You have your new logic circuit running and everything works as =
designed.=20
Then you turn off the power and the circuit continues to work. What is =
going=20
on?
In today=E2=80=99s world, with all the plug-and-play devices and =
emphasis on power=20
conservation, there are usually multiple voltage sources. To save =
energy, we may=20
turn off certain parts of a circuit to conserve power. This is =
especially true=20
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=20
output has a voltage on it, then the part will power up Vcc through the=20
diode. The input or output voltage will be transferred to the Vcc =
pin and=20
the Vcc voltage will be equal to the voltage on the input or output =
-1VT. This=20
is called "back powering." Since the voltage is now on the Vcc pin, it =
can=20
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=20
outputs. These diodes not only serve as ESD protection but also provide =
a=20
voltage clamp to prevent the inputs or outputs going above Vcc or below =
Gnd.=20
There are several different configurations of these diodes depending on =
the=20
family of products to which they belong. All devices will have diodes to =
Gnd on=20
the input and output pins. Some will have the protection diodes =
configured in a=20
way to prevent the device from being back powered.
Any device that has the Ioff feature will set the inputs and outputs =
to high=20
impedance when Vcc is 0V and cannot be back powered. Here are some=20
configurations and specs that will tell you if the diodes are there, and =
thus=20
your device will be back powered.
The best way to check for diodes is to look at the absolute =
max spec=20
Iik and Iok. If current limits are (+/-), then there is a diode to Vcc =
and Gnd.=20
If there is only a (=E2=80=93) current then there is only a diode to =
Gnd. Take a look at=20
these examples:
(Note: Even though it does not show a clamp diode there is actually =
an ESD=20
circuit where the diodes are configured back-to-back so back power =
cannot=20
occur.)
1) No clamp to VCC on input or output
This device cannot be back powered
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
In absolute max spec
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
OR
In absolute max spec
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
In recommended spec
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
OR
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
2) No clamp to Vcc on input, but there is a Vcc clamp =
on=20
output &nb=
sp;
This device can be back powered from the=20
outputs.  =
; =
&=
nbsp;
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
In absolute max spec
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
In recommended spec
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
3) Clamp to Vcc on both input and output =
This device can be =
back powered=20
from inputs or outputs
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
In absolute max spec
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
In recommended spec
![3D"](3D_http_/e2e.ti.com/resized-image.ashx/__size/550x0/__key/communitys%3d.html)
One other way the part can get back powered is if you switch the part =
off by=20
opening Gnd. The Gnd can come through the diode if any of the =
inputs or=20
outputs are connected to Gnd. We do not recommend ever powering down by =
opening=20
the Gnd connection.
So, next time you consider powering down sections of your circuit for =
energy=20
conservation, keep these things in mind. Do you have any other tips?
To read more, read this app note on understanding and interpreting =
standard=20
logic data sheets.
To learn more about TI's logic devices, check out our full logic portfolio of standard logic ICs =
including=20
switches, translation, I2C, interface, little logic, gates, inverters,=20
flip-flops and FIFOs.