L9945 datasheet(126/151 Pages) STMICROELECTRONICS

126 / 151 page

The following behaviors correspond to the different sensing methods (refer to Figure 31):

•

In case Rshunt is selected, the Vsense behavior during the OFF==>ON transition corresponds to an

ascending transient. In case the FET current crosses the OC threshold with a positive slope, tBLANK_OC is

stopped and tOC is started. The OC detection will be mainly based on tFIL_ON and tOC parameters. The

former is a deglitch filter to avoid small overshoots on the shunt resistor due to inductive effects, while the

latter represents the actual OC blanking time. An OC event lasting tFIL_ON+tOC will be detected and the

output switched OFF: the programmed tBLANK_OC will have no effect on the OC reaction time. Only in case

Peak & Hold configuration is selected and the OC event occurs while tBLANK_OC is still running, an ‘OC pin

failure’ (000) will be reported instead of simple ‘OC failure’ (001). However, reaction time won’t be affected.

•

In case DSM is selected, the Vsense behavior during the OFF==>ON transition corresponds to a falling

transient. Once tBLANK_OC has expired, the VDS will be compared to the OC threshold and, if greater, the

output will be shut OFF. Hence, tBLANK_OC must be sized to allow VDS settling. In order to ensure maximum

FET protection against critical OC events during blanking time, an OC threshold crossing with a positive

slope will stop the tblank_oc and engage the tOC filter.

Figure 31. Rshunt and DSM method diagram

GADG3108170806PS

OC THRESHOLD

VSH

RSH method, tblank running

OC EVENT

SWITCH ON

NORMAL VALUE

SHUT OFF

OC THRESHOLD

VDS

DMS method, tblank running

OC EVENT

SHUT OFF

SWITCH ON

NORMAL VALUE

6.3.4

OC threshold selection

The voltage corresponding to the OC threshold can be selected independently for each channel programming the

OC_config_xx bit. Such threshold is highly flexible: it is encoded on 6 bits (64 available values) and falls in the

[50 - 1000] mV range.

Table 39. Overcurrent threshold selection electrical parameters

Symbol

Parameter

Min.

Typ.

Max.

Unit

VOC_RANGE

Range

1000

OC_RES

Resolution

Bit

tSETTLING

Threshold settling Time

µs

Refer to Table 40 for the encoding tables for LS and HS configurations. Overcurrent threshold must be

programmed according to the Eq. (3):

Eq. (3): Overcurrent threshold

VOC=ROC×IOC

(3)

where VOC is the value programmed in the OC_config_xx bit, IOC is the maximum current for the given

application and ROC can be either RSH (shunt resistor) or RDSon (DSM).

Note:

For H-Bridge configuration, refer to Overcurrent detection strategies for H-Bridge and Current limitation for H-

Bridge to understand how OC thresholds can be programmed and exploited also for current limitation feature.

L9945

ON state diagnostics

DS12275 - Rev 8

page 126/151

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