MLX16305 Datasheet, PDF (21/29 Pages) Melexis Microelectronic Systems – Interlock switch sensor interface IC

MLX16305
Interlock switch sensor interface IC
14. DIAGNOSTICS
14.1. Shorts
Long wires to the remote switches are prone to shorts to Ground, to supply or between the wires. The
MLX16305 allows continuous failure mode diagnostics on each SENSE pin individually. The circuit works
also if one of the SENSE pins is shorted to GND or Vbat (up to +25V); this means that such a short does
not influence the measurement on other SENSE pins.
A. Shorts to GND
A short between a SENSE pin and GND can be diagnosed either in Current Mode (CM) or in Voltage
mode. The short will cause the voltage source to go in current limitation. As described in the ‘Overcurrent
limitation’ paragraph above, the voltage over Rref (VRREF) will rise to VREF.
• In Current Mode (CM) over current is detected as
VMEASOUT = 0.9* VREF
• In Voltage Mode over-current is detected as
VMEASOUT < VREF/2 .
This value may however also indicate an open or closed switch. Therefore it is recommended to
diagnose Shorts to GND in CM.
B. Shorts to Supply (Vsup)
A short between a SENSE pin and a high voltage level, like VIGN or even directly to the battery supply
VBAT, can be diagnosed in Voltage Mode:
Method 1: SENSE pin not actively supplied
• Set all SENSE pins high impedant (MUX_SUP[0:2]=000b),
• Then select the respective SENSE pins one by one in Voltage Mode (VM).
=> The weak pull down on each SENSE pin should yield
VMEAS < VMEAS_DIS.
Any higher value indicates a short to a Supply voltage.
Method 2: SENSE pin actively supplied
When there is a short to Supply on a pin that is supplied by the voltage regulator, then the
MEAS_OUT voltage will be larger than Vref/2.
VREF/2 < VMEASOUT < VREF
C. Shorts Between wires
A short between the wires connecting the sense pins to the remote switches can be diagnosed in a similar
way as shorts to supply:
• Select a SENSE pin. This channel will be supplied with VREF, and act as the supply in the ‘Short
to Supply’ case.
• Then select the other SENSE pins one by one in Voltage Mode (VM).
The weak pull down on the other SENSE pin should yield VMEAS < VMEAS_DIS. Any higher value
indicates a short to the selected SENSE pin. To have an unambiguous diagnostics first check ‘Shorts to
Supply’ before checking ‘Shorts between Wires’.
14.2. Open Wire
In an Open Wire condition, the regulator can not supply any current (Isense= 0mA). This condition can
not be checked in Voltage Mode. In Current Mode Ioffset will generate a signal on RREF. Therefore for
the analog measurement chain, an open wire conditions is identical to an offset measurement. Mind that
due to ±VMeas_Ierr2 it is possible that after subtraction of the Offset, a negative value may result!
3901016305
Rev. 005
Page 21 of 29
Data Sheet
Oct/06