how to use an LT1618

[hoopa]

Hello,

For a project I have to use an lt1618.
My Vin = 4V with 1.1A and I want to convert that to Vout = 5V in 0.6A.

I first used this design and with the right R1 and R2 i'm able to get Vout =5V but the issue is i can't regulate my current at all, i'm getting Iout=Iin.

I'm guessing i'm using the wrong design.
Can anyone help me and tell which application is the i need?
Here a link to the datasheet.

--- Updated Apr 28, 2022 ---

https://www.analog.com/media/en/technical-documentation/data-sheets/1618fas.pdf

 

[KlausST]

Hi,

you don´t "regulate" the current.
This is a fixed voltage output supply .. and the current is deteremined by the load (resistance).

The given "0.6A" is the maximum you may / should draw from this supply.

Klaus

 

[barry]

Hello,

For a project I have to use an lt1618.
My Vin = 4V with 1.1A and I want to convert that to Vout = 5V in 0.6A.

I first used this design and with the right R1 and R2 i'm able to get Vout =5V but the issue is i can't regulate my current at all, i'm getting Iout=Iin.

I'm guessing i'm using the wrong design.
Can anyone help me and tell which application is the i need?
Here a link to the datasheet.

--- Updated Apr 28, 2022 ---

https://www.analog.com/media/en/technical-documentation/data-sheets/1618fas.pdf

--- Updated Apr 28, 2022 ---

You can't regulate your current because what you have there is a VOLTAGE regulator. The output current is a function of your load.

 

[hoopa]

Oh so i'm using the wrong regulator, i can only fix my Vout. My Iout is based on the load.
Is there another convertor that could do that? Regulate Vout and Iout at the same time?

 

[barry]

Oh so i'm using the wrong regulator, i can only fix my Vout. My Iout is based on the load.
Is there another convertor that could do that? Regulate Vout and Iout at the same time?

I can't think of a scenario where you'd want that. Can you tell us what you're trying to accomplish?

 

[hoopa]

I can't think of a scenario where you'd want that. Can you tell us what you're trying to accomplish?

It's a school project.
I wanted to use the lt1618 as a led driver powered by a battery of 4V at 1.1A.
To increase the autonomy of my system i was thinking of limiting my Iled.

 

[KlausST]

Hi,

Either you make a "constant voltage" supply or a "constant current" supply.
You can´t have both at the same time.

Since there is ohm´s law.

Examples:
you may build a 5V (constant voltage supply)
* connect a 100 Ohms load and get 5V/100R = 50mA current
* connect a 10 Ohms load and get 5V/100R = 500mA current
* connect a 1 Ohms load and get 5V/100R = 5000mA current but only theoretically. It is overloaded.

you may build a 100mA (constant current supply)
* connect a 1000 Ohms load and get 1000 Ohms x 0.1A = 100V. but theoretically only. You may distroy your supply by overly high switching voltage.
* connect a 100 Ohms load and get 100 Ohms x 0.1A = 1V ... will work
* connect a 1 Ohms load and get 1 Ohms x 0.1A = 0.1V ... theoretically only, because the boost converter can´t regulate much below input voltage. The expectable ouput voltage may be about 3.4V and 3.4A killing your diode or coil.

Klaus

--- Updated Apr 28, 2022 ---

It's a school project.
I wanted to use the lt1618 as a led driver powered by a battery of 4V at 1.1A.
To increase the autonomy of my system i was thinking of limiting my Iled.

No, you don´t design a LED supply by it´s input current.
You design it to match the LED = output current.

Klaus

 

[hoopa]

No, you don´t design a LED supply by it´s input current.
You design it to match the LED = output current.

Klaus

Then the best would be to use the LT1618 as a constant current supply and not a constant voltage supply if I want to limit my led and increase my autonomy?

 

[barry]

Then what you want is a voltage regulator (as you have) with a current limiter. You might be able to find one with built-in current limiting, or add it as an external circuit.

Also, there are dedicated LED drivers...

 

[hoopa]

Then what you want is a voltage regulator (as you have) with a current limiter. You might be able to find one with built-in current limiting, or add it as an external circuit.

Also, there are dedicated LED drivers...

I don't get the choose what i can use, i have to use what i was assigned.

I was taking the project from the wrong end, i understand know what i should do.
Thanks to both of you!

 

[cupoftea]

You can use the LT7805 which will regulate vout, and then clamp iout to a set maximum if you want.
Download the free LTspice simulator and put your cct in there.
If you need help i will do it for you.

