“Power is the lifeblood of technology that drives our modern world, and we increasingly rely on high-tech products to help us in our busy work and life outside of work. Managing and controlling power is an important topic in high-tech, low-power applications. Whether developing complex multi-rail power systems for larger devices or utilizing the last fraction of a battery-powered device, power management is a top priority for designers.
Bryson Barney, Applications Engineer, ON semiconductor
Power is the lifeblood of technology that drives our modern world, and we increasingly rely on high-tech products to help us in our busy work and life outside of work. Managing and controlling power is an important topic in high-tech, low-power applications. Whether developing complex multi-rail power systems for larger devices or utilizing the last fraction of a battery-powered device, power management is a top priority for designers.
In systems large and small, load switches play an important role in power management and load protection. There are several factors to consider when selecting a load switch for a specific application, and new devices with improved performance and functionality are now available.
The main function of any load switch is to connect/disconnect the power source and the load. A discrete solution using only a simple pass MOSFET can be used to achieve this function; however, fully integrated load switch devices are typically smaller than discrete solutions, reducing device count and providing greater functionality, including and detection and protection of overcurrent fault conditions. Reverse current protection is sometimes included so that any current trying to flow from the load to the source is blocked. This is especially useful for USB-C, the latest power delivery standard used by many applications.
Figure 1: Typical Integrated Load Switch Block Diagram
All integrated load switches include at least four pins – VINVOUTGND, and EN(Enable), although other pins are usually added to support more functions and provide protection for system outputs, as shown in Figure 1.
In addition to providing simple switch control and protection, the load switch can control the speed at which the load is turned on by controlling Vout. By controlling the charging of the FET gate, the inrush current is controlled during the rise time of Vout. This soft-start protects the load from current spikes caused by the uncontrolled connection of the load to the power supply, especially if the load is not purely resistive.
Some load switches also include a bleed resistor to allow rapid discharge of any energy stored in the load, eliminating floating nodes on the load supply pins when the load switch is turned off.
Load Switch System Usage and Configuration
One of the most common applications of load switches is to manage separate power domains in a system. This control is especially useful in battery-operated equipment, where preventing any unnecessary energy consumption is critical to obtaining maximum battery runtime.
Figure 2: Load switches manage power delivery for multiple load domains
Load switches can be used to control power in various parts of the system that are divided into logical domains, as shown in Figure 2. In practical use, parts that only need a short time, such as sensors or transmit/receive circuits, are powered on only for the short time they need to be activated.
Additionally, for systems with multiple power supplies 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, from mains or a backup battery.
Figure 3: Power multiplexing allows a single load to utilize multiple power supplies
Complex systems often require multiple power rails, which in many cases must work in a certain order for the system to function properly. For example, it may be necessary to fully start the core processor before enabling the radio transmitter to avoid confusing or incomprehensible transmissions.
Figure 4: Load Switching Can Facilitate Self-Driven Power Sequences
While an external microcontroller can be used to implement the power-up sequence to enable the load switch with the correct timing, the load switch can also provide an independent sequence if the “power good” output is used, as shown in Figure 4. Multiple load switches can be “daisy chained” by linking the Power Good pin of one load switch to the EN pin of the next load switch. If a larger delay between phases is required, a capacitor can be added to the ground control line to increase the delay.
Choosing the Right Load Switch for Your Application
There are many types of load switches, giving designers a wide range of options when choosing a device for a specific application. While initially choosing may seem difficult, evaluating options and finding the best device is relatively straightforward by considering six main parameters.
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 R of the device under considerationON. The lower switch resistance, operates from VINto VOUTThe lower the voltage drop, which reduces the power dissipation and heat dissipation of the load switch.
In terms of the device’s maximum allowable load current and input voltage (VIN) range, it is necessary to specify the correct ratings for the application. While making sure you have a sufficient current rating for the application, care should be taken not to over-rate it, as larger FETs have higher gate capacitance and require more energy to turn on. Additionally, higher rated devices can also be larger and more expensive.
Quiescent current (the energy used when the load switch is on) should be as low as possible, as is any leakage current (the current from the source to the load when the load switch MOSFET is off).
Finally, for a specific application, the speed of response is critical to the time it takes to switch as well as any failsafe operation. This response time is affected by the size of the MOSFET, the larger the device size, the more charge required and therefore the slower the operation. Likewise, overdesigning an oversized FET can result in worse transient performance.
ecoSWITCH™ – a comprehensive range of load switches
ON Semiconductor’s ecoSWITCHTMFamily of load management devices provide designers with best-in-class on-resistance RON. The family includes more than 20 different products and continues to expand with the introduction of new load switches from ON Semiconductor.
The NCP45560 is just one device in the ecoSWITCH family. Housed in a space-saving 3mm x 3 mm DFN12 package, the device can switch up to 24A of current efficiently and continuously, with an on-resistance as low as 4.1mΩ. The NCP45560 is ideal for power management and hot-swap applications requiring a small footprint solution, combining many protection features while providing significant space and cost savings over discrete solutions.
As power supply requirements become more complex and the need for small portable battery powered devices to operate as efficiently as possible, load switches are gaining popularity as it enables designers to implement energy efficient, sophisticated power solutions, Even in the smallest devices.