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OMAP4 Junction Temperature Calculation

Brad Griffis
Guru*** 125430 points
Other Parts Discussed in Thread: 4460

I’m reading swpa188b (OMAP4460 and OMAP4470 Thermal Management). Is there a formula to compute the junction temperature based off these two sensors?

Alternatively, is there any way for us to do some kind of quick simulation internally to come up with the “max delta” between the on-chip sensor and the “hot spot” for a given CPU frequency?

  • 0
    TI__Intellectual 815 points

    Hey Brad,

    Looking at an ICS/JB kernel release, you can check out drivers/staging/thermal_framework/governor/omap_die_governor.c

    Using just the on-chip (bandgap) sensor, the formula used is (I modified a few variable names for clarity):

    hotspot_temp = bandgap_sensor_temp + absolute_delta
    where absolute_delta = (sensor_temp * OMAP_GRADIENT_SLOPE_4460) / 1000 + OMAP_GRADIENT_CONST_4460

    *Note that the numbers used are in milliCentigrade. The table adc_to_temp[] in drivers/staging/thermal_framework/sensor/omap_temp_sensor.c and pcb_temp_sensor.c lists the corresponding temp values to adc values returned by the temp sensor.

    If using bandgap and pcb temp sensor (your case) and the pcb sensor placement follows the spec in the thermal app note (section 5.4.3.1) then this equation is used:

    hotspot_temp = bandgap_sensor_temp + absolute_delta
    absolute_delta = ((average_bandgap_sensor_temp - pcb_sensor_temp) * OMAP_GRADIENT_SLOPE_W_PCB_4460) / 1000 + OMAP_GRADIENT_CONST_W_PCB_4460

    *Note that for this case, the sw release uses an average sample of bandgap temperatures in the calculation.  average_on_die_temperature() in omap_die_governor.c covers this.

    As for the max delta, it increases as the temperature increases.  Since your customer wants max temp at 105C, you could raise the omap's temperature by running at max frequency on the ICS/JB release, then note the bandgap/pcb temp sensor values when the hotspot hits 105C.

    Regards,

    Alex