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LM25066A: Temperature sensing

christopher2
Intellectual 515 points
Part Number: LM25066A
Other Parts Discussed in Thread: LM25066

Can I use a typical small signal diode like 1n4148 do-35?

Recommendation from datasheet is MMBT3904 transistor with shorted Collector and base. I tried with the diode but the circuit is off after i connect the diode.

The idea was to make something like a bridge over the mosfet and glue it directly to the mosfet case.

Is the temperature sense current (from the diode pin of LM25066) to low for the measurements?

  • 0
    TI__Guru** 105361 points
    Hi Christopher,

    A diode connected bipolar transistor (BJT) rather than a standard true diode is recommended. This is because BJTs have consistent temperature coefficient which results in smaller errors over temperature. Please refer app note www.ti.com.cn/.../sboa277.pdf
    Also, the PCB parasitics play role in measurement. Did you made new PCB? I did not find temperature dependency information for 1n4148 diode, Have you by any chance?

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Intellectual 515 points

    Yes, I did my own test pcb. As soon I connect the diode the LM goes off. The diodes are quite linear from the graph in Fairchild datasheet www.onsemi.com/.../1N914A-D.pdf
    I could estimate thermal factor of 1,9mV /C. Some people write the PN junction has 2 - 2.2mV/C , the accuracy here is not the priority. +-5C would be ok.


    But perhaps it is related with the voltage drop at so low current. At 100uA the 1N4148 has about 0.5V drop and 0,4V at 10 uA.
    If I understood correct the LM25066 sends current pulses 9/250uA and senses Voltage.

  • 0 in reply to christopher2
    TI__Guru** 105361 points
    Hi Christopher,

    yes, the controller sends pulsed current and measures voltage. As per my understanding, the voltage should be lower at higher temp & 250uA current when compared to voltage at ambient temp & 9uA current. The MMBT3904 has wider variation in Vbe with respect to temperature. We never tried with small signal diodes but i like your idea, please note if we use more diodes like a bridge the pulsed current 9/250uA will be shared.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Intellectual 515 points

    What I mean with the bridge is a just a single DO-35 diode bridge over my MOSFet. Just to get MOSFET temperature not a PCB temperature as they can not be thermal coupled. MMBT3904 SOT23 case has not thermal pad, so such sensing is not accurate.

    Do you suggest to use two serial connected diodes to increase the voltage drop? Or 4 in configuration like 2S2P?

    Is there any calibration procedure to set proper temperature reading?

    Or the values are fixed?As for example

    0.65V=0000hex
    0.35V=FFFFhex

    Thus probably my board gets of when I connect the diode because it is seeing overtemperature. This is my guess.

  • 0 in reply to Rakesh
    Intellectual 515 points
    need more clearification
  • 0 in reply to christopher2
    TI__Guru** 105361 points
    Hi Christopher,

    I am checking on the internal threshold details. Can you confirm whether the device turned OFF even at room temperature with diode as temperature sensing element.
  • 0 in reply to Rakesh
    Intellectual 515 points

    Yes, at room temperature.