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Common applications of logic
Learn how to overcome common challenges by utilizing logic devices to aid in optimizing system design. Some design challenges are prevalent across many systems. In this section, these common challenges are overcome utilizing logic devices to aid in optimizing system design. These videos include an understanding of the problem being solved, and a description of one or more solutions to these issues.
Combining Power Good Signals
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Hello. And welcome to The Logic Minute. In this video, we will look at how to use logic to get power good signals from the power supply to where they need to go. Microcontrollers, processors, and FPGAs typically require very well regulated power inputs. Because of this, they should be held in reset until the system power is properly regulated.
The power good output of a DC/DC converter is set up specifically to handle this task. It provides a weak open drain driver to hold the reset pin low until the output is regulated. Because the output is typically open drain, a pull-up resistor is required to bring the line high once regulation is achieved.
Then, the system can start working as usual. If there is more than one supply rail that needs to be regulated, the open drain power good outputs can be tied together directly to produce a wired AND configuration. The reset pin will remain low until all power good pins go into a high impedance mode. There is a hidden problem in this configuration, however.
The power good outputs usually have very limited current drive, less than 1 milliamp in many cases, which means the pull-up resistors must be chosen as very large values. A large value of pull-up resistor will make the line very sensitive to increases in load capacitance.
But what can increase the load capacitance? It's very common in system design to physically separate the power supply from the controllers due to power supply noise considerations. This creates long traces and increased capacitance. It's also very common to need to hold multiple devices in reset. Each device input adds capacitance to the line. In most real systems, it's a combination of these two issues that results in the need for a solution to this problem.
If the converters are close enough together in the power supply, their outputs can be combined as previously discussed, then put through a buffer to increase the drive strength. If the converters are separated in the supply, their outputs can be combined by an AND gate externally to reduce the loading on each power good output, and then drive the high capacitance reset line into the system.
This is the most common configuration for power good lines and systems today. Please explore the other videos and training material on our site. Thank you for watching. And if you have any questions, come over to the E2E forums to ask us directly.
