Let your latest logic run and everything works as designed. Then turn off the power and the circuit is still working. How is this going?
In today's world, plug-and-play devices are used and power is emphasized, so there are usually multiple voltage sources. In order to save energy, we may save power by turning off some parts of the circuit. This is especially true for battery-powered devices, which can help you save as much power as possible.
If the input or output of the device has a clamping diode and the input or output is supplied with a voltage, the component can power up Vcc through the diode. The input or output voltage is transferred to the Vcc pin and the Vcc voltage is equal to the input or output voltage of -1 VT. This is called “reverse power supply. Since the voltage is now at the Vcc pin, it can be transferred to and powered by other devices in the circuit.
How do I know if the device has a diode that can supply power to the device in the reverse direction?
All standard logic devices have ESD protection diodes on the input or output. These diodes are not only available for ESD protection, but also provide voltage clamping to prevent input or output from exceeding Vcc or below ground. These diodes come in many different configurations depending on which product family they belong to. All devices have ground diodes on the input and output pins. Some devices' protection diodes are configured to prevent the device from being powered back.
Any device with Ioff characteristics will set the input and output to high impedance when Vcc is 0V and will not be reverse powered. Here are some configurations and specifications that will tell you if there are diodes and if the device will be powered back.
The best way to determine the diode is to look at the absolute maximum specifications Iik and Iok. If the current limit is (+/-), then there is a diode connected to Vcc and ground. If there is only (–) current, then there is only one diode connected to ground. Take a look at these examples:
(Note: Even without the clamp diodes shown, there are actually ESD circuits where the diodes are back-to-back, so there is no reverse power supply.)
1) No VCC clamp at input or output
This device cannot be powered backwards
Under the absolute maximum specification
or
Under the absolute maximum specification
Under the recommendation
or
2) There is no Vcc clamp at the input, but there is a Vcc clamp at the output.
The device can be powered backwards from the output.
Under the absolute maximum specification
Under the recommendation
3) Both input and output have Vcc clamp
The device can be powered backwards from the input or output
Under the absolute maximum specification
Under the recommendation
Another way to get reverse power to the device is to turn off the part by turning on ground. If any input or output is connected to ground, the ground can pass through the diode. It is definitely not recommended to turn off the power by opening the ground connection.
Therefore, these things should be remembered the next time you consider the shutdown part of the energy-saving circuit. Do you have any other tips?
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