I'm working on programmable power module based on LT3763. The main power input is +48 Vdc and I have on board also +7.5 V dc.
My question: is it possible to power LT3764 Vin with mentioned bias of +7.5 V while power MOSFETs is powered by +48 Vdc? Idea is to reduce dissipation of internal regulator. Vout should be from 0 to +20 V (i.e. above bias voltage).
Many thanks in advance,
Yes you can use the 7.5V rail to power the IC
Base on our simulation connecting IVINP and IVINN to 48Vin or 7.5Vin does not affect its performance or introduce additional heat.
The way that the component is mounted on the board has a big impact…
In the meantime I've tested this in practice and can say that I cannot or, I can but Vout cannot be higher then what is applied on the Vin pin. In the process I was managed to burn two LT3763 and hi-side MOSFET trying to push Vout under load over +7.5 V.
When LT3763 works under load (e.g. output current is 2 A) its package temperature quickly rise above 100 oC. External fan cooling can holds it below thermal shutdown threshold but I'm concerned about long term reliability. Perhaps 2-layer PCB is not enough and 4-layer with huge internal ground plane is required for efficient thermal dissipation.
I'd like to learn more how to use lower the Vout voltage on Vin. Is it possible that some extra circuit is needed?
The IVINP and IVINN are left connected to the main power in (i.e. +48 Vdc) since I'm not using them. Perhaps that is a main reason for failure: a great difference between that inputs an Vin on the +7.5 V? Should I tie them also to the +7.5 V?
Please find the schematics here (Pg.3 or 4).
The way that the component is mounted on the board has a big impact on thermals. Make sure that the ground pad is properly soldered on the board.
The PCB layout also has effect on this. Can you share your PCB file ?
This project is open source and all it details are available, like Gerbers, Eagle files and images of both PCB sides.
Thanks for mentioning simulation. Please find in attachment LTspice where Vin is set to 8 V and 48 V. It's clearly seen that with lower Vin it cannot reach set output voltage. I've tested with IVINP and IVINN tied to 8 V, 48 V or just tied together but that didn't make any change.
I forget to say the LT3763 thermal pad is well soldered to the ground plane. Please also note that I've tested only the "top" of two channels that has better ground plane then the "bottom" channel.
I'll try the different approach here and that is Vout "tracking".
LT3763 Vin,min is 6 V therefore we need offset of at least 6 V, i.e. for Vout, Vin has to be 6 V or higher. With tracking a considerable part of total dissipation is moved to external BJT that can be easier to cool. Number of extra parts are minimal and simulation results are promising. Now I have to check that in practice.