Using low threshold voltage to extend battery life

For years, semiconductor manufacturers in the power management business have struggled to keep up with the demands of end-system users. The increasing number of portable electronic products that are becoming more versatile and require peak performance requires designers to achieve the highest possible efficiency within the physical dimensions of the device. While the battery industry has worked hard to develop alternative battery technologies that have higher power than traditional nickel-cadmium (NiCd) batteries, they are still far from meeting the energy demands of the new generation of portable devices. As a result, portable applications have had to seek innovative developments in low-power circuit design that allow design engineers to make the end system use battery resources as efficiently as possible. In portable devices, components are a major part of the power budget, and it is clear that to keep up with the changing demands, semiconductor device manufacturers need to continue to innovate to help reduce the power consumption of portable products.

Taking mobile phones as an example, reducing the operating voltage of key components in handheld devices such as analog and digital baseband chips is one way to reduce power consumption. When the DSP or microprocessor is not required to perform at its maximum performance, the core supply voltage can be reduced and the clock frequency can be reduced. More and more new generation low-power applications use this technology to save system energy as much as possible. The formula PC~(VC)2.F describes the power consumption of a DSP core, where PC is the core power consumption, VC is the core voltage, and F is the core clock frequency. Reducing the internal clock frequency can reduce power consumption, and reducing the core supply voltage can reduce power consumption even more.

What role can advanced silicon and packaging technologies play?

There are many design factors that affect the performance of emerging power-hungry portable devices. This article will focus on the power MOSFET, the most common power switch in low-voltage applications , to illustrate the impact of the latest silicon technology breakthroughs on increasing power demand. To illustrate the impact of these technological advances, it is necessary to understand some key parameters of the power MOSFET.

Channel On-Resistance ( rDS(on)) is controlled by the lateral and longitudinal electric fields of the channel. Channel resistance is primarily determined by the gate-source voltage difference. When VGS exceeds the threshold voltage (VGS(th)), the FET begins to conduct. Many operations require a switch ground point. The resistance of a power MOSFET channel is related to the physical dimensions determined by the formula R = L/A, where  is the resistivity, L is the channel length, and A is W x T, the cross-sectional area of ​​the channel.

In a typical FET structure, L and W are determined by the device geometry, while the channel thickness T is the distance between the two depletion layers. The position of the depletion layer varies with the gate-source bias voltage or the drain-source voltage. The position of the depletion layer varies with the gate-source bias voltage or the drain-source voltage. When T decreases to zero under the influence of VGS and VDS, the two opposite depletion layers are connected together, and the increased channel resistance (rDS(on)) approaches infinity.

Figure 1 is a curve showing the relationship between rDS(on) and VGS characteristics. Region 1 corresponds to the situation where the accumulated charge is not enough to produce a reverse direction. Region 2 corresponds to the condition where there is enough charge to reverse part of the P region and form a channel, but this is not enough because the "space charge" effect is also important. Region 3 corresponds to the situation where the charge is limited, and when the gate potential increases, rDS(on) does not change significantly.

Figure 1: rDS(on) vs. VGS characteristics

Threshold voltage (VGS(th)) is a parameter used to describe how much voltage is needed to turn on the channel. VGS controls the size of the saturation current ID. Increasing VGS will make the constant ID smaller, so a smaller VDS is required to reach the inflection point of the curve (as shown in Figure 2).

(Text in the picture: At rated RDS(on) and 1.5V voltage, the driver circuit can turn on the MOSFET without a level conversion circuit )

Figure 2: Relationship between rDS(on) and Id at different gate voltages (Source: Vishay Siliconix)

High-speed performance and low-power operation can be achieved by using transistors with low threshold voltages . Using low-threshold power MOSFETs in the signal path allows the supply voltage (VDD) to be lowered, thereby reducing switching power dissipation without sacrificing performance. This is why, in response to the growing demand for reduced power consumption and longer battery life, many ASICs for portable electronic systems operate with core voltages around 1.5V. However, until now, the lack of power MOSFETs that can turn on at such low voltages has made it difficult for designers to realize the power-saving benefits of voltages below 1.8V without using level-shifting circuits , which make the circuit more complex and increase power consumption. Vishay Siliconix has pioneered a series of breakthrough power MOSFETs that are guaranteed to turn on at 1.5V, thus solving this problem.

(Text in the picture: At rated RDS(on) and 1.5V voltage, the driver circuit can turn on the MOSFET without a level conversion circuit)

Figure 3. Reducing VGS(th) allows the driver to turn on the switch with a lower output voltage, reducing the required level shifting circuitry.

From past experience, we need a threshold voltage of at least 1.8V to compensate for the negative temperature coefficient of the threshold point in all power MSOFEs. If the device is operated at 125°C (which is likely to happen in portable applications), existing MOSFET designs have to increase the threshold voltage of the MOSFET to prevent the MOSFET from self-turning on, because even if the applied VGS is 0V, the MOSFET with a low threshold voltage may self-turn on.

In particular, portable devices and mobile phones have an insatiable demand for multimedia capabilities. Designers are striving to provide greater data processing capabilities while meeting the special power requirements of next-generation portable devices. However, there is no doubt that power MOSFETs using advanced silicon processing and packaging technologies will be able to provide the power efficiency, ultra-small size and low cost that designers expect, turning these multimedia mobile phones from a concept into a reality.