Recommended LTM8024 frequency in use case and VIN2 connection

  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
Parents
  • +1
    •  Analog Employees 
    on Jun 21, 2021 12:26 PM

    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.

Reply
  • +1
    •  Analog Employees 
    on Jun 21, 2021 12:26 PM

    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.

Children
  •   Thank you for your fast asnwers and suggestions on starting values! I will definitly simulate it more in LTspice.

      My biggest concern was that 3.3V was to low for VIN2, even though it is stated that when VIN1 is above 3V, it is sufficient if VIN2 is above 2V. 

      Regarding capacitance, delay due to foldabck is not an issue, it is important that it is not stuck due to some protection.

      Kind regards,

      Nikola