized to be driven with a lower gate voltage from the driver, 4.5 V rather than the full 12-V bus voltage. Moxey stated higher frequencies enable the use of smaller phase-output inductors and capacitors, but must be balanced against higher switching losses.
Moxey also stated that further gains in frequency will be possible in the future through integrated packaging of the power stage and the analog controller chip. This represents a class of device between a complete power module and a system-on-chip converter. Moxey stated there can be high cost when using the former, and that the latter supports only a limited current range from 1 A to 3 A. This will make the FET plus Driver Multichip Module family an attractive option for PC power designers.