Question about Heating / Radiant Heat condition of LTM4611

Question

Hello, 
 My customer is reviewing to use LTM4611 in their product. 
 For this, they are considering the heating condition of LTM4611. 
 Q1) Would you provide materials (simulation/test result/......) about the heating/radient condition of LTM4611? 
 And they will use a heatsink ( 375424B00034G of AAVID compamy) which is recommended by ADI(LTC). 
 So they want to know if they only add this heatsink to LTM4611, they can get a stable power supply and reduce power consumption. 
 - Their Max. load conditions are like below. 
 1. Input 4V 
 2. Output 2V and 11A 
 Q2) Would you advise whether they can get a stable power only to add a heatsink (your recommended in Datasheet)? 
 Q3) Would you provide materials (simulation/test result/......) about the heating/radient condition of LTM4611 at their max. load condition? 
 Please advise me. 
 Regards, 
 Se-woong

SYoung · Accepted Answer

Hi Se-woong, 
 
 On the datasheet's page 20 there are measured curves available for load current de-rating as reference. Based on your expected max. ambient operating temperature, as long as your junction temperature does not exceed the max (125C) then thermally your operating condition should be fine. 
 
 For example based on available de-rating curves for similar conditions (figures 13-14, 3.3Vin to 2.5Vout) with heat-sink and similar PCB layout construction (demo board DC1558A) the maximum ambient temperature allowed for an 11A load is ~85C with no forced airflow and a little over 90C if airflow is available. Normally that is good amount of margin for most applications. If your expected max. ambient temperature is below 85C and the layout has decent thermal resistance (ie demo board) then thermally the operating condition would be fine.

For your specific conditions (4Vin to 2Vout) loss data is not readily available, it would be recommended to evaluate the module performance using the module's demo board. Otherwise a rougher estimation can be made based on datasheet efficiency curves for similar conditions and some quick calculations based on the thermal resistances provided. Pages 17-19 also give more detail on how these estimations can be made. 
 
 As another quick example estimation based on efficiency curve data for another similar condition (5Vin to 1.8Vout) at 11A the efficiency at room temperature is ~89%, so loss is ~2.45W. When the junction temperature is at maximum (125C) the loss goes up by approx. 1.4x to ~3.43W. From page 21 the junction-to-ambient thermal resistance with BGA heat-sink reduces to ~12C/W with no airflow, so the max allowed operating ambient temperature for this condition would be 125C - 12C/W x 3.43W ~ 84C. If up to 400LFM were available then max ambient can go higher to ~94C. 
 
 Based on the above if your actual board has a decent (ie similar to demo board, adequate copper & vias) thermal resistance and max. expected ambient temperature is not higher than ~80C or so, then without airflow your operating condition should be fine for the LTM4611. Again the demo board provides the best reference for evaluating the LTM4611. 
 
 Thanks.)

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Question about Heating / Radiant Heat condition of LTM4611