The load switch is a space-saving integrated power switch. These switches can be used to "disconnect" power-hungry subsystems (when in standby mode) or for point-of-load control to facilitate power sequencing. After smartphones became popular, people created load switches; because mobile phones added more features, they needed higher-density circuit boards, leaving less space. An integrated load switch solves this problem: returning board space to the designer while integrating more functions.
Figure 1: Common discrete implementation of power switches
What are the advantages of integrated load switches compared to discrete circuits?
As shown in Figure 1, a typical discrete solution consists of a P-channel Metal Oxide Semiconductor Field Effect Transistor (MOSFET), an N-channel MOSFET, and a pull-up resistor. Although this is a proven and effective solution for switching power rails, it has a larger footprint. Now we can get a more compact solution, such as the load switch of Texas Instruments' TPS22915, which has a footprint of less than 1mm2! Using the dual-channel TPS22968 can significantly save space. Figure 2 shows a customer's implementation compared to this TI solution. The TPS22968 enables the customer to reduce their footprint by more than 80% while integrating more features such as controlled slew rate and fast output discharge.
Figure 2: Dimensional comparison of dual-channel discrete switch and integrated load switch (TPS22968)
Why do I need a controlled slew rate?
All TI load switches have a controlled slew rate to reduce inrush current, which is also referred to as "soft start" function. By slowly increasing the charging rate of its output capacitor, the load switch can prevent the supply voltage from "dropping" due to rapid charging of the load capacitor. For more information on reducing inrush current, read the application note: Managing Inrush Current.
Figure 3: Simplified functional block diagram of the TPS22908 integrated load switch.
What is a quick output discharge?
The fast output discharge feature of most load switches ensures that a load that has been disconnected or disabled will not be floated. As shown in Figure 3 above, a low "on" input can turn off the channel element through the inverter and open the discharge field effect transistor (FET). This creates a path from VOUT to GND, ensuring that the load can quickly return to a known 0V “off†state.
Integrated load switch options
The integrated load switch is a space-saving power switch solution that integrates more functions such as fast output discharge, voltage monitoring, and reverse current protection. With a variety of package options (from WCSP to QFN and SOT-23), Texas Instruments' load switch portfolio has a solution for each power supply design.
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