L9660 datasheet(22/49 Pages) STMICROELECTRONICS

22 / 49 page

Functional description

L9660

22/49

DocID024714 Rev 2

Figure 7. Deployment drivers diagram

The L9660 is protected against inadvertent turn on of the firing drivers unless the

appropriate conditions are present. Non-typical conditions do not cause driver activation.

This includes the case where VRESx and/or VSDIAG pins are connected to a supply up to

40V and VDD is between 0V and VDD min. Under these conditions the L9660 ensures that

driver activation does not occur. No flow of current shall be allowed through the SQHx and

SQLx pins.

Driver activation

The firing of a squib driver requires the appropriate FEN function to be active and two

separate sixteen bit writes be made over the SPI. The FEN function is defined as the result

of the FENx pin OR’d with the internal FENx latch. The FENx pin going high initiates the

FEN function. With the FEN 1 function being active and the appropriate Arm and Fire

commands sent then Squib_0 & 1 drivers would be activated. With the FEN 2 function being

active and the appropriate Arm and Fire commands sent then Squib_2 & 3 drivers would be

activated.

The first write is to ARM the drivers in preparation of receiving the fire command. The ARM

command stops on all channels any diagnostics that are active. Any combination of squibs

can be armed. The second write is a FIRE command that must directly follow the ARM

command and that activates the desired driver pairs assuming the FEN function is valid. If

there is a parity mismatch the data bits are ignored and the squib drivers do not have their

status changed, and the two write sequence must then be started again. If there is a

mismatch in channels selected then only those channels selected in both the Arm and Fire

commands are activated.

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