Can a Circuit Be Powered by Both USB and DC Supply Simultaneously?

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Discussion Overview

The discussion revolves around the design of a power system for a circuit that can be powered simultaneously by both USB and DC supply. The focus is on how to manage power sources effectively while ensuring functionality for various applications, including the need for communication via USB and the potential for using DC power alone.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant seeks advice on how to design a circuit that can be powered by both USB and DC supply, mentioning the need for the USB connection for communication.
  • Another participant suggests using PMOS MOSFETs to create a seamless switch to DC power when it is applied, raising the question of whether the switch needs to be glitchless during operation.
  • A later reply clarifies that glitchless switching is not a requirement since the circuit will be enclosed, and power sources will be connected based on the application.
  • There is a discussion about using a specific chip that can automatically select between two input voltages, with some uncertainty expressed about its functionality when switching between 5V DC and 5V USB.
  • One participant advises against using the automatic mode of the chip, suggesting instead to implement logic to control the enable signals based on the presence of the DC connection.

Areas of Agreement / Disagreement

Participants generally agree on the need to switch power sources but have differing views on the necessity of glitchless operation and the best approach to manage the power selection. The discussion remains unresolved regarding the optimal implementation of the power management strategy.

Contextual Notes

There are limitations regarding the assumptions about the operational environment of the circuit, such as the conditions under which power sources will be connected and the specific requirements for switching behavior.

xortan
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Hello everyone,

I am designing the power system of a new circuit. It will receive an analog video signal and then be able to split that video signal to multiple outputs. I would like to have multiple ways to power the circuit depending on the application. One way to power the device will be by apply a DC voltage to the circuit and the other way will be by using the 5V from the USB.

The USB needs to be connected at all times for the communication. We may use this circuit in other places as just a receiver/splitter and the USB will not be required and it will be powered from the DC voltage. We may have an application where we don't want the DC voltage and will power just from USB. For the current application we will have the USB and adapter connected at the same time.

I am having problems trying to find a chip where I can have an adapter plugged in and a USB at the same time. If someone could please point me in the right direction of how to accomplish this it would be greatly appreciated. I am fairly new to electronics design (only graduated a year ago).

Thanks!
 
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You want to switch to DC power (and not draw from USB +5) when there is voltage applied to the DC input jack.
Does it need to work dynamically without missing a beat? For example, if I am watching video and plug in the DC supply, does it have to switch glitchlessly?
That makes it a bit harder. (need to be careful)

I'd use pmos mosfets to implement a SPDT switch that seamlessly switches to DC when DC is plugged in.

Can you use this?
http://www.micrel.com/index.php/pro.../dual-high-side-pwr-sw/article/5-mic1344.html
 
Yes, I would like to switch to DC power when there is voltage applied to the jack and not draw from the USB +5V.

I don't think it needs to work dynamically like that. The circuit will always be placed inside an enclosure and the USB cable or DC supply will already be wired up depending on the application. There is no situation where a customer will be viewing video and have to open the enclosure and supply power to the device. If it can be switched without a glitch I think that is an added benefit but not a requirement.

That chip looks interesting. It states it will automatically select between the higher of the two input voltages. I don't know how I can make that work when switching between a 5V DC supply and 5V from the USB.
 
xortan said:
That chip looks interesting. It states it will automatically select between the higher of the two input voltages. I don't know how I can make that work when switching between a 5V DC supply and 5V from the USB.

Don't use automatic mode. Use some logic to drive ENA and ENB based on whether DC is connected.
 
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Okay, I understand. Thanks so much for the help! I really appreciate it. :)
 

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