MLX16305 Datasheet, PDF (24/29 Pages) Melexis Microelectronic Systems – Interlock switch sensor interface IC

MLX16305
Interlock switch sensor interface IC
16. Application Notes
16.1. System definition example with 2-wire Hall Switches
The MLX16305 can be used with so called 2-wire Hall effect switches, like the MLX90275. These require
typically a minimum operating voltage of 3.5V, and have 2 well defined current states. For instance:
• Ioff: [5; 6.9]mA
• Ion: [12; 17]mA
An example is elaborated for
• Vsup > 8.5V
Mind that on the Safety ECU often higher voltages are available from the squib drivers. These can also
be used in stead of the battery voltage to further extend the operating zone.
And furthermore
• Vref = 5V Î Vsup-Vref = 3.5V Î Isensemax = 40mA
• Rref=1kΩ Î Isenselim > 40mA
Since the output voltage in Current Mode is undefined in case of a short to supply, we start in Voltage
Mode to diagnose this fault condition first.
VI_CTRL=0
1. Short to VBAT
All SENSE channels are disabled (ENABLE=0) and scanned one by one. If any of the output values
rises above VMEAS_DIS, this means the weak pull down is overruled by a short to the supply.
2. Short between wires.
In a similar way if one channel is supplied with Vref, all other channels can be checked on a short to
this supplied channel. If any of the output values rises above VMEAS_DIS, this means the weak pull
down on the selected channel is overruled by a short to the supplied channel. In a 6 channel
application, 15 different wire-to-wire shorts can be checked and diagnosed one by one.
VI_CTRL=1
In Current Mode (CM) 4 more conditions can be diagnosed.
STEP 1:
The first step is to guarantee that each measurement is in the target operating zone under any corner
condition. Two tolerances have to be taken into account when calculating the border values for each
condition: The offset and the measurement error.
Remark for simplicity of the example we did not calculate tolerances on external components (like Rref)
and other noise sources like ADC supply. These have to be added in a real system definition.
As calculation examples the Voltage range corresponding to the OFF state is calculated:
The minimum worst case situation is calculated using
• Ioffmin = 5mA
• Ioffsetmin = 0.055mA
• Error = - 5%
Î VMEASout (OFFmin) = (Isense/10 * 0.95 + Ioffsetmin) * Rref * 0.9
= 0.477 V
The maximum worst case situation is calculated using
• Ioffmax = 6.9mA
• Ioffsetmax = 1mA
• Error = + 5%
Î VMEASout (OFFmax) = (Isense/10 * 1.05 + Ioffsetmax) * Rref
* 0.9
= 1.076 V
3901016305
Rev. 005
Page 24 of 29
Data Sheet
Oct/06