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.
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.
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?
I want to know how to prevent startup before achieving 1.5v. Is that possible?
Ultimately I am trying to understand what triggers startup because I want to make sure I'm sizing my parts properly and I'm afraid of getting partly through a startup condition and getting stuck. I have observed a partial startup condition before where high current is drawn by the LTC3105 and startup doesn't complete.
My question surrounds the formula you provided for calculating the input capacitor size.
Your formula, when calculating for a 0.250V startup results in a cap of 1350+ uF.
That same formula, calculating a VIN startup target to be 1.5V, results in a cap of 33uF.
The 33uF cap stores considerably less energy than the 1350uF cap.
The 33uF cap will charge to 0.250V very rapidly, and store very little energy when it reaches that voltage.
At what point does the LTC3105 attempt to start? When the input voltage is 0.250? Something else?
The LTC3105 can't know VIN, it can only know the cap voltage, which in both the case of VIN=1.5 and VIN=0.250 the voltage sensed by LTC3105 will read 0.250V at some point.
My concerns are that when VIN=1.5, the 33uF input capacitor will charge to 0.250 and the LTC3105 will attempt to start at that moment (prematurely), and it will fail, due to insufficient stored power in the capacitor.
I realize 0.250V may not be literally the voltage which starts the LTC3105, but I don't know what the correct value is.
Can you please reply to the questions I've asked above so that I can understand the issue?
I hope I have made clear my concerns and what I don't understand.
Please let me know.
There is SHDN pin which can be controlled externally to enable/disable IC. You can pull down this pin if you do not want to enable the IC before 1.5V. You should take care in selecting the device for controlling the SHDN pin. It should have very low leakage current else the internal pull up resistor to enable the converter will become low and the part will never startup.
Answering to your concern above, LTC3105 can start up with 0.25V Vin but first charges AUX. In the startup mode the part only charges up the AUX. When AUX is higher than 1.4V, AUX starts charging LDO and once LDO voltage is charged to regulation, AUX will start charging VOUT. If Vin is 1.5V the input cap of 33uF, the input cap will charge up to 1.5V.The AUX voltage charging will be faster and VOUT pin will not start charging unless AUX voltage is higher than 1.4V and LDO is in regulation. I do not think there should be any concern in starting up with 1.5V Vin with 33uF input cap.
I would recommend you to read the operation of the IC in page 8-9 of datasheet. Since the part has a very low startup voltage, its operation modes are different than the usual startup.
Thank you, I will try it.