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What is Latch-up?

Latest Updated:08/01/2005

Question:

Latch-up

Answer:

Latch-up is a phenomenon whereby VDD and VSS in a CMOS device become structurally shorted due to the effect of noise, etc., and may occur if a voltage exceeding the absolute maximum rating (i.e., higher than VDD or lower than GND/ VSS) is input to the device.

In a CMOS structure, current flows only in the P-channel and N-channel in normal conditions. However, if a high voltage is input, since a CMOS structure is equivalent to a thyristor structure (PNPN) formed by parasitic transistors, current flows into the lower layers of the chip, turning the parasitic transistors on, and passes from the power supply to the ground, which causes a short circuit.



 VIN becomes higher than VDD.
 Tr1 is turned on.
 Tr2 is turned on.
 Current flows to R2, and the potential difference between the ends of the resistor becomes the voltage at which Tr6 is turned on.
 Tr6 is turned on.
 Current flows to R1, and the potential difference between the ends of the resistor is generated. As a result, Tr2 remains on, even if VIN becomes lower than VDD.
 Due to the occurrences of  to ], current continues to flow between VDD and VSS, causing latch-up.

Latch-up can be caused by various factors, such as static electricity, noise, or application of a voltage to the input pin outside the power supply voltage range (e.g., overshoot).
Once latch-up occurs, even if the cause is removed, the continuous flow of current between VDD and VSS set in motion by the above mechanism does not stop.
Since latch-up means the device has a short circuit, it may lead to degradation of the device's characteristics or may even seriously damage the device.
It is important to remember that the absolute maximum ratings are specifications that should not be exceeded, even for a moment.
Latch-up is a mechanical short circuit, so when it does occur, it cannot be overcome unless the power supply is turned off and then turned on again. A reset does not work.
However, since latch-up degrades the device's reliability, it is not recommended to continue using a device once latch-up occurred.

Latch-up is commonly prevented by absorbing noise using a noise-clipping diode.
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