Recommended LTM8024 frequency in use case and VIN2 connection

Question

Hello, 
 
 Could you, please, provide me the answers to some questions regarding LTM8024? 
 
 Here are the conditions at which I want to use it: 
 VIN1=12V car battery 
 VOUT1=3.3V @3A max average 
 VOUT2=1.2V @1A max average 
 BIAS connected to AUX1=3.3V 
 
 1. Would it be on the edge if I connect VIN2 to VOUT1 or it would work? What would be your recommendation regarding the connection in this case? 
 
 2. What would be the recommended value for RT (or switching frequency) and is it the same if: 
 a) VIN2=VIN 
 b) VIN2=VOUT1? 
 
 3. Would VOUT1 one have a start-up issues if the load input capacitance is ~270uF in addition to the Buck Cout recommended in the datasheet (VOUT2 would be disabled during this period by the means of RUN2 pin)? 
 
 Thank you in advance! 
 
 Best regards

MarkusHP · Accepted Answer

1. LTM8024 channel 1 can deliver enough power to serve as input to channel 2 as well. I would check efficiency+thermals for both scenarios before deciding on teh preferred setup. 
 2. With VIN2=Vout you can use higher frequencies as step down ratio/Duytcycle is not at the edge. For example 3MHz are not doable for 12V down to 1,2V. But 3MHz are doable for 12V down to 3,3V and 3,3V down to 1,2V. It's your choice depending on other constraints (efficiency, thermals, externals,...) You should start with the datasheet examples. There will be a reasonable large frequency range left which is doable from timing and general operation. I think 1MHz is a good starting point. 
 3. Startup with additional capacitance will be delayed, especially as LTM8024 offers frequency foldback in overcurrent/startup operation.But I do not expect any general issues. 
 LTSpice will be of great help to get more accurate answers to my statements (on all your questions) 
 
 kind regards 
 Markus.)

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Recommended LTM8024 frequency in use case and VIN2 connection