LT3083 as current booster

Before I completely give up on LT308X parts maybe I can try to use one LT3083 as I need 4.5A & hoped that this was also a better way of using the LT3083 as using it in voltage out mode is proving problematic - it's not regulating correctly

According to the LT3083 datasheet this example gives 4.5A current output @ 3.3V

Two questions:

- I presume I could use this with an any adjustable voltage regulator - it doesn't have to be fixed voltage output & there's no special adjustment to make for this?

- the LT3083 is operating in current mode which is determined by the connection of the SET pin through 20mohm to OUT pin through 10mohm R. But the SET pin has 33K R to ground just as if it was operating in voltage mode (2.5V output, I believe) - so what is this 33K for - if the LT3083 was operating in voltage out mode (3.3V) this would be 66K, not 33K ?

According to the LT3083 datasheet this example gives 4.5A current output @ 3.3V
[edited by: dweeb99 at 12:08 PM (GMT 0) on 8 Feb 2020]

Top Replies

  • 0
    •  Analog Employees 
    on Feb 10, 2020 5:33 PM over 1 year ago

    You can use any LDO you like to replace the LT1963-3.3.  The 33k is just a 1mA preload as far as I can tell.  The LT1963-3.3 can supply 1.5A maximum, and the LT3083 will supply twice the LT1963-3.3 current due to the 20mOhm to 10mOhm ratio.

  • Thanks but I looked into LT3081 datasheet & the same current boost diagram is there but 33K is now 6.2K for same LT1963 3.3V output so don't think it's a preload of 1mA

    The 42ohm for LT3083 is 8.2ohm in LT3081

    And 10mohm on OUT pin is now 20ohm in LT3081

    Anybody have a handle on the configurations? I would like to scale this up for different output voltages but need to understand the role of surrounding components

    It really is a pity that there isn't more explanation of the workings of these examples in the datasheet - one explanation would have saved a lot of questions on what are relatively expensive parts!!

  • Getting closer to the answer, I believe:

    3.3V through a voltage divider of 8.2ohm & 6.2K ohms gives a voltage of 3.296V i.e drop of 4mV on LT3081
    3.3V through voltage divider of 42ohm & 33K ohm gives a voltage of 3.296V i.e drop of 4mV on LT3083

    So this 4mV is on the SET pin in both cases

    Not sure why the same voltage divider isn't used on both configs to give the same 4mV drop?
    Is it because of the different max current outputs of LT3081 (1A) Vs LT3083 (3A)? But how, exactly?

    Is there some compliance I'm not aware of?

  • Still working my way through this but NOT making progress!

    I'm surprised nobody already has a handle on this or used it in this way (or wants to offer advice/help)?

    Here's the datasheet text on using LT3081 as a current source (not in other LT308X datasheets, AFAIK)

    The 50µA reference current from
    the SET pin is used with one resistor to generate a small
    voltage, usually in the range of 100mV to 1V (200mV is a
    level that rejects offset voltage, line regulation, and other
    errors without being excessively large). This voltage is
    then applied across a second resistor that connect from
    OUT to the first resistor

    So, apart from the 4mV reduction in the above voltage dividers, there's also the text above to work through.
    To achieve 200mV across Rset, it needs to be 4.0Kohm & 1V from 20Kohm Rset - so Rset between 4K & 20K are the min/max guardrails

    So for the 8.2ohm/6.2Kohm voltage divider, I'm not sure how to treat this? Does the 50uA still apply on Rset or is it a higher current based on the 3.3V through the 6.2Kohm to ground? I reckon this would give 530uA added to 50uA for 580uA. Now running this through 8.2 wouldn't give anything between 200mV & 1V

    Now running this through 8.2 would NOT give anything between 200mV & 1V

    The 3.3V isn't going through a voltage divider of any note to reduce the 3.3V to between 200mV & 1V

    What is Vset in the scenario of the current booster schematic?

    Can anyone steer me in the right direction?

    I'm surprised there isn't more support on this??

  • Well, it's working when powering a circuit that draws current but when charging battery or supercapacitors it does not regulate even when I have resistor from Out to GND drawing 5mA current.

    I'm pretty much ready to give up on these parts.

    Can't say there's much support here which surprises me