L9945 datasheet(37/151 Pages) STMICROELECTRONICS

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5.6.2

Peak & Hold diagnostics

When in peak & hold configuration, diagnostic is performed independently on each channel, as if a low-side or

high-side configuration was applied. The external microcontroller monitoring the device must properly combine

the diagnostic information of the high-side channel to the one read on the low-side channel in order to determine

the eventual fault type. Refer to Table 36 for the diagnostic codes.

5.6.3

ON state diagnostics

ON state diagnostic latches are updated only when both channels are switched ON, that is, only when the current

is supposed to actually flow in the load. In case an OC event occurs on a channel while the other is switched OFF,

OC protection is triggered and the external FET is protected, but the diagnostic code of that channel is not

updated. OC event will be eventually confirmed once both channels are switched ON (OC or OC pin will be

reported).

When in peak & hold configuration, L9945 protects the external FET against overcurrent as in the HS/LS

configuration. However, an additional diagnostic code is available for this configuration:

•

If an overcurrent event occurs while the output is switching ON (tblank_oc timer still running), an OC pin

failure is stored in the diagnostic latches → code "000"

•

If an overcurrent event occurs while the output is fully ON (tblank_oc timer expired), an OC failure is stored

in the diagnostic latches → code "001"

In order to detect a short across the load (SCL) failure, when an OC/OC pin event occurs simultaneously on HS

and LS, an OC pin failure is latched for both sides.

5.6.4

OFF state diagnostics

When both HS and LS are commanded OFF by the control signal, an intentional open load occurs on both load

pins (refer to Internal regulator for open load (OL) detection). In order to avoid false OL detection, OL fault is

masked in this condition. The diagnostic code reported in such case can be selected by programming the

PHx_diag_strategy bit:

•

If PHx_diag_strategy = 0, "No OL/STG/STB failure" is reported → code "110";

•

If PHx_diag_strategy = 1, "No diagnostic done" is reported → code "111".

OL detection is guaranteed in case HS and LS have different states. In normal HS/LS configuration, the OFF

diagnostic filter timer tDIAG is started every time a channel is switched OFF. In peak & hold configuration, two

additional events generate a start condition for tDIAG:

•

HS OFF, LS OFF → ON;

•

LS OFF, HS OFF → ON.

Therefore, an eventual OL fault is detected as soon as one of the two transistor is switched ON (after tDIAG)

While HS is in the OFF state, if the voltage on the load node rises above VOL, the fast discharge current is

activated to prevent false STB detection (refer to OFF state diagnostics and Fast charge/discharge currents).

Referring to Figure 19, Table 24 shows the possible faults in peak & hold configuration, along with the diagnostic

strategy. Refer to Table 36 for the diagnostic codes.

Note:

Table 24 has been compiled under the assumption of all channels starting from "No failure" state (100). Such

state is reached during normal operation of the circuit when no failure has been detected by both OFF and ON

state diagnostics. Therefore, the diagnostic strategy can be applied if at least one ON/OFF switching cycle has

been completed without failures. In case the starting state was "No diagnostic done", the "No failure" state can

be replaced with "No OL/STG/STB failure" (110) for OFF state diagnostics and with "No OC failure" (101) for ON

state diagnostics. Diagnostic codes follow a priority concept. Diagnostic latches are reset in case of NRES/POR

assertion or in case of SPI readout. Refer to Diagnostics overview to understand priority and FSM algorithm.

L9945

Peak & Hold

DS12275 - Rev 8

page 37/151

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