Integrated or discrete load switch: which one should you use? (Part 1 of 2)

MOSFET-based load switches, whether as an integrated circuit (IC) or built with discrete components, continue to be popular choices for many engineers. They are favored in power-switching applications, such as battery-powered portable devices including mobile handsets and notebook PCs. The center of the load switch, either integrated or discrete, is a MOSFET.

Today, there are two major types of MOSFETs, "trench" and "planar". Because of the different semiconductor fabrication technologies, the trench MOSFET is usually used as a component for the discrete load switch, whereas the planar MOSFET is more suitable as a building block of the integrated load switch IC. This article takes an easy-to-understand and non-mathematical approach in explaining the operational differences between a trench MOSFET and a planar MOSFET. Parameters such as R_DSON, gate charge, gate-source voltage (V_GS), input-voltage (V_IN) range, load current, and die size will also be investigated.

MOSFET-based load switch
Whether integrated or discrete, the MOSFET-based load switch consists of the following blocks:

• A MOSFET, usually an enhancement-mode type. The MOSFET operates in the saturation region to pass the current from the power source to the load like a "switch" (as opposed to an amplifier).
• A gate-drive and level-shift circuit, which provides a voltage to the gate of the pass element, to switch it ON or OFF. It is called "level-shift circuit," referring to the fact that an external enable signal is "level shifted" to create a high-enough gate voltage (V_G) to fully switch the pass element ON and OFF.

MOSFET types in a load switch
Although the integrated load switch is a relatively recent invention, the MOSFET has been in existence since the early days of the semiconductor industry. One of the most popular applications for the MOSFET is power switching, where the MOSFET operates in its saturation region, with its ON and OFF state controlled by the voltage between the Gate and Source (V_GS).

In power-switching applications, a P-channel MOSFET is typically preferred to an N-channel device. The main reason for this is that the P-channel MOSFET is easier to drive than its N-channel counterpart. For the N-channel MOSFET, the voltage between the Gate and Source (V_GS) must be higher than the voltages of the Drain (V_D) and Source (V_S) by a threshold (V_TH), to fully turn on the channel. Since the V_D is typically tied to the input voltage and therefore equal to V_IN (which is usually the highest voltage in the system, especially for small devices like a mobile handset), it is impossible to generate a V_G that is higher than the V_D (V_IN), unless a voltage-multiplication circuit such as a charge pump is used.

The charge pump is typically tied to V_IN (V_D); it multiplies the V_IN and generates a V_G which is higher than V_IN. Designing a charge pump is often not trivial, since it needs an oscillator, so in practice designers prefer not to use the N-channel MOSFET. On the contrary, for a P-channel MOSFET, the V_G needs to be lower than VS (tied to V_IN) by V_TH, and no voltage-multiplication circuit is needed as it is always easy to generate a voltage that is lower than the highest voltage in the system (V_IN).

This makes the P-channel MOSFET the favorite choice for power switching applications and therefore, much more widely used than N-channel MOSFET. N-channel MOSFETs are typically used in high-current power-switching situations where its smaller size offers enough benefit to offset its higher design-complexity disadvantage. This article focuses on P-channel MOSFETs and P-channel MOSFET-based load switches.

Most of the MOSFETs used today are of the "planar" type, meaning that the carrier terminals (Drain and Source) and the Gate terminal are on the top of the device, Figure 1. The planar MOSFET is widely used in ICs.

Figure 1: A typical planar MOSFET (Source: Reference 3)
(Click on image to enlarge)

In recent years, MOSFETs which are used primarily in discrete (non-IC) applications are the "trench" type, meaning the MOSFET itself is a "vertical" device which uses both carrier terminals on the top of the MOSFET as Source, and the substrate as the Drain, Figure 2.

Figure 2: A typical trench MOSFET (Source: Reference 3)
(Click on image to enlarge)

As a result, the planar and trench MOSFETs are specifically designed to address different applications. The planar MOSFET is primarily used as a building block within an IC which offers a particular function, while the trench MOSFET is primarily used as a discrete component as part of a board-level circuit. They are fabricated using very different processing technologies which make the planar MOSFET desirable for ICs, while the trench MOSFET is more suitable for discrete components. Additionally, because of the processing-technology differences and their intended usage modes, they are somewhat different in several of the key parameters as well. It is the designer's responsibility to understand the parameter requirements for his application and choose either the IC solution or the discrete solution accordingly.

Planar vs. trench MOSFET: key parameters for load switch
When choosing a MOSFET, the first parameter the designer should usually consider is the on-resistance between Drain and Source while the MOSFET is ON (R_DSON). This is because several of the key performance requirements for power switching, including low voltage drop, high current capacity between V_IN and V_OUT (I_D), and lower power dissipation (hence higher efficiency), are directly related to the R_DSON. Since load switches typically handle more than 1 A peak current, it is desirable to have a R_DSON that is as low as possible, and no higher than 100 mΩ, to achieve the lowest voltage drop and highest efficiency. Since a trench MOSFET typically has higher cell density (up to 50 percent) than a planar MOSFET, its R_DSON tends to be lower by about 15 percent when compared its planar counterpart with similar die size.

However, recent semiconductor processing-technology improvements on planar MOSFETs have greatly reduced, or even eliminated, the RDSON gap between planar and the trench MOSFETs. For example, Micrel's highly successful MIC940xx high-side load switch family uses an advanced "propeller" planar MOSFET technology that significantly increases the effective width of the MOSFET, and as such, decreases the RDSON to the level that is comparable to, or even better than, that of the latest trench MOSFET. Currently, the typical RDSON of a discrete P-channel trench MOSFET is between 45 mΩ and 100 mΩ, which is basically the same range as that of the MIC940xx family products fabricated on the proprietary propeller P-channel planar MOSFET technology.

