MSP430G2955: WHAT IS MSP430G2XXXX JUNCTION TEMPERATURE?

Suyoung Lee said:

I USE MSP430G2955 MCU FOR TEMPERATURE TRANSMITTER.

Please do not shout using ALL CAPS. It is hard to read such posts.

Suyoung Lee said:

I CAN'T FOUND MSP430G2955 JUNCTION TEMPERATURE.

As msp430 chips are comparably low power, you can assume that junction temperature does not differ much from specified ambient temperature (free air temperature).

If you want to measure temperature, you need external temperature sensor anyway.

Sorry about using All Caps. 

My product is temperature transmitter that ouput signal is 4~20mA

My company Q.A team measured MCU temperature on board when board is working. 

Measured temperature is 31.4 degrees. And room temperature is 23 degrees.

I guess air temperature which inside of product was rising. because another part of my product fully out 22mA.

MCU temperature increased by 8.4 degrees.

The ambient temperature of my product is -40 to 85 degrees. And the ambient temperature of MSP430G2955 is -40 to 85 degrees. 

Because of the problem with product safety when using the part, Q.A team not allow using MSP430G2955. But I can't change MCU. My project almost done.

How can I solve this situation?

Operating free-air temperature means that the part is just "cooled" by the ambient air without having any cooling mechanism like a heatsink on it. Of course you cannot measure the temperature of the die from outside, but you can read out the internal temperature via the built-in ADC. This measurement is not very precise, but it may give you an idea. The pass transistor in a loop-powered device can heat up the housing of the device, of course. It depends on the layout and on the used transistor if this will cause trouble or not. Always place this part away from any analog circuitry and also try to use a location where heat can dissipate in some way.

Anyway - I have thousands of loop-powered 4 to 20mA MSP430G2955 based devices running in environments like the one you described. No problem at all. But I'm running the parts at 1MHz only - what is your frequency? As already mentioned before - the MSP430 is a ultra-low-power device, so heat dissipation normally is no big deal here.

One option is to move the MCU to another board so that there is little temperature rise.

The other option is see if that MCU has an extended temperature range version:
www.ti.com/.../extended-temperature.page

Dennis Eichmann said:

I'm running the parts at 1MHz only - what is your frequency?

Let's do a quick math. At 16MHz particular CPU draws 4.5mA of 3.3V supply. It is roughly 15mW (milliwatts) which according to thermal calculator, heats 500mm^2 PCB  by 1.2 oC. This is at 16MHz and 100% duty cycle, wihout any LPM.  I doubt that CPU is main heat source here, thou size and type of PCB is unknown.

Either heat dissipation of circuit shall be fixed or enhanced msp430 shall be used.

[edit] Even at very poor junction to ambient temperature coeff 50 C/W, 15mW will result in 0.75 degree difference between PCB and junction - in case CPU of msp430 is heat source. Of course, you can run current through pins such way increasing dissipated power, but to know how much power is dissipated - you measure consumed (by msp430) current and do the math.

I use frequency at 1MHz. And I know it doesn't matter using MSP430G2955 at 85 degree. But the Q.A team, they just read datasheet. they don't know how it works. They want to use MCU that have an ambient temperature above 100 degrees.

Suyoung Lee said:

But the Q.A team, they just read datasheet. they don't know how it works. They want to use MCU that have an ambient temperature above 100 degrees.

Your QA team is right. It does not matter - heat source is msp430 or not. Chip and product max temperatures cannot be the same because in such case your product shall not dissipate, thus consume any energy. You can't cheat physics. You shall pick another microcontroller.

Just to add some clarification about this topic, these two threads explain why some devices have same junction and ambient temperature specs:

https://e2e.ti.com/support/microcontrollers/msp430/f/166/t/711979

https://e2e.ti.com/support/microcontrollers/msp430/f/166/t/593263

Regards,

Luis R