L9908 datasheet(92/153 Pages) STMICROELECTRONICS

92 / 153 page

Note:

•

The OFD circuit can detect a SCB or SCG condition affecting the three phases in general, due to the low

DC resistance of the motor it cannot identify at which phase the failure is present.

•

Correct functionality of OFD is based on a current mismatch between pull-up and pull-down current. In

case of additional external pull-up/down contribution the following inequation must be satisfied:

IPU_TOTx>IPD_TOT_PHy+IPD_TOT_PHz

(21)

Where:

Ipu_totx is the equivalent pull-up current injected into the phase x (assuming phase x being configured as

OFD_EN = 01)

Ipd_toty and Ipd_totz are the equivalent pull-down currents injected respectively into the phase y and z

(assuming phase y and z being configured as OFD_EN = 10)

Figure 66. OFD currents in motor phases

Blanking and Filtering

To avoid false error detections during the diagnosis settling time and to increase noise robustness, the OSD

implements a two-step digital filtering: an Up/Down counter of length equal to T_ofd_flt on the comparator`s

output filters out the high frequency noise while a blanking time masks the filters output by a programmable delay

time T_ofd_blank.

Figure 67. OFD enabling and masking time start - IPU example

L9908

Off State Diagnosis (OFD)

DS13546 - Rev 3

page 92/153

L9908 Datasheet(PDF) 92 Page - STMicroelectronics