LDO Voltage Regulator Current Source Not Working As Expected.

Thread Starter

MattIsHere

Joined Dec 23, 2015
12
So everyone know famous LM317 can be configured as Current Regulator like this:



However the dropout voltage of LM317 is pretty high and so I am looking for some LDO chip to do the same thing. Here I got some RT9018 chip to do this:

http://www.richtek.com/Products/Linear Regulator/Single Output Linear Regulator/RT9018ART9018B.aspx

I have successfully configured RT9018 as Voltage Regulator like what it supposed to be. However I can't get it working as a current regulator. I connect the load exactly like what I do for LM317 but it just don't working. Is it not all the voltage regulator can regulate current or is there something I am missing? Thanks for helping.
 

dl324

Joined Mar 30, 2015
11,573
Your first post. Welcome to AAC!

Post a schematic for the circuit that isn't working and give current and load requirements.
 

GopherT

Joined Nov 23, 2012
8,012
So everyone know famous LM317 can be configured as Current Regulator like this:



However the dropout voltage of LM317 is pretty high and so I am looking for some LDO chip to do the same thing. Here I got some RT9018 chip to do this:

http://www.richtek.com/Products/Linear Regulator/Single Output Linear Regulator/RT9018ART9018B.aspx

I have successfully configured RT9018 as Voltage Regulator like what it supposed to be. However I can't get it working as a current regulator. I connect the load exactly like what I do for LM317 but it just don't working. Is it not all the voltage regulator can regulate current or is there something I am missing? Thanks for helping.
Are you connecting Vin to the EN pin? The EN pin is "enable" so the regulator can be turned on/off. It myt have a voltage applied to make the chip work.
 

Thread Starter

MattIsHere

Joined Dec 23, 2015
12
Are you connecting Vin to the EN pin? The EN pin is "enable" so the regulator can be turned on/off. It myt have a voltage applied to make the chip work.
According to the datesheet:
"For RT9018A, the EN pin function pulls high level internally. So the regulator will be turn on when EN pin is floating."

I have successfully configure it as a voltage regulator so the problem is changing it from voltage regulator to current regulator. I am not sure if all the voltage regulator can be configured as current regulator.
 

GopherT

Joined Nov 23, 2012
8,012
According to the datesheet:
"For RT9018A, the EN pin function pulls high level internally. So the regulator will be turn on when EN pin is floating."

I have successfully configure it as a voltage regulator so the problem is changing it from voltage regulator to current regulator. I am not sure if all the voltage regulator can be configured as current regulator.

It is possible that the adj needs a ground reference (directly or through a resistor), so it would not be able to work as a current source.
 

Thread Starter

MattIsHere

Joined Dec 23, 2015
12
It is possible that the adj needs a ground reference (directly or through a resistor), so it would not be able to work as a current source.
You maybe right. I included the block diagram for you to determine the problem better:

block.JPG

If you are right. I think I have to look at another chip to do the job. How could I know whether the adj need a ground reference? Can it be determined by looking at the block diagram?

Thanks
 

Thread Starter

MattIsHere

Joined Dec 23, 2015
12
Hi Dennis,

I am using this LDO as a LED drive (LED Vf about 3.3v). I am planning to use single li-ion cell and power source (3.7-4.2v), I want to get the maximum current at about 1A. Thanks
 

dl324

Joined Mar 30, 2015
11,573
You don't have enough headroom for even a simple current source. Even at full battery voltage of 4.2V, a medium power transistor in a current source would be near saturation @ 1A. Not enough voltage to consider a MOSFET.
 

Thread Starter

MattIsHere

Joined Dec 23, 2015
12
Hi Dennis,

As I said before. I am using a 5V power supply which provide enough headroom when I test the circuit. I don't really care about the input voltage as I can use 2 cell. I just want to get the LDO working as a current regulator now. Thanks
 

dl324

Joined Mar 30, 2015
11,573
As I said before. I am using a 5V power supply which provide enough headroom when I test the circuit.
It wasn't clear to me that this was just an exercise to use the LDO as a current source.
I don't really care about the input voltage as I can use 2 cell. I just want to get the LDO working as a current regulator now.
It won't work. The regulator you've chosen just uses the resistor divider to feedback a portion of the output voltage to the control logic. It doesn't work like an LM317.

There are ways to build a current source that don't require a voltage regulator. That's where I'm trying to guide you. But you don't have a high enough voltage for any I can think of.
 

Thread Starter

MattIsHere

Joined Dec 23, 2015
12
It wasn't clear to me that this was just an exercise to use the LDO as a current source.
It won't work. The regulator you've chosen just uses the resistor divider to feedback a portion of the output voltage to the control logic. It doesn't work like an LM317.
How can I know whether the regulator I choose can limit the current like LM317?

There are ways to build a current source that don't require a voltage regulator. That's where I'm trying to guide you. But you don't have a high enough voltage for any I can think of.
I don't understand . LDO only have around 0.2-0.3v drop at 1A. With a fully charged cell (4.2v) why it cannot provide enough headroom for 3.3Vf LED? (4.2v-0.3v=3.9v which > 3.3v)
 

dl324

Joined Mar 30, 2015
11,573
How can I know whether the regulator I choose can limit the current like LM317?
Read the datasheet.
I don't understand . LDO only have around 0.2-0.3v drop at 1A. With a fully charged cell (4.2v) why it cannot provide enough headroom for 3.3Vf LED? (4.2v-0.3v=3.9v which > 3.3v)
The LDO you selected doesn't have a constant current mode like LM317, so the fact that it only drops 0.3V@1A isn't significant. It regulates voltage, not current.
 

crutschow

Joined Mar 14, 2008
25,450
I think a basic 3-terminal LDO regulator such as the LT1086 with a 1V drop should operate similarly to the LM317 as a current regulator.
But note that the minimum operating voltage drop would still be about 1V plus the 1.25V reference voltage or 2.25V.
If you want a lower dropout than that, you may have to go to an op amp/transistor type of constant-current circuit which can reduce the dropout to well less than a volt.
 
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