L9026 datasheet(32/60 Pages) STMICROELECTRONICS

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Figure 15. OFF diagnosis circuit

9.3

Open load at ON

On each channel it is possible to request the open load at on diagnosis.

This diagnosis is designed having in mind a specific case of high side channels driving LEDs kept fully ON.

Moreover, the device will ignore the request of an open load at on diagnosis in case the channel is configured as

low side. This means that the request diagnosis register will not be updated in case it is requested the diagnosis

on a low side configured channel.

Such a diagnosis is not recommended in case of loads driven in PWM mode (to avoid eventual interferences with

the PWM driving signal) and inductive loads (to avoid unwanted power dissipations during switch OFF transients).

9.3.1

Open load at ON (diagnosis mechanism)

The diagnosis cannot be run on more channels simultaneously, but it can work only on one channel at a time.

This can be activated by setting the bit corresponding to the desired channel in the DIAG_OPL_ON_EN register.

During the diagnostic loop the channel is switched off every tOPLCYC for a maximum time of tOPLEN.

Such a time is designed to be short enough to result in a negligible variation of the luminescence of LED during

the test.

During the short switch-off time required to perform the diagnosis a pull-up current generator IL(OL) is applied to

the source of the channel, as can be seen from Figure 16.

If the load is present the source voltage drops suddenly, the output of the comparator is set and channel

immediately restarts.

If an open load condition exists, instead, the pull-up current IL(OL) is capable of keeping the source voltage to

battery and the Vds value remains stable for the whole diagnosis time interval.

In this case once tOPLEN, is elapsed, the comparator output is still low and the flag related to the fault is set; the

channel is anyway restarted.

The OPL ON diagnosis lasts just a tOPLEN time in the worst case scenario, i.e. when the open load is detected.

Once an SPI frame commanding the diagnostic is received, the diagnosis will start a Toplcyc time after the

corresponding request has been accepted.

As long as the diagnosis request bit remains high the open load test will be performed every Toplcyc and the

results of the diagnosis will be reported in the register DIAG_OPL_ON: if an open load on channel ‘n’ is detected,

the corresponding bit DIAG_OPL_ONn is set.

L9026

Open load at ON

DS13397 - Rev 5

page 32/60

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