L9780 datasheet(28/56 Pages) STMICROELECTRONICS

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Diagnostic

L9780

28/56

DocID026356 Rev 3

From a design point of view, to HD pin are connected an 80 μA sink current generator

(normally on, except for particular diagnostic conditions) and a 190 μA source current

generator (normally off, except for particular diagnostic conditions); possible ranges for

generated currents are specified in Table 13.

5.4.1

Heater short to battery

When the external heater is in “on” condition, the heater drain is expected to be low (the

external heater is considered “on” when the HG voltage value is higher than a specific

threshold, defined in Table 13, typical value 4V). If the heater drain voltage value is higher

than the heater short to battery threshold (selectable via SPI using bit STBHTH, as defined

in Table 13), a possible heater short to battery condition is detected. Possible heater short to

battery condition is confirmed by the logic (and reported in the SPI output register) only if the

duration of the fault is greater than a filter time, defined as a multiple of the clock period; in

other words, when the possible heater short to battery condition is detected, a logic counter

starts and when the count reaches the value defined in Table 17 (and selectable via SPI

using bit STBHFT[1..0]) the fault condition is considered validated. This counter is reset

when the possible short to battery fault is cleared and after every power on reset.

Actions

Once the heater short to battery fault is validated (the duration of possible fault is greater

than the filter time) the following actions are taken:

Latch HG off;

Set STBH bit in SPI output register.

These actions remain valid until cleared by the user.

5.4.2

Heater short to ground

When the external heater is in “off” condition, the heater drain is expected to be high. If the

heater drain voltage value is lower than the heater short to ground threshold HSG (3.69 V

typical value, possible range specified in Table 13), a possible heater short to ground

condition is detected. Possible heater short to ground condition is confirmed only if the

duration of the fault is greater than a filter time, defined as a multiple of the clock period; in

other words, when the possible heater short to ground condition is detected a logic counter

starts and when the count reaches the value defined in Table 17 the fault condition is

considered validated. This counter is reset when the possible short to ground fault is cleared

and after every power on reset.

Actions

Once the heater short to ground fault is validated (the duration of possible fault is greater

than the filter time) the following actions are taken:

Set the STGH bit in SPI output register;

Latch HG off;

Invoke a test of heater open circuit.

These actions remain valid until cleared by the user or modified after the open circuit test.

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