MLX16305 Datasheet, PDF (20/29 Pages) Melexis Microelectronic Systems – Interlock switch sensor interface IC

MLX16305
Interlock switch sensor interface IC
13.1. Overtemperature considerations
Depending on the layout of the PCB, the thermal resistance of a narrow body SO16 package can be as
low as 82^oC/W (ideal reference value for a 4 layer pcb).
For the below calculations we use 100^oC/W.
The increase in junction temperature (Tj) due to energy dissipation in the linear voltage regulator can be
calculated as follows:
• Assuming worst case operating conditions VSUP=18V and Tamb = 85^oC.
• And VREF=5V and RREF = 1kOhm,
EXAMPLE 1: 40mA continuous
Assuming the maximum supply current is drawn from a Sense pin, and the voltage is regulated to
Vref (no short condition)
Ö Dissipation due the this: 40mA * (18-5)V = 0.52W
Additionally the current sources supply RREF with Isense/10 + Ioffset (<1mA)
Ö Maximum Dissipation due the this: (40mA/10 + 1mA) * (18-5)V = 0.07W
Therefore the maximum power dissipation on the MLX16305 is 0.59W.
Ö The maximum junction temperature is then 85^oC+100^oC/W*0.59W = 144^oC.
This is outside the operating range. Furthermore the MLX16305 may shut down due to
overtemperature since Tj > T_shutdown.
EXAMPLE 2: Duty Cycling
In practice the controller will scan the different SENSE outputs one by one. As described in
above (diagnostics flow) the regulator can be expected to be active for example for 0.5ms per pin,
including 250us settling time.
At Tamb = 85^oC, Tj<90^oC can be guaranteed as follows:
The heating of 59^oC in EXAMPLE1 can be reduced by waiting 5.5ms before selecting the next pin
The flow then becomes:
0.5ms evaluating pin1 => 5.5ms off => 0.5ms evaluating pin2 => 5.5ms off => …
Or also: evaluate all 6 pins consecutively (6*0.5ms) and switch off for > 33ms
Example3: Short to GND
Short to ground implies that Vsense = 0V.
When the regulated supply is applied, then the SENSE voltage will be kept at 0V by the short, and
the internal current limitation will limit the current to for instance 50mA. Therefore the power
dissipation from the SENSE pin is
Ö Worst case Dissipation due the short: 50mA * 18V = 0.9W
The controller will immediately diagnose the failure, and switch off the supply.
Therefore no dissipation will be due to this. Worst case the MLX16305 will go in overtemperature
and shut down itself.
3901016305
Rev. 005
Page 20 of 29
Data Sheet
Oct/06