What is your LED load?.......Is it a 5V LED string?.....you know you cant drive a led voltage less then vin from a boost converter.

 

[KlausST]

Hi,

ou can use the LT7805

As I understand the challange is to use LT1618.

Klaus

 

[cupoftea]

Thanks Klaus...Hoopa's project is now done, using LT1618, as in the attached LTspice simulation.....its a free sim you can download from analog.com

As others have said, there are other chips which will do this with far less components...i think microchip and diodes.com have drivers which can do 2 leds at 600mA from 4vin very easily.

 

[Easy peasy]

A boost converter cannot limit current - look at the circuit - if you short
the output only the sense resistor and the diode and the choke present some small
resistance to a lot of current flow from your source - there is no controlled series
pass device - add a large BJT as a series current limiter after your booster - OR,
design a buck that operates from a higher source - a buck can have current limit

 

[cupoftea]

Yes good point from Easy Peasy.....here it is re-done, this time with LT1618 as a SEPIC..in LTspice again...Sorry its a bit rough.....ill come back and tidy it up if you are interested?...again attached in LTspice....it doesnt start settling out till after 6ms..........sorry, i just guessed the Vref level in the LT1618.....does work though...settles out at 600mA.

iF f you want it re-done with one LT1618 instead of 2...ill come back and read the LT1618 datasheet and re-do it...if you want?

 

[KlausST]

Hi,

My personal opinion:
It's a school project ... they should learn to find solutions on their own. We should discuss problems and give hints about "why" and "how to solve them".
But not give a detailed and completely developed solution. This prevents from learning something.
Doing internet research, spending time, gaining knowledge, getting satisfied by finding a solution ... even if it's not perfect...

Otherwise it's similar to buying a LED driver circuit from Amazon...

Klaus

 

[hoopa]

Thx everyone, i couldn't answer during the day.
After your advices and a day of trying this is what I made..

Vin is my battery at 3.7-4.2V.
By setting R1 at 300k and R2 at 100k I get L+ (my led power supply) at 5V and by setting R4 at 0.25 I limit my Iout at 250mA. It works and to the job.

In a concern for autonomy i want to shutdown my LT with my system when i'm not using it.
I pilot the shutdown with a simple MAX16054.

But here I have a new issue, when I use my LT1618 the whole system is working great i have Vled at 5V and my Iled at 0.25 but when i shutdown my LT i get for some reason Vled=Vin, as if shuting down the LT turned it into a simple wire.

 

[hoopa]

You can use the LT7805 which will regulate vout, and then clamp iout to a set maximum if you want.
Download the free LTspice simulator and put your cct in there.
If you need help i will do it for you.

What is your LED load?.......Is it a 5V LED string?.....you know you cant drive a led voltage less then vin from a boost converter.

Ty for LTspice, i didn't know it and it's really useful!

 

[KlausST]

Hi,

But here I have a new issue, when I use my LT1618 the whole system is working great i have Vled at 5V and my Iled at 0.25 but when i shutdown my LT i get for some reason Vled=Vin, as if shuting down the LT turned it into a simple wire.

this is what a typical boost converter does.

See the path:
Vin --> R4 --> L1 --> D1 --> L+
there is no switch to shut down the output.

/SHDN just shuts down the BOOST part, indeed most parts of the IC to reduce IC current consumption.

.. but there is not "switch output off"

A simple P-Ch MOSFET will do.
Or an N-Ch Mosfet in GND path of the LED.

Klaus

--- Updated Apr 29, 2022 ---

added:

BTW: R4 just limits the switch current. Thus the output current will vary with input voltage.

 

[hoopa]

Hi,

this is what a typical boost converter does.

See the path:
Vin --> R4 --> L1 --> D1 --> L+
there is no switch to shut down the output.

/SHDN just shuts down the BOOST part, indeed most parts of the IC to reduce IC current consumption.

.. but there is not "switch output off"

A simple P-Ch MOSFET will do.
Or an N-Ch Mosfet in GND path of the LED.

That is what I thought.
I was thinking of using a P-ch MOSFET on Vin before the IC, i could pilot the mosfet with the on/off controller. One pushbutton would begin and shut my whole system.

BTW: R4 just limits the switch current. Thus the output current will vary with input voltage.

Yes but my input voltage variation isn't that big I don't think it will cause an issue.