Buck - Boost design using LTC3112


we have a requirement to design Buck-Boost converter which takes 2.5V to 12V as input and generates 3.3V @ 1A to 2A.

This has 2 scenarios of operation.


default mode:

Input voltage : 10.8 to 12.6V, Typ-12V

Output: 3.3V @ 1A rare peaks 2A

Scenario 2 : 

When main power lost auxillary power comes to the Input

Input voltage : 2.5V to 5.5V- Typ is 4.6V.

Output : 3.3V @ 1A

I see LTC3112 suits well to this application.

I couldn't able to find LTSpice application circuit for quick validation.

Here are my questions:

1. Is this suitable device for this input range? if not suggest alternate

2. I see Output current is dependent on duty cycle in boost mode - I need data on at what input voltage in this range (2.5V to 5.4V), LTC3112 can supply max load current and how much is the ~current.

3. If you can provide a LTSpice circuit - it will be helpful for quick check. I tried to change the values in demo circuit but without success

  • I see LTC3119 more opt for this.

    Please answer for Questions 2 & 3 keeping LTC3119 as regulator.

  • 0
    •  Analog Employees 
    on Oct 24, 2019 8:57 PM in reply to mk365


     LTC3112 starting operating voltage is 2.7V so it might not be apt for the application.

    Given the operating range for the application, LTC3119 is a suitable part. Since the boost ratio is not much in this case, LTC3119 should be able to support the required max load even while in boosting. For your reference, on page 9 of the datasheet, there are efficiency graphs for various output voltage wrt load current at different input voltages. This would give you an idea of the max current capability at a desired input voltage.

     LTspice has an application circuit for LTC3119 in its library. First add component LTC3119 in the LTspice schematics page --> After adding right click the model --> Select first option i.e. 'Open this macromodel's test fixture'. This will open the application reference circuit which you can update and simulate based on the requirement. Page 27 and 28 of the datasheet has few application circuits for 3.3V output voltage. You can use these values to simulate the circuit.

    Alternatively, we have demo board DC2129A available for LTC3119. You can use the demo board for your application after configuring it for the requirement.

    Let us know if you have further questions.


  • OK,

    I will design the required configuration using LTC3119 and come back.


  • I see a graph for Boost ratio vs Output current in Page 10.

    Is the same graph available for Output 3.3V. or can we use this same graph for 3.3V output case also.

  • 0
    •  Analog Employees 
    on Nov 7, 2019 2:27 PM in reply to mk365


    That graph is  for Vout=12V. Since boost ratio for your application very small (boost from 2.5V to 3.3V) , the part should be able to support the max load current(1A). If you wish to know the max current for 3.3V out at 2.5V input ,you can refer the first three efficiency vs load current graphs at page 9 for 3.3V out at different input voltages.