TPS7B69-Q1: thermal performance evaluation under load dump test

Hi Sherry,

So the calculations all seem fine. when computing thermal performance we generally compute it at a steady state reading which would be at the 12.5Vin, 69.6C. At the worst case conditions listed it would reach up to 187C which is 12C above thermal shutdown. Steady state thermal shutdown would be reached at 32.7Vin. Recovery would be reached at 28.2Vin where 150C is the temperature.

For this case It looks like this transience would only be at temperatures above shutdown on the order of 15-25ms and 20-30ms when above 150C. So overall the part should be completely fine. Heat is an energy reading so there is a time component to it, it has to saturate the part before damage would occur. Since the total overtemp Power dissipation time is fast, the part will be fine. The part may not even be saturated enough to enter thermal shutdown in that brief window, it seems like it won't get there for another 150ms, but our official position is that we cannot assure proper operation outside of the datasheet specs.

Even if the part is held at a much higher dissipation power, OverTempProtection will kick in to save the part from damage, although this obviously means regulation is halted until safe operation is once again assured. Once again I don't think it will trip on this timescale, but officially OTP will kick in to save the part, at the expense of the output's regulation

I will link below a great resource to send to them to explain ways to improve thermals on PCBs with LDOs:

Additional reading on how to improve thermal performance in LDO's

Hope this helps,

John Schneider

Hi Sherry,

Sorry if my response was unclear. What I meant to convey is that heat is not an instant change based on the wattage that is being consumed. The part, just like an oven or toaster, requires the energy to "saturate" or spread throughout it's mass, before the steady state temperature is reached. Just like a toasters coils do not instantaneously reach their top temperature, the part, while dissipating top wattage, will not be at that temperature instantly.  This is why heat sinks and thermal pads are important considerations for LDO's, the thermal mass allows the temperature to rise much more slowly, as it has more mass to spread through, and consequently the larger area allows dissipation to occur more quickly. Providing the design with two points of improved thermal performance.

Since we do not have robust data on how long that time to saturate is, I cannot give you specifics on how long it will take, but I can say that I do believe that because the overvoltage is only held for a very brief window(30ms max), based on the information you gave me, it will not allow enough energy to be consumed by the part to cause a overtemperature protection to kick in.

The part has built in OTP which means as long as ambient is not too hot, the part will protect itself, so that is not a concern

Regulation however could be halted if OTP happens to turn on and shuts the part off. For this they would need better thermal design on their layout in order to avoid OTP during a harsh transient condition.

Hope this clears up what I originally meant.

Regard,

John