You have your new logic circuit running and everything works as designed. Then you turn off the power and the circuit continues to work. What is going on?
In today’s world, with all the plug-and-play devices and emphasis on power conservation, there are usually multiple voltage sources. To save energy, we may turn off certain parts of a circuit to conserve power. This is especially true for battery-operated devices where you need to conserve as much power as possible.
If the device has a clamp diode on the input or output, and the input or output has a voltage on it, then the part will power up Vcc through the diode. The input or output voltage will be transferred to the Vcc pin and the Vcc voltage will be equal to the voltage on the input or output -1VT. This is called "back powering." Since the voltage is now on the Vcc pin, it can transfer to other devices on that circuit and power them up as well.
How do you know if the device has a diode that may back power the device?
All standard logic devices will have ESD protection diodes on the inputs and outputs. These diodes not only serve as ESD protection but also provide a voltage clamp to prevent the inputs or outputs going above Vcc or below Gnd. There are several different configurations of these diodes depending on the family of products to which they belong. All devices will have diodes to Gnd on the input and output pins. Some will have the protection diodes configured in a way to prevent the device from being back powered.
Any device that has the Ioff feature will set the inputs and outputs to high impedance when Vcc is 0V and cannot be back powered. Here are some configurations and specs that will tell you if the diodes are there, and thus your device will be back powered.
The best way to check for diodes is to look at the absolute max spec Iik and Iok. If current limits are (+/-), then there is a diode to Vcc and Gnd. If there is only a (–) current then there is only a diode to Gnd. Take a look at these examples:
(Note: Even though it does not show a clamp diode there is actually an ESD circuit where the diodes are configured back-to-back so back power cannot occur.)
1) No clamp to VCC on input or output
This device cannot be back powered
In absolute max spec
In recommended spec
2) No clamp to Vcc on input, but there is a Vcc clamp on output
This device can be back powered from the outputs.
In absolute max spec
In recommended spec
3) Clamp to Vcc on both input and output
This device can be back powered from inputs or outputs
One other way the part can get back powered is if you switch the part off by opening Gnd. The Gnd can come through the diode if any of the inputs or outputs are connected to Gnd. We do not recommend ever powering down by opening the Gnd connection.
So, next time you consider powering down sections of your circuit for energy conservation, keep these things in mind. Do you have any other tips?
To read more, read this app note on understanding and interpreting standard logic data sheets.
To learn more about TI's logic devices, check out our full logic portfolio of standard logic ICs including switches, translation, I2C, interface, little logic, gates, inverters, flip-flops and FIFOs.
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