Hi.
Does it apply also when used as a current regulator ?
Am after a 300mA constant current IC capable of doing its work at 2V supply.
Which regulator should be used ?

 

Does it apply also when used as a current regulator ?

Yes.

Am after a 300mA constant current IC capable of doing its work at 2V supply.
Which regulator should be used ?

Don't know of any regulators that will operate down to 2V. You don't have much headroom, but you might be able to make one with a PNP