Power is the lifeblood of technology in our modern world, and we increasingly rely on high-tech products to help us in our busy work and leisure time. Managing and controlling power is a crucial theme in high-tech, low-power applications. Whether developing complex multi-rail power systems for larger devices or utilizing the last bit of power from battery-powered devices, power management is a top priority for designers.
Load switches play a vital role in power management and load protection in systems of all sizes. Several factors need to be considered when selecting a load switch for a specific application; in addition, new devices with improved performance and functionality are now available.
The primary function of any load switch is to connect/disconnect the power supply and the load. This function can be achieved using a discrete solution with only a single-path MOSFET; however, fully integrated load switching devices are typically smaller than discrete solutions, reducing the number of components and offering more robust functionality, including detection and protection against over-temperature, undervoltage, and overcurrent fault conditions. Sometimes, reverse current protection is also included, blocking any current attempting to flow from the load to the source. This is particularly useful for the latest power supply standard, USB-C, which is used in many applications.
Figure 1: Typical integrated load switch block diagram
All integrated load switches include at least four pins—VIN, Vout, GND, and EN (Enable), although additional pins are often added to support more functions and provide protection for the system output, as shown in Figure 1.
In addition to providing simple switching control and protection, the load switch can also control the speed at which the load turns on by controlling Vout. Inrush current is controlled during the rise time of Vout by controlling the charging of the FET gate. This soft-start protection shields the load from current spikes caused by uncontrolled connections between the load and the power supply, especially when the load is not purely resistive.
Some load switches also include a leakage resistor to support the rapid discharge of any energy stored in the load, eliminating floating nodes on the load power pins when the load switch is turned off.
Load switch system usage and configuration
One of the most common applications of load switches is managing independent power domains within a system. This control is particularly useful in battery-powered devices, as preventing any unnecessary energy consumption is crucial for achieving the longest possible battery runtime.
Figure 2: Load switches can manage power transmission across multiple load domains.
Load switches can be used to control the power of various parts of a system that are divided into logic domains, as shown in Figure 2. In practical applications, parts that only require power for short periods, such as sensors or transmitting/receiving circuits, are only powered on for the short time they need to be activated.
Furthermore, for systems with multiple power sources for a single load, as shown in Figure 3, a load switch can be used. In such a system, the power delivery to the load can be controlled, for example, by drawing power from the main power supply or a backup battery.
Figure 3: Power multiplexing supports a single load using multiple power supplies.
Complex systems often require multiple power rails, and in many cases, these rails must operate in a specific sequence for the system to function correctly. For example, it may be necessary to fully power on the core processor before activating the radio transmitter to avoid confusing or incomprehensible transmissions.
Figure 4: Load switching can benefit self-driven power sequences
While an external microcontroller can be used to implement the power-on sequence to enable the load switches in the correct timing, the load switches can also provide independent sequences using the "powergood" output, as shown in Figure 4. Multiple load switches can be daisy-chained by linking the PowerGood pin of one load switch to the EN pin of the next load switch. If a greater delay between phases is required, a capacitor can be added to the ground control line to increase the delay.
Select the appropriate load switch for the application
There are many types of load switches available, giving designers a wide range of choices when selecting devices for specific applications. While the initial selection may seem difficult, it becomes relatively straightforward by considering six key parameters, evaluating options, and finding the optimal device.
Energy efficiency is important and is often a key reason for using load switches in applications. Designers should pay close attention to the on-resistance RON of the devices under consideration. The lower the switching resistance, the lower the voltage drop from VIN to VOUT during operation, thereby reducing the power consumption and heat dissipation of the load switch.
Regarding the maximum permissible load current and input voltage (VIN) range of the device, the correct ratings need to be specified for the application. While ensuring sufficient current ratings for the application, care should be taken not to over-rated ratings, as larger FETs have higher gate capacitances and require more energy to turn on. Furthermore, higher-rated devices may also be larger and more expensive.
The quiescent current (the energy used when the load switch is turned on) should be as low as possible, just like any leakage current (the current from the power supply to the load when the load switch MOSFET is turned off).
Finally, for specific applications, response time is critical to both the time required for switching and the time required for any fault protection operations. This response time is affected by the MOSFET size; larger devices require more charge and therefore operate more slowly. Similarly, over-designing an excessively large FET can lead to worse transient performance.
ecoSWITCH - A comprehensive range of load switches
ON Semiconductor's ecoSWITCH™ series of load management devices provides designers with best-in-class on-resistance (RON). The series includes over 20 different products and continues to grow as ON Semiconductor introduces new load switches.
The NCP45560 is just one device in the ecoSWITCH family. This space-saving 3mm x 3mm DFN12 package enables continuous, energy-efficient switching of up to 24A of current with an on-resistance as low as 4.1mΩ. The NCP45560 is ideal for power management and hot-swappable applications requiring a small footprint, offering numerous protection features while significantly saving space and cost compared to discrete solutions.
Summarize
As power demands become increasingly complex, there is a growing need for small, portable, battery-powered devices to operate as efficiently as possible. Load switches are gaining popularity because they enable designers to implement highly efficient and sophisticated power solutions, even in the smallest devices.
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