What is a Load Switch?


Load Switches are space saving, integrated power switches.  These switches can be used for “disconnecting” a power hungry sub-system when in standby, or for point of load control to facilitate power sequencing.  Load Switches were created after smartphones gained popularity; as phones added more features they required higher density circuit boards and space became scarce.  The integrated load switch solves this problem: giving board space back to designers, while integrating additional functionality.

Figure 1: Common discrete implementation of a power switch

What are the benefits of an integrated load switch compared to a discrete circuit?

Shown in Figure 1, the typical discrete solution consists of a P-Channel MOSFET, N-Channel MOSFET, and pull-up resistor.   While this is a proven and effective solution for switching power rails, it carries a large footprint.  More compact solutions are available such as Texas Instrument’s load switches like the TPS22915 with a footprint of under 1mm2!  The space savings become even more dramatic with the dual-channel TPS22968.  Figure 2 shows a comparison between one customer’s implementation compared to the TI solution.  The TPS22968 enables this customer to reduce their footprint by over 80%, while integrating additional functionality such as controlled slew rate and quick output discharge.

Figure 2: Size comparison of dual channel discrete vs. integrated load switch (TPS22968)

Why do I need a controlled slew rate?

All TI load switches feature a controlled slew rate to mitigate inrush current, also called “soft start” functionality.  By slowly ramping up the charging of its output capacitance, load switches are able to prevent supply voltage “droop”, caused by quickly charging a load capacitor.  For more information about mitigating inrush current, read the app note: “Managing Inrush Current.”

 

Figure 3: Simplified Functional Block Diagram of TPS22908 Integrated Load Switch

What is Quick Output Discharge?

Quick output discharge, present in most load switches, ensures that a disconnected or disabled load will not be left floating.  As shown above in Figure 3, a low ON input turns off the pass element and turns on the discharge FET via the inverter.  This creates a path from VOUT to GND, ensuring that the load is quickly returned to a known 0V “off” state.

Options for integrated load switches

Integrated load switches are space efficient solutions to power switching, capable of integrating additional functionality such as Quick Output Discharge, Voltage Monitoring and Reverse Current Protection.  Offered in a variety of package options from WCSP to QFN and SOT-23,Texas Instruments’ load switch portfolio has a solution for every power design. 

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