L9678P datasheet(130/203 Pages) STMICROELECTRONICS

130 / 203 page

Remote sensor interface

L9678P, L9678P-S

130/203

DocID029274 Rev 3

The converter default count value is 42, therefore,

Ibase = 2.5 mA + 42 * 300 µA

Ibase = 15 mA

Itrip = Ibase + threshold where threshold is a fixed at 12 mA

Thanks to this implementation, Ibase can span from 2.5 mA up to 41 mA covering PSI-5

specification range. As an example, for a remote sensor that operates at 10 mA base

current, Itrip = 23 mA.

7.2

Remote sensor interface fault protection

7.2.1

Short to ground, current limit

Each output is short circuit protected by an independent current limit circuit. Should the

output current level reach or exceed the ILIMTH for a time period greater than TILIMTH the

output stage is disabled and an internal up-down counter will count in 25 µs increment up to

TILIMTH. The filter time is chosen in order to avoid false current limit detection for in-rush

current that may happen at interface switch-on. When the output is turned off due to current

limit, the appropriate fault code STG is set in the Remote Sensor Data Register (RSDR).

The fault timer latch is cleared when the sensor channel is first disabled and then re-

enabled through the Remote Sensor Configuration Register (RSCR). This fault condition

does not interfere with the normal operation of the IC, nor with the operation of the other

channels. When a sensor fault is detected, the RSFLT bit of the GSW is set indicating a fault

occurred and can be decoded by addressing the RSFSR register.

In order to fulfil the blanking time requirement at channel activation as per PSI-5

specification, a dedicated masking time is applied to the current limitation fault detection

each time a channel is activated.

7.2.2

Short to battery

All outputs are independently protected against a short to battery condition. Short to battery

protection disconnects the channel from its supply rail to guarantee that no adverse

condition occurs within the IC. The short-to-battery detection circuit has input offset voltage

(10 mV, minimum) to prevent disconnecting of the output under an open circuit condition. A

short to battery is detected when the output RSUx pin voltage increases above VSUP

supply pin voltage for a TSTBTH time. An internal up-counter will count in 1.5 µs increment

up to TSTBTH. The counter will be cleared if the short condition is not present for at least

1.5 µs. Short to battery protection blocks the battery condition to guarantee that no adverse

condition occurs within the IC. The channel in short to battery is not shut down by this

condition. Other channels are not affected in case of short of one output pin. As in the case

previously described, the STB fault code can be read from RSDR bits and any fault will set

the RSFLT bit of the global status word register (GSW). The STB bit is cleared upon read.

7.2.3

Cross link

The device provides also the capability of a cross link check between outputs, in order to

reveal conditions where two output channels are in short. This functionality is allowed by

enabling one output channel, while asking for voltage measurement on any of the other

ones.

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