LT8410-1 Booster Circuit Current Draw

Hi,

I'd like to get some support regarding Linear's LT8410-1 boost converter. 

More specifically I am measuring a large disrepancy between simulated current consumption (using LTspice) and the reality of my circuit. 

To be more precise the boost converter is set to convert about 3.4V to 13.25V or 26.5V. I am measuring about 150uA current drawn from my supply in the first case and about 3mA in the second case. LTspice simulates an average current draw of ~130uA in the first case and about 250uA in the second case. 

The situation is a bit more complicated but I would like to discuss this in private if possible with one of the support engineers.

Thank you for any support.

  • Hello Dulcevida,

    Thank you for your reply. 

    I have run the simulation and can confirm what you are saying but.....what started all this is in your very last sentence. 

    'That yields a duty cycle of <1%, for a very low average current from the battery.'

    When I run the simulation I am measuring the average current out of the source V1. This in the case of 50V output with 3.4V input is in the order of 200uA and generally agrees with bench measurements. 

    In the case of 100V output the simulation shows an average current of about 420uA coming out of the source. 

    Although the switch current may peak at 3 and 8mA respectively as you mention, the average source current is much much smaller than this. 

    The question then arises is the average source current that we should be looking at or the rms? The rms is shown as indeed much higher in the simulation and closer to the actual bench measurement (about 3.5mA both in simulation and bench.) However the rms when the output is 50V is simulating as 1.2mA which is way higher than in the bench measurement. 

    Also all of the above are with 4uA load at 105V output and 2uA load at 53V (due to load resistor). 

    When the load is removed, the average current out of the source drops to 300uA (in simulation) which is CONSISTENT with the datasheet value shown for quiescent current in regulation with no load for the part:

    At 34.5V output the current is shown as about 300uA.

    So my opinion is that there is still something wrong with my circuit that causes excessive current draw from the supply at 100V. 

    It would be great if you could help me more on this. 

    Thank you.

  • Also Dulcevida,

    An additional note:

    Take a look at the end of the datasheet of Lt8410. Essentially this is the circuit I used. According to the efficiency vs load graph at 9uA load the efficiency should be about 40% with VIN=5V and VOUT=100V.

    This means that Iin=450uA.

    Would there be such a deterioration in efficiency from Vin=5V to Vin=3.4V to justify an 6 fold increase in input current?

  • 0
    •  Analog Employees 
    on Jul 19, 2018 11:49 PM

    Hi Garibaldi, 

    I believe the LT8410 circuit you mentioned would work for you with no problem. This part has >3 times the switch current capability.

    Efficiency does degrade with lower inputs, but I think the main issue is the lT8410-1 just does not have enough juice to make it to the finish line, so it gets stuck peddling as fast as it can.

    Please try the LT8410 and let me know if that solves your problem. 

  • Hello Dulcevida,

    Thank you for your reply. 

    I selected LT8410-1 in the first place because of its 8mA current limit. I am only using a very small battery and would not like to drain it more than 8mA at anytime for the booster (since there are other loads on it).

    I don't think that using the LT8410 will have much different results. Response time will be better but I am not sure that consumption will be lower. The LT8410-1 does exactly what I need it to do. It has great response time and very good load and line regulation. My circuit does reach 105V from 3.4V which is also great. Also at 50V output the bench measurement matches simulation. 

    The question is very simple:

    the graph above shows the simulated final output voltage at 105V and the average battery draw at 308uA. The load is 4uA @105V.

    My bench measurement shows current draw of about 3.5mA. 

    The simulated switch voltage with no load can be seen below. The OFF time is about 830nsec:

    The actual bench measured switch voltage (blue trace) with no load can be seen below:

    The OFF time is more than a microsecond and switching frequency is less than a MHz. It looks to me that the part is operating in the discontinuous mode. 

    All of the above seem to be very consistent with the datasheet. Hence, either my circuit/layout has some other fault or the simulation is not reliable or the datasheet has some problematic information.

    Which one is true?

  • 0
    •  Analog Employees 
    on Jul 20, 2018 7:52 PM

    Hi, Garibaldi,

    The only evidence I have of this application working well is shown in the datasheet circuit, with lt8410. The included datasheet efficiency curve indicates the input current should be lower than 3.5mA with no load. We have not tested this circuit with the lt8410-1, and based on what I see from your bench test results it does not work well. Your bench test waveform indicates high ON time, and close to 1MHz switching frequency. It also indicates normal switching behavior, which implies to me the layout is probably not the issue.

    You indicate you want consumption to be lower, and I am saying you will get precisely that, if you copy the datasheet circuit as is.

    I will gladly offer additional assistance if you try the suggested lt8410 application and you find it does not work.