LTC3105 minimum power required for startup

Hi,

What is the minimum power required for startup for the LTC3105? The datasheet shows a minimum startup voltage of 0.25V typical (0.4V max), but gives no indication of current/power requirements to move from startup to normal operation. In fact I couldn't find anything relating to the minimum current/power requirements for the source. The only reference I found was in the applications section where it states under the industrial current loop example:

"In this application, a large input capacitor is required due to the very low available supply current (less than 4mA)"

This suggests that a typical application should be supplying tens of milliamps and that the IC is not suitable for microwatt type applications (e.g. small single cell PV). Is this the case? I understand the ADP5091 (which provides detailed cold start power requirements in the datasheet) is aimed more at ultra low power sources, but I'd like to know where the cutoff is for the LTC3105.

Thanks.

  • +1
    •  Analog Employees 
    on Oct 24, 2018 4:19 PM

    Hi,

    You can refer to the graph for No-Load Input current vs Input Voltage, on page 5 of the data sheet of LTC3105. The minimum input power required for IC is high as it is primarily for solar cells with higher input current capability . Depending on your application requirement of minimum startup voltage, LTC3106(Can refer Page 6 of the datasheet for 'No-Load Input current vs Input Voltage' graph)  or ADP5091 could be better options as they are designed for very low power input sources.

    Thanks

  • Hi Anisha, I have a further questions about startup current.

    I tried reading the No-Load graph and it's unclear to me how to apply it in this case.

    I understand the graph to approximately be saying "the lower the input voltage, the higher the input current must be."  I guess you're saying that applies to startup.

    However, what I want to do is size a capacitor such that startup succeeds, and I want to know how to do that.  I can't tell how to do that with only this chart.

    If I have a 0.250V input voltage, what value capacitor do I need to ensure startup when using a 10uH inductor?

    I have tried a 22uF cap, and that isn't sufficient.  A 470uF cap was sufficient.  So it's somewhere between.  How do I calculate that value?

    Also, I'm using a 10uH inductor (the COILCRAFT MSS5131-103MX specified).  I see the spec also says, in the Component Selection section, "Low DCR power inductors with values between 4.7µH and 30µH are suitable for use with the LTC3105. For most applications, a 10µH inductor is recommended. In applications where the input voltage is very low, a larger value inductor can provide higher efficiency and a lower start-up voltage."

    Can you tell me how the situation changes with a 20 or 30uH inductor in use instead of a 10uH?  Is less current required using a 30uH at 0.250V than 10uH at 0.250V?  How much less?  How do I calculate that?

    Thanks.

    doug

  • 0
    •  Analog Employees 
    on Mar 13, 2019 4:31 PM in reply to douglas.malnati

    Hi Doug,

    Figure 1 on Page 9 of the datasheet gives the startup sequence of the converter. First VAUX and then VLDO needs to be charged up before Vout to start up.  So the input capacitance should be selected such that it should be able to charge up both AUX and LDO during the startup. Recommended values for capacitor at AUX pin is 1uF cap and LDO pin is 4.7uF. Input capacitance for LTC3105 can be calculated by finding the energy required by LDO and AUX cap to charge up. The formula below can be used to calculate the energy transfer,

    To calculate the capacitance it is recommended to take worst case values, such as efficiency=20% at no load during startup, VAUX=20% higher than VLDO, Ven=90mV.

    So for Vout=3.3V, VLDO = 2.2V and considering startup voltage=0.25V, input capacitance of 1350uF or higher is recommended.

    Thanks

    Anisha

  • Hi Anisha, thank you for the reply.

    I will look to make the change regarding the capacitor.

    Separately, can you please also answer the question I had about the inductor?

    Thanks.

    doug

  • 0
    •  Analog Employees 
    on Mar 13, 2019 8:31 PM in reply to douglas.malnati

    Hi Doug,

    10uH is the recommended inductor value. However using a higher inductor value reduces the number of switching as the output current slew rate becomes slower. This will reduce switching losses and might have slightly better efficiency. However the increase in efficiency might not be very significant.

    Also I have a correction to make in the capacitor calculation mentioned above. The input cap value was calculated with efficiency factor of 40%.

    For starting up at Vin=.25V, it is recommended to program the MPPC pin value for Vin<0.225V.

    Thanks

    Anisha