Extending Battery Life with Low Threshold Voltage

　　Reducing energy consumption and extending battery life are the goals that every engineer strives for when designing portable electronic products. Battery technology advances very slowly, so designers of portable products focus on power management to extend battery life. For many years, semiconductor manufacturers in the power management business have struggled to keep up with the needs of end-system users. More and more portable electronic products are becoming more and more versatile, and these products require peak performance, requiring designers to achieve the highest possible efficiency within the physical dimensions of the device. Although the battery industry has worked hard to develop alternative battery technologies with higher power than traditional nickel-cadmium (NiCd) batteries, it is still far from meeting the energy needs of the new generation of portable devices. Therefore, portable applications have to seek innovative developments in low-power circuit design, allowing 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 obvious that to keep up with the changing needs, 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 power consumption of the core, 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 high-power 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 requirements. To illustrate the impact of these technological advances, it is necessary to understand some key parameters of the power MOSFET.

　　The on-resistance (rDS(on)) of the channel is controlled by the lateral and longitudinal electric fields of the channel. The channel resistance is mainly determined by the gate-source voltage difference. When VGS exceeds the threshold voltage (VGS(th)), the FET begins to conduct. Many operations require switching ground points. The resistance of the 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

　　The 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. An increase in VGS will reduce the constant ID, 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 compromising performance. This is why, in order to meet the growing user demand for reduced power consumption and extended battery life, many ASICs used in portable electronic systems operate with core voltages of around 1.5V. However, until now, due to the lack of power MOSFETs that can turn on at such low voltages, designers have difficulty realizing the benefits of voltages below 1.8V in reducing power consumption without using level-shifting circuits, which makes the circuit more complex and also increases power consumption. Vishay Siliconix has pioneered a series of breakthrough power MOSFETs that can be guaranteed to turn on at a voltage of 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 features. Designers must strive to provide stronger data processing capabilities while trying to meet the special power requirements of the next generation of portable devices. However, there is no doubt that power MOSFETs using advanced silicon wafer processes 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 reality.