AN2716 Datasheet, PDF (19/28 Pages) STMicroelectronics – STCC08 application guidelines

AN2716
Figure 16. AVF signal detection
Timer
> T/4 ms
T/2 ms
Electromagnetic compatibility (EMC) tests
V _AC
5.2.3
ZVS delay
ZVS delay
Read AVF and timer off
Read AVF and timer of
ZVS detection => Timer on
ZVS detection => Timer on
The demonstration board and mains wires are placed 10 cm above the ground reference.
The mains wire is shorter than 1 m. Each operating cycle has been tested (load off and on).
The demonstration board supports burst levels higher than 4 kV without spurious triggering
of the ACS or ST7Lite3 MCU loss whatever the coupling mode (to L, N, etc.).
Advice to improve the application immunity
To improve the application performance in term of EMC, the software must be EMC oriented
(for more information please refer to the application note AN1015):
● Auto-recovery routine. At each reset interrupt, the program must check if the data
stored in the RAM are as scheduled or not. A reset can occur without the supply
voltage having fallen below V_RM (data retention parameter). In this case, a full start-up
is not necessary, and the program can keep working with the previous RAM data. This
is helpful to avoid missing load control and the AC switch status when a reset occurs
due to an EMI problem, for example. If the checked RAM registers are not as expected,
then a complete initialization procedure is launched. If the RAM area is adequate, then
a “smart reset” can be performed. Only the registers that are used to store internal sub-
routine variables are cleared. Only the main registers keep their previous values (AC
switch status, AC switch control …).
● Using the watchdog properly. Enable the watchdog as soon as possible after reset
and never refresh the watchdog in an interrupt routine.
● Secure the unused program memory area. Fill the unused memory locations with
code that forces a watchdog reset or jumps to a known program location if you do not
want to generate a reset.
● Input filtering. It is recommended that the AVF signal be read during several AC line
cycles.
Doc ID 14460 Rev 2
19/28