For many years, manufacturers of circuitry aimed at power management applications have struggled to keep pace with the demands of the end system users. An increasing number of portable electronic products, with heightened levels of functionality, demand peak performance and challenge designers to achieve the highest efficiency possible within the device’s physical bounds. Although the battery industry has been making efforts to develop alternative battery technologies with a higher energy capacity than that of conventional Nickel-Cadmium (NiCd) batteries, it is nowhere close to delivering the power requirements of new-generation portable devices. Therefore, portable applications have led to innovative developments in low-power circuit designs so that design engineers can ensure that the end system utilizes battery resources as efficiently as possible. Obviously, to keep up with that demand, semiconductor manufacturers continue to drive innovation to help reduce power consumption in portable devices.
When maximum performance from a DSP or microprocessor isn’t necessary, the core supply voltage can be lowered to operate at a reduced clock frequency. More and more new-generation low-power applications are implementing this technique to maximize system power conservation. The formula PC~(VC).F describes the power consumption of a DSP core, where PC is core power consumption, VC is core voltage, and F is the core clock frequency. Lowering the internal clock frequency can reduce the power consumption; lowering the core supply voltage can reduce it even further.
While these design parameters will provide users with a compelling and functional package, they also will impose stringent requirements on power management circuitry. This is forcing manufacturers and designers to develop new architectures that can deliver more power with greater efficiency for increased battery life at about the same level of cost.
How advanced silicon and packaging technology can help
While there are many design factors that affect the performance of new power-hungry portable devices, this article focuses on power MOSFETs -- the most common power switches for low-voltage applications -- to illustrate the impact of the latest silicon breakthroughs on increasing power requirements. In order to explain the impact of these advancements, it helps to understand some critical parameters of power MOSFETs.
Channel on-resistance (rDS(on)) is controlled by the electric field present across and along the channel. Channel resistance is mainly determined by the gate-to-source voltage difference. When VGS exceeds the threshold voltage (VGS(th)), the FET starts to turn on. Many operations call for switching a point to ground. The resistance of a power MOSFET channel is related to its physical dimensions by the formula R=ρ L/A, where:
ρ is resistivity,
L is the length of the channel, and
A is W x T, the cross-sectional area of the channel.