L99DZ120 datasheet(92/161 Pages) STMICROELECTRONICS

92 / 161 page

Application information

L99DZ120

92/161

DS11567 Rev 5

The safety block is surrounded by a GND isolation ring realized by deep trench isolation.

The LS driver must work down to a lower voltage than the other circuits. The block has its

own internal supply and an own oscillator for monitoring the failure signals (WWD, V1 fail,

SPI fail & Tj) which are Manchester encoded and decoupled by high ohmic resistances. In

case of fail-safe event, both LS switches LS1_FSO and LS2_FSO are switched on.

In case of entering V1_standby mode or Vbat_standby mode both fail safe low-side

switches are switched on to minimize the current drawn by the fail safe block (e.g. oscillator

is switched off and Manchester Encoding is deactivated). Short circuit protection to VS is

active in both standby modes limiting the current to IOLimit for a filter of tSCF.

After this filter time the fail-safe switches are switched off and LSxFSO_OC (SR 3) is set. To

reactivate the low-side functionality this bit has to be set back by a read and clear command.

In case of VS loss the fail safe switches are biased by their own output voltage to turn on the

low-side switches down to VOUT_max.

To allow verification of the Fail-Safe path, the low-side switches LS1_FSO and LS2_FSO

can be turned on by SPI (Configuration Register 0x3F bit 4: FS_FORCED)

Figure 44. LSx_FSO: low-side driver “passively” turned on, taking supply from output

pin (if main supply fails), can guarantee VLSx_FSO < VOUT_max

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