2024-11-03 05:55 PM
LDL212-3.3v DFN (2x2) Running Superhot
With LDL212
The power dissipation PP can be calculated using the formula:
P=(VIN−VOUT)×IOUTP = (V_{IN} - V_{OUT}) \times I_{OUT}
Where:
P=(6.65V−3.3V)×0.22A=0.726WP = (6.65V - 3.3V) \times 0.22A = 0.726W
The junction temperature can be calculated using the formula:
TJ=TA+(P×θJA)T_{J} = T_{A} + (P \times \theta_{JA})
Where:
TJ=25°C+(0.726W×65°C/W)=25°C+47.19°C=72.19°CT_{J} = 25°C + (0.726W \times 65°C/W) = 25°C + 47.19°C = 72.19°C
TC=TA+(P×θJC)T_{C} = T_{A} + (P \times \theta_{JC})
Where:
TC=25°C+(0.726W×15°C/W)=25°C+10.89°C=35.89°C
All these numbers are theoritical
But in reality the LDL runs in 44 degree zone and easily goes beyond 55 degrees with just 220mA of current , where the exposed pad is connected to GND Zone with an aproximate area of 150^2 mm.
Attaching the landing pattern as well
Solved! Go to Solution.
2024-11-05 02:08 AM
2024-11-03 11:29 PM
Welcome @amansinghaljpr, to the community!
Well, you are burning almost 750mW in a DFN 2x2. Have you determined the thermal resistance of the 150mm2 GND cooling area, which has to be added to Rthjc of the device?
Have you considered adding further cooling through thermal vias near the LDL212?
Regards
/Peter
2024-11-04 10:56 AM - edited 2024-11-04 10:59 AM
Hey @Peter BENSCH , thanks for reply.
I just wanted to verify a couple of things .
1) Are my calculations correct ?
2) I dont know the thermal resistance of that area , its just a big chunk of copper area connected to the exposed pad intended to increase the surface area for heat dissipation. I thought i had overdesigned , but after looking at the temp i guess some major issues going on
3) Please enlighten me about any numbers i am missing out on
Also is 750mW too much heat ? I thought a DFN 2x2 package should be capable enough to handle it
I have added the schematic i am using as well just for your reference
2024-11-04 11:36 AM
Speaking of heat distribution: if you know the current consumption of your 3.3V load very well, you can possibly change R23 from 0ohms to a larger value. At 8.2ohms, for example, a voltage of 1.804V drops at 0.22A, i.e. a power loss of 397mW. The power dissipation that the LDL212 has to burn is then reduced by this value. Of course, you then have to use a resistor with a corresponding power capacity - and ensure that it is cooled. So the problem remains the same, it is just distributed.
Hope that helps?
Regards
/Peter
2024-11-04 05:40 PM - edited 2024-11-04 05:59 PM
@Peter BENSCH Thanks a lot for the prompt reply peter .
So here's what i understand
1) I am still yet to calculate or find out the thermal resistance of that PCB copper area , i know the thickness of the copper but i don't know the exact calculations for that . So I'll probably test that
2) With the given power dissipation and the package size , is it okay expect for the temps to go upward of 55 degrees with whatever cooling capacity is given ? Which i just mentioned. (55 degrees was measured on top of casing of the ic )
3) Even if the DFN-2x2 package is capable of handling such powers , i could not find any recommended landing pattern or PCB layout guideline for the LDO , can you point me to any such ST whitepaper or Application note ? So that i can keep a note of this in the future . I thought i am consuming just 220mA and the LDO can take up to 1.2 A , i should probably be good with some copper area as a heat dissipation area .
4) As of now if i don't have to fabricate a new PCB , i am already on my way of testing the series resistor to share the heat dissipation by dropping the input voltage . Would you recommend adding a heatsink on top of it ? Or what can be some other methods or suggestions you can give which do not involve a new design .
And would you consider this a design "mistake" or just an "oversight" because we didn't have tools for thermal simulations and all to figure this out earlier before fabrication.
2024-11-05 02:08 AM
2024-11-06 10:06 AM
Hey so the resistor Idea worked for me and the heat is much better distributed between the series resistor and the LDO . From the next time I'll try to improve the layout to use the LDO without the series resistor