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Hello, everyone. This presentation is about getting a deeper understanding of load switch features and how to easily design them into your system.

A load switch is a device that enables and disables DC power rails to a load, and offered different power protection features to fit your system needs. They're typically placed after your DC-to-DC converter and on the same board as the load.

Load switches are great solutions for extending battery life, saving space, simplifying and sequencing and controlling inrush current. The different features that are offered are slew rate control, power good, thermal shutdown, short-circuit protection, reverse-current protection, current limit, and quick output discharge.

You can see here on the right that you're able to reduce your BOM count by replacing multiple components with just one integrated solution. We'll now delve a bit deeper into each feature.

Inrush current is caused by having output capacitance that requires charging when the power rail first turns on. This can spike to a significant amount of current, but can be controlled by limiting your rise time. Load switches can reduce and inrush current by controlling the slew rate of their output.

An example of how this works is let's say your output capacitance is 100 microfarads. Your input voltage is 5 volts. But you want a max of 1 amp of inrush current. With the inrush current calculation, you can find your required rise time to prevent your current from exceeding 1 amp.

Power Good, or the PG pin, of some of our devices allow you to power sequence easier and allow logic control, because the pin indicates when the output voltage has risen to 90% of its final value-- and in some cases, well after it has reached 90%.

As you can see in the top right image, the PG pin is being used to prevent the second device from turning on until the output of the first device is stable. We'll talk more about power sequencing and logic control later on in this presentation.

Thermal shutdown protects the load switch junction temperature from reaching levels that could damage it. It has auto retry once the junction temperature falls below the falling threshold, as you can see in the table pulled from the TPS22950 for thermal shutdown. You can also see this in the scope shot of the oscillations that occur when a device enters thermal shutdown while having an auto retry feature.

Short-circuit protection and current limit are two different features that often get confused for one another. The difference between them lies in the trigger method. Short-circuit protection triggers when V IN minus V OUT is greater than a specified value-- usually V Short Circuit, and then enters a regulation state.

Current limit triggers from an integrated sense circuit that forces the device into a regulation state when the specified current limit is exceeded. Devices with current limit will have short-circuit protection, but devices with short-circuit protection may not have current limit.

There are two different types of current limiting with our load switches-- latch off current limit and regular current limit. Latch off means that once the current limit engages, it holds for a short period of time before it turns off. And the Enable pin must be toggled to turn it back on.

Regular current limit holds the current limit until the fault is either removed or thermal shutdown is hit. It will then retry once falling below the thermal shutdown falling threshold.

I LIMPEAK is the overshoot of current that occurs before the current limit is engaged.

We have two different types of devices with current limit-- ones with fixed current limit and ones with an adjustable current limit through resistance to ground on an island pin, as you can see on the right-hand side.

Load switches may have reverse current protection, which will require a voltage drop from V OUT out to V IN of some specified value for the reverse current protection to engage. Because of this, it will allow some reverse current to flow. Typically, the larger the R ON of the device, the less reverse current that is allowed.

You can see in the example on the right how this works. There are two different types of reverse current protection for devices, some devices that enable it only when the ON pin is low, and devices that will enable it regardless of the ON pin condition.

Quick output discharge is available for some of our devices, which ties the output of the device to ground when the ON pin is below its falling threshold voltage. This helps ensure that the output is in a known state, and that downstream devices are off or not floating.

The cons of QOD come into play for power muxing applications and charging super caps, because you will always have a leakage path on the devices that are turned off.

The way you calculate your expected fall time for a device with QOD is by R load and parallel with R QOD times load capacitance times approximately 2.

Power sequencing is important for some applications that require specific rails to be enabled before or after other rails are turned on. The PG pin or the QOD pin can help with sequencing.

We briefly explained earlier how the PG pin can prevent another device from turning on until that device's output rises first. But you can also use the QOD of device 1 to ensure that device 2's rail is held at 0 volts until device 1's Enable pin has risen above its rising threshold. You can see this in the figure on the right.

Some systems may need logic control to operate. For example, in a power multiplexing application, they may need break before make logic to ensure there isn't a path from one supply to the other through our device. We can use the PG pin to ensure one device is enabled at a time.

This diagram shows two TPS22953 load switches in a power muxing application with break-before-make logic. When SNS voltage goes beyond the V IH SNS level, and Enable is above its V IH level, the PG pin inserts high. The inverter holds the Enable of the second device low. When the second supply rises, and the first device falls, the PG pin asserts low, and the Enable on the second device is able to go high.

Check out our load switch portfolio on ti.com/loadswitch. While you're there, feel free to take a look at our 11 ways to protect your power path, and Basics of Load Switches application reports.

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