In addition, there are three key parameters of the MOSFET that are important for the designer from an application's perspective: gate-source voltage (V_GS), gate charge (Q_G), and drain-source voltage (V_DS).

The gate-source voltage (V_GS) is important so the designer can determine how high a gate voltage (V_G) the level-shift circuit needs to provide, in order to fully turn on the MOSFET. For a P-channel trench MOSFET, its threshold voltage (V_TH) is typically between 2 V and 4 V, because of the thicker gate oxide. In comparison, the V_TH of a planar MOSFET can be below 1 V because of the thinner gate oxides. Since the gate voltage (V_G) must be another 1 V to 2 V higher than the source (on top of V_TH) to get to the desirable R_DSON and I_D levels, the V_GS would be at least 3 V or higher (with V_GS equal to the absolute value of the difference between V_G and V_S and > V_TH) for the P-channel trench FET. In contrast, it can be 2 V or less for the P-channel planar FET.

For example, to achieve the minimum level of R_DSON (between 45 mΩ to 100 mΩ typical) specified on its datasheet, a late-generation P-channel trench MOSFET from the leading manufacturers typically requires no less than 4.5 V V_GS to fully turn itself on.. This means that for the P-channel trench MOSFET, a level-shift circuit must be used, since the digital source (such as a microcontroller) which controls the ON and OFF state of the MOSFET almost never has an output with sufficiently high voltage level, especially in battery-powered portable devices. Also, since V_G for a P-channel MOSFET is always lower than the V_S by at least V_TH, for the P-channel trench MOSFET, the input voltage it switches (V_IN or V_S) is almost never below 1.8 V. On the other hand, the P-channel planar MOSFET can switch an input voltage (V_IN or V_S) which is lower than 1.8 V.

The gate charge (Q_G) is needed so the designer can calculate how high a gate current (I_G) the gate-drive circuit needs to provide, in order to rapidly turn on the MOSFET. Because of the much-thicker gate oxides, and hence higher intrinsic capacitances, the trench MOSFET typically has higher QG than the planar MOSFET. The Q_G determines the dynamic responses of the MOSFET, such as its turn -n time (t_ON) as a function of the I_G. The relation between the Q_G and t_ON and I_G is:
Q_G = I_G × t_ON (derived from i = dQ/dt)

For example, a typical P-channel trench MOSFET has a gate charge of 10 nC, and if a rapid turn on time of 1 μs is required, the I_G is calculated as:

I_G = Q_G/t_ON = 10 nC/1 μs = 10 mA

One the other hand, a typical P-channel planar MOSFET, with its much thinner gate oxides, has a gate charge that can be several orders of magnitude lower than its trench cousin. For example, Micrel's MIC940xx family products feature a supply current (I_G included) of less than 1 μA to achieve a 1 μs t_ON.

The drain-source voltage (V_DS) is important because it determines how high an input voltage (V_IN) can be switched. Because of the vertical "trench," there simply is more space in a trench MOSFET to put on thicker gate oxides (from a semiconductor processing standpoint), which means properties such as higher voltage are easier to achieve, without significantly increasing the silicon size. This is the reason why most of trench MOSFETs have a typical V_DS of about 12 V to 20 V.

On the other hand, it is not so easy to thicken the gate oxides on the flat surface of the planar MOSFET. Therefore, the more viable way to accommodate high V_DS requirement is to increase the MOSFET size. This is the reason why most of the planar MOSFETs (in ICs) have a typical V_DS of about 5 V to 12 V.

The P-channel trench and the P-channel planar MOSFETs also differ in die (silicon) size. Again, the reason is the very different manufacturing processes they employ. Some independent studies show that for the similar RDSON range, the die size of a P-channel trench MOSFET can be up to three times as large as that of a complete integrated load switch with P-channel planar MOSFET.

Conclusions
The P-channel planar MOSFET has the advantages on parameters such as Gate-Source voltage (V_GS), gate charge (Q_G), gate current/supply current (I_G/I_SUP), die size, component size, solution size and design complexity. The P-channel trench MOSFET has the advantages on input voltage (V_IN) and load current (I_D). Both have comparable R_DSON. Table 1 outlines the parameter comparisons between the two.

Table 1: Comparison between P-channel trench MOSFET and P-channel planar MOSFET
(Click on image to enlarge)

Due to the differences between the planar MOSFET and trench MOSFET results, they are used differently. The planar MOSFET is typically used as a building block of an IC, such as an integrated load switch, while the trench MOSFET is usually used as a component on a board-level circuit, such as a discrete load switch. The differences between the integrated load switch and discrete load switch will be further investigated in Part 2 of this article series, which has an application perspective, click here.

References

1. "A primer on high-side FET load switches (Part 1 of 2)," Qi Deng, Micrel Semiconductor, Planet Analog, click here
2. "A primer on high-side FET load switches (Part 2 of 2)," Qi Deng, Micrel Semiconductor, Planet Analog, click here
3. "Power MOSFET Basics," Vrej Barkhordarian, International Rectifier, click here

About the author
Qi Deng is a Senior Product Marketing Manager for Mixed-Signal Products at Micrel, Inc., San Jose, CA.