Lt3942 intvcc power dissipation

Question

Page 20 in the data sheet states that the user should figure out power consumption on the intvcc pin, but gives no real guidelines. Why wasn&rsquo;t a chart prepared or some type of assistance with acceptable values? This seems like a real oversight on this part data sheet. 
 
 In my case, I want to use 24Vin and 2 MHZ switching speed.

MarkusHP · Answer

Hello Erik, 
 even without being an LT3942 expert: The operating conditions of this part might change so much in terms of overall power losses(LED current, PWM dimming, efficiency) and thermal relief (layout, amount and thickness of layers, exposed pad, amount of vias, defined ambient temperatures etc....) that these requested chart would be hard to establish or give meaningful infos. 
 For sure your 24V Vin AND 2MHz switching frequency is a 'bad' start for the internal LDO for the required power, but as said upfront with a good layout, or a smaller LED current, or PWM dimming you can do a lot to ease the thermal situation of the LT3942. But for sure it might be the case that your desired operating conditions are not doable in total in terms of power losses and you have to decrease one or more of your requirements in order to keep the junction temp of LT3942 in its allowed region..

MarkusHP · Answer

Hello Erik, 
 the LT3942 is for sure a topnotch part, but for every DC/DC converter the overall operating conditions play a huge part in the final answer if your initial requirements are doable or not. This is what I wanted to state with my answer. Worst case for the internal LDO you have (24V-3.6V)*40mA = 816mW Losses inside the part just from the LDO. You can establish the resulting deltaT based on the thermal resistances value from the datasheet info. There are some more contributors to the internal losses adding to the heating of the part and these contributors depend heavily on your operating conditions: 
 
 Output current 
 Dim ratio 
 
 And on top Layout and operating ambient temperature come into play as well. How should we deliver a meaningful diagram with so many parameters? 
 
 Please go ahead with some LTSpice simulations (Another top notch tool from our company) and get a demoboard for real world measurements. These two tasks will give you a good feeling if your final application is doable or not.

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Lt3942 intvcc power dissipation