Lt3942 intvcc power dissipation

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..

intvcc only provides power for internal circuitry and gate drivers, see page 20.  There isn't enough info about the ldo and chip usage to make an educated guess as to proper use.. This is an LT part after all, LT parts and datasheets are usually top notch.

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.

In my mind the missing information is this, that there is no info or estimate at all related to internal power consumption.  At minimum I would expect the development board to have this information detailed.  The D3103A dev board is rated from 3 to 36V, so is it in reality out of spec then?  There isn't even a good way to measure internal current draw, so not sure how the end designer is supposed to arrive at something meaningful.

In my mind the missing information is this, that there is no info or estimate at all related to internal power consumption.  At minimum I would expect the development board to have this information detailed.  The D3103A dev board is rated from 3 to 36V, so is it in reality out of spec then?  There isn't even a good way to measure internal current draw, so not sure how the end designer is supposed to arrive at something meaningful.