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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

2.1.2 Latch-Up (LU)

2.1.2.1 Static Latch-Up (LU) test:

The Latch-up is a phenomenon which is defined by a high current consumption resulting from

an overstress that triggers a parasitic thyristor structure and need a disconnection of the

power supply to recover the initial state.

NOTES

1 The overstress can be a voltage or current surge, an excessive rate of change of current or

voltage, or any other abnormal condition that causes the parasitic thyristor structure to be-

come self-sustaining.

2 Latch-up will not damage the device if the current through the low-impedance path is suffi-

ciently limited in magnitude or duration.

This test conforms to the EIA/JESD 78 IC latch-up standard.

True LU is self-sustaining and once triggered, the high current condition will remain until the

power supply voltage is removed from the device. A temporary LU condition is considered to

have been induced if the high current condition stops when only the trigger voltage is re-

moved.

s

Power supply overvoltage (applied to each power supply pin) simulates a user induced

situation where a transient over-voltage is applied on the power supply.

s

Current injection (applied to each input, output and configurable I/O pin) simulates an

application induced situation where the applied voltage to a pin is greater than the maximum

input due to ringing.

Table 7 shows how LU test result is presented in ST datasheets.

Table 7. Example of the LU test result on ST72F521

1.

Class description: “A” class is an STMicroelectronics internal specification. All its limits are higher

than the JEDEC specifications, that means when a device belongs to Class A it exceeds the JEDEC

standard. “B” Class strictly covers all the JEDEC criteria (international standard).

2.1.2.2 Dynamic Latch-Up (DLU) test:

The product is evaluated for its LU susceptibility to ESD discharges when the microcontroller

is “running.”

Symbol

Parameter

Conditions

LU

Static latch-up class

pending on the temperature

range of the device)

A