MOSFET
Optimizing MOSFET Selection in Handset and Portable DC Load Applications
Trench 4 process targeting low voltage P-Channel MOSFET
There are many applications where load switches are used for sequencing, protection, power management and distribution, etc. and each of these has their own characteristics. As portable electronics such as cell phones, portable media players, personal navigation devices, etc. continue to shrink in form factor, there is a continued need for smaller electronic components to match this trend. At the same time, the expectation of component performance remains the same. This trend is no different for power applications using MOSFET devices.
By Ryan Zahn, Low Voltage MOSFET Product Line Manager – Handset / Portable Products and Isauro Amaro, Low Voltage MOSFET Applications Manager at ON Semiconductor.
In those areas where a switching regulator is required, there are several topologies based on cost, from the very basic P-ch MOSFET paired with a Schottky diode to the use of two N-ch MOSFETS to provide the highest efficiency possible. In those circuits, the industry has developed a figure-of-merit (FOM) to compare one device from another and it is defined as FOM = Qg X Rds(on). In the case of a load switch application where switching losses do not play a significant role, the main point of comparison has been the Rds(on) at a certain bias voltage (Vgs), but that does not really tell you the whole picture. We believe a better figure of merit to describe a load switch is the Rds(on) the part can provide for a given package or footprint area, hence FOMp = Rds(on) x Footprint. As a result, the part with the lowest Rds(on) and the smallest footprint , will be able to provide the best power density for a given DC load in portable devices, where real estate and battery life are key requirements.
Figure 2: A N-channel High Side Load Switch Power applications within handset and portable products often use MOSFETs for a variety of uses. Some common sockets and the types of MOSFETs used can be seen in Figure 1. The most prevalent socket includes a DC load switch for power management of different feature sets, battery charge / discharge, and general power architecture management. Figure 1: Typical System Architecture

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