TPS2812: Formula for power dissipation

Hello Hirotaka,

Thank you for the interest in the TPS2812. I am an applications engineer in the High Power Driver group at TI and will work to answer your questions.

For the thermal resistance the TPS2812 was specified in a power dissipation rating at specific ambient temperatures. The Theta j-a can be derived from the power dissipation numbers. There are however different results based on the ambient. Use the following results based on the operating ambient of the system. Also the different packages have different thermal resistance.

Ambient

Pkg      25 deg C     70 deg C       85 deg C

P           92                   79              71

D         137         118        105

PW         192         166        148

                 Theta J-A

For the power dissipation, please refer to the UCC27524A datasheet section 9.2.2.2. The duty cycle does not affect the power dissipation in the driver, as the MOSFET charge is charged and discharged on each cycle regardless of the duty cycle.

This formula is the calculation of the driver power dissipation based on the external gate resistance, and driver internal resistances.

For the driver internal resistances, they can be calculated based on the parameter table high level (or low level) output voltage and test condition current.

For the driver RON or driver internal turn on resistance, it will be the VDD-9.1V high level output divided by the test current of 100mA. Which is 0.9V/100mA or 9 Ohms.

For the ROFF or driver internal turn off resistance, the 1V divided by the test current of 100mA results in 10 Ohms.

I hope this helps with the application.

Regards

Richard Herring 

Hi Hirotaka,

Thanks for reaching out, and good question! I work with Richard and will futher help answer your questions on TPS2812.

To follow up on Richards post above I gave an example of how to calculate theta_ja from the table and try to answer maximum operational current question.

As you already understand that by subtracting the junction and ambient temperatures we can find the relative temperature between the two and then divide out by theta_ja to see the maximum power the thermal resistance will help to dissipate.

The Toperating_max I am using is Tj_max=150  found on page3 of the DS. This makes the theta_ja slightly higher resistance or temperature rise for a given power dissipation.

For example for the first table entry:

Pmax = (Toperating_max – Tamb_max)/ theta_ja

From the table (using worst case package P, Tjmax up to 150C and Ta=25C)

1.09= (150 – 25)/ theta_ja

theta_ja = 114.67

note:To find the temperature rise as you know we need the total power dissipation from the formulas you mention however you also have to account for the regulator current.

-Total power dissipation : (Ig^2 × Rds) + (Vcc × Icc) +(VIN-VOUT)*IOUT

also note: Maximum operational current comes from supply bias current of 5uA and the regulator (capable of supplying up to 20mA) current of 150uA.

Thanks,

Hi Matsumoto-san,

Gald that we can help with your questions!

This VIN-VOUT comment is if you will be using the internal regulator to power other circuitry (for something other than VCC).

VIN and VOUT refer to REG_IN and REG_OUT

The Internal Regulator extends the input supply range from 14V to 40V.

Since there is a regulator internal to this part it will consume some power if it is being used.

If there is a voltage on REG_IN and nothing connected to REG_OUT there will still be, as much as, 100uA of supply current. The bias current for full operation is 150uA max.

The regulator is recommended to only supply up to 20mA. Therefore IO would be 0mA<IO<20mA (see screenshot below).

However if the regulator is being used for VCC then this limits the continuous output current from the average 100mA to 20mA therefore power dissipation is less of a problem. But it is still something to be aware of.

Please let me know if you have any more follow up questions!

Thanks,