TPS62740: Ommit exposed GND pad vias in fooprint

Hello Nikos,

Thank you for posting your queries.

From the electrical point of view, I don't see any concern about not placing the three vias on the thermal pad. But, please ensure the pad is connected to the GND pin (3) of the IC.

The primary purpose of the vias is to serve as a thermal relief by connecting the thermal pad of the IC to an internal GND plane of the PCB.

If based on your evaluation shows that the junction temperature (Tj) will not exceed 125 deg. C (allow at least 10% ~ 20% margin to accommodate the unit to unit variation) at worst-case conditions in your application, then removing the vias should be possible. Please refer to the formula below on how to calculate for the Tj with the WSON package.

Tj =  Tc +  (ψJT * Pdiss)

where: 

Tc = case temperature

ψJT = junction-to-top characterization parameter (refer to the datasheet)

Pdiss = IC power dissipation 

Best regards,

Excel

Thank you for your support Excel!!

Given that my application consumes 10mA, the Vin = 3.5V and the Vout = 1.8V, then from figure 14 the efficiency is around 90%

Pout = I * Vout = 0.01 * 1.8 = 0.018mW

Pdiss = Pin - Pout =  (0.018 / 0.9) - 0.018 = 0.02 - 0.018 = 0.002 = 2mW

Is that correct?

Also how do I calculate the case temperature?

Hi Nikos,

Thanks for the quick feedback.

For the Pdiss_IC calculation, the inductor power and load switch losses need to be deducted to the input power as well. This is assuming a negligible power losses on input and output capacitors. By the way, the efficiency graph in the datasheet is only applicable if the load switch (pin #6) part of the IC is not used.

Pdiss_IC = Pin - Pout - (Ind_DCR x Iout) - (RLOAD * ILOAD)

where:

Ind_DCR: inductor maximum DC resistance

Iout: output current

RLOAD: high side MOSFET on-resistance

ILOAD: output current on Load Switch

Use a thermal camera or thermocouple (w/ temperature meter) to measure the case temperature of the device in actual application.

I hope I was able to answer your queries.

Best regards,

Excel