Sitting in front of a TV is easy. Changing the channel is easy. Recording four shows at once while watching one show on your TV and streaming another show to your tablet is excessive – but also easy! It’s all thanks to the power of set-top boxes (STBs), such as the one shown in Figure 1.
Figure 1: A typical STB
STBs take in a cable/satellite signal; translate that signal into video; and then transmit that video to a TV, hard drive or wireless device. Designing these systems can be complicated but designing the power distribution can be easy, especially when you use load switches and eFuses.
Why bother turning different loads on and off?
STB designers usually follow standby power requirements so they can improve the system’s power efficiency. These requirements limit the amount of power that the STB can draw when it is inactive, so different subsystems need to be off in order to draw a minimal amount of power. Some regions even have specific power requirements, such as such as Energy Star. Figure 2 shows the common STB subsystems that can be controlled to improve standby power.
Figure 2: Load switch and eFuse applications in STBs
Now let’s take a look at some of the subsystems you can switch on and off.
Load switches can control power to each of these subsystems, and both the TPS22918 and TPS22975 can be used depending on the current load. Both are plastic devices that come with an adjustable output rise time suitable for different capacitive loads. The TPS22918 can support loads up to 2A and the TPS22975 can support loads up to 6A.
What about using switches for additional features?
Aside from power savings, several other features require a switch:
All of these applications are modeled in the block diagram in Figure 3.
Figure 3: Recommended load switches and eFuses in STBs
Where can you get started?
The Power Switching Reference Design for Set Top Box shows all of the different load switches and eFuses used for each subsystem. With the added DC/DCs, the design helps create a complete solution for STB power delivery.
Figure 4: Power Switching Reference Design for Set Top Box
With the trend in STB moving to smaller form factors, it’s easy to see why designers are looking for more integration in their systems. So keep your STB power design small and easy, and use load switches and eFuses to accomplish your varying power switching needs!
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