MOSFET-based load switches, whether as integrated circuits or built with discrete components, continue to be popular choices for many engineers for applications with power-switching needs, such as battery-powered portable devices, including mobile handsets and notebook PCs. Depending upon factors such as level of design complexity, parameters of the components, and solution footprint size, among other factors, one choice typically has some advantages over the other. This article takes an easy-to-understand and non-mathematical approach to explain the operation differences between an integrated load switch and a discrete load switch, from an application perspective.
The article is a natural extension of the Part 1, (click here) so readers should first study that part to understand the fundamental operational differences between a P-channel trench MOSFET which is usually used as a component for the discrete load switch, and a planar MOSFET which is more suitable as a building block for the integrated load switch.
Planar vs. trench MOSFET: IC vs. discrete
There are reasons why the planar MOSFET is used in ICs and the trench MOSFET is used in discrete components. The key reason is that, as discussed in Part 1, the trench MOSFET and the planar MOSFET are manufactured using very different silicon-fabrication technologies.
A typical trench MOSFET may require less than ten mask steps in silicon fabrication, which is relative simple to make from semiconductor-processing standpoint. This means that the die size (physical size of the MOSFET) can be made rather large to accommodate requirements such as higher voltage and higher current, while still maintaining cost-effectiveness. These properties make the trench MOSFET a very good choice for discrete-power MOSFET.
However, if the control circuit (gate drive and level shift) required by load switch was to be added on the same piece of semiconductor (an integrated circuit), the silicon fabrication process becomes complicated, because of the trench MOSFET's "vertical" nature. For an IC, both the trench MOSFET and the control circuit would be fabricated on the same silicon substrate, so certain "isolation" has to be provided on the substrate to separate the two.
This is not trivial from processing technology standpoint, and the silicon size would have to be increased to accommodate the isolation. Also, because of the additional IC-level components and their routing needs, the number of mask layers would increase significantly, sometimes to more than twenty. Both the silicon size and the increased number of mask layers would add more cost, and make a trench MOSFET-based IC a less attractive proposition. As such, the trench MOSFET is seldom seen on an IC, but typically offered as a discrete component.
The planar MOSFET, on the other hand, is a good building block for integrated circuits such as a load switches. Since the planar power MOSFET does not "cut" into the substrate, the same substrate can be shared with the control circuit without isolation that is otherwise required for the trench MOSFET. This makes the silicon fabrication process a lot easier, and even if a load switch may require twenty or more mask steps to accommodate the higher complexity introduced by the additional control circuit. The die size can still be kept relatively small, and therefore an economical cost.
With this understanding, we are ready to discuss and compare the discrete load switch and the integrated load switch from an application perspective.
Discrete load switch with P-channel trench MOSFET
A typical, simplified P-channel MOSFET based load switch is illustrated in Figure 3. The main building blocks are the P-channel trench MOSFET itself, and a gate drive and level-shift circuit. Although the gate drive and level shift circuit in the figure is simplified and might not represent the true implementation that can be found in a typical load-switch solution, it is conceptually correct and provides the fundamentals for our discussion.
The gate-drive and level-shift circuit, for a "small" N-channel MOSFET (Q2) is selected by these factors: it needs to operate on a logic level that is compatible to the microcontroller, and it also needs to sink enough gate current (IG) from the gate of the main P-channel trench MOSFET (Q1). The values of the divider resistors, R1 and R2, are selected in a way so that when the Q2 is ON, the VG should be "level shifted" to a value that can fully turn-on Q1. The pull-up resistor, R1, is required to have Q1 fully turned off when Q2 is OFF. A capacitor, C1, can be used to further fine-tune the timing responses, as well as provide a slew-rate control (or soft-start) feature.
Figure 3: Typical, simplified discrete P-channel trench MOSFET-based load switch
(Click on image to enlarge)
Integrated load switch with P-channel planar MOSFET
The integrated load switch of Figure 4, from schematic standpoint and therefore basic-function standpoint, is essentially the same as the discrete load switch. The main switch is a P-channel planar MOSFET, and it also includes a gate-drive and level-shift circuit with a schematics which is identical to that of the discrete-MOSFET-based load switch.
However, all circuits, including the P-channel planar MOSFET and the gate-drive and level-shift circuit, are "integrated" on a single piece of semiconductor material.
Figure 4: Typical, simplified integrated load switch with P-channel MOSFET
(Click on image to enlarge)
Discrete load switch vs. integrated load Switch: application comparison
As we have discussed, from a semiconductor-processing standpoint, the discrete load switch and the integrated load switch are quite different. From an application perspective, the following parameters are what the designers should often consider.
- RDSON
- Gate-Source Voltage (VGS)
- Gate Charge (QG)
- Gate Current (IG for discrete) and Supply Current (ISUP for integrated)
- Input Voltage (VIN) Range
- Load Current (ID)
- Component Size
- Solution Size
- Design Complexity
The RDSON is important from an application's perspective, as this parameter directly impacts voltage drop between VIN and VOUT, load current (ID), solution efficiency, and thermal capability requirements for packaging. For both the discrete load switch with P-channel trench MOSFET and the integrated load switch with P-channel planar MOSFET, the RDSON is essentially in the same range, between 45 mΩ and 100 mΩ.
The Gate-Source voltage VGS is important, as it determines how high a voltage the level-shift circuit needs to provide. To achieve a reasonable RDSON, for the discrete load switch with P-channel trench MOSFET, the VGS must be greater than 3 V, and sometimes even 4.5 V. For the integrated load switch with P-channel planar MOSFET, VGS can be below 1.8 V. As a additional benefit, the integrated load switch with P-channel planar MOSFET can pass much lower VIN to the VOUT than the discrete load switch with P-channel trench MOSFET can, by the same margin. This is very desirable for battery-powered portable systems, since most of the power rails are below than 3 V. Here, the advantage is clearly with the integrated load switch.
The Gate charger QG is important, as it determines how high a gate current, IG, the gate-drive circuit needs to provide to turn on the MOSFET within a certain amount of time. To achieve a fast turn-on time (tON), such as 1 μs, for the discrete load switch with the P-channel trench MOSFET, the IG must be greater than 10 mA. For battery-powered portable systems, this IG is too high for the microcontroller to supply directly. For the integrated load switch with the P-channel trench MOSFET, on the other hand, the entire supply current (ISUP) to the IC with the IG included is typically no greater than 1 μA. The advantage is again clearly with the integrated load switch.
The discrete load switch with P-channel trench MOSFET has the advantages of higher input voltage (VIN) and higher load current (ID), which is primarily a result of its thicker gate oxides and cell density. The most popular P-channel trench MOSFET has a high VIN, usually between 12 and 20 V, and an ID of up to 5 A. However, these features do not matter much in low-voltage, low-current, battery-powered portable applications such as a mobile handset, which usually does not require more than 5 to 7 V for VIN and 2 to 3A for ID, which are what a typical, integrated load switch with P-channel planar MOSFET can provide.
The discrete load switch and integrated load switch differ in die (silicon) size, and for that final component (packaged device) size as well. Again, the reason is the rather different manufacturing processes they employ. Independent studies indicate that for a similar RDSON range, the die size and the component size of a P-channel trench MOSFET can both be up to three times as large as that of an entire integrated load switch using P-channel planar MOSFET.
For the total solution, including the external gate-drive and level-shift circuit, the footprint size of a discrete load switch with P-channel trench MOSFET can be up to four times as large as that of an integrated load switch. For battery-powered portable systems which require small footprint solutions, the integrated load switch is a much better choice. The advantage is clearly, one more time, with the integrated load switch.
Finally, a designer choosing to use an integrated load switch for power-switching applications does not need to choose the main P-channel MOSFET and the components that make up the gate-drive level-shift circuit individually. This reduces the design time since a large number of device parameters no longer need to be analyzed, and it also helps avoid lots of potential design mistakes.
Conclusions
The integrated load switch with P-channel planar MOSFET has the advantages in Gate-Source voltage (VGS), gate charge (QG), gate current/supply current (IG/ISUP), die size, component size, solution size, and design complexity. The discrete load switch with P-channel trench MOSFET has the advantages in input voltage (VIN) and load current (ID). Both have comparable RDSON. Table 2 outlines the parameter comparisons between the two.
Table 2: Comparison between discrete load switch and integrated load switch
(Click on image to enlarge)
From an application perspective, the integrated load switch is an excellent choice for battery-powered portable systems which require low-voltage operation, low-current consumption, fast-timing responses, small footprint, and ease of use. As such, they can typically be found in mobile handsets, personal music and multimedia players, portable game consoles, GPS modules, ultramobile personal computers, and laptop personal computers. The discrete load switch is suitable for systems which require high-voltage and high-current operation such as power supplies, desktop computers, servers, and communications equipment. Of course, there isn't a clear-cut choice on which type of load switch should go into which application, and the designer will need to make the decision on what is needed and application priorities.
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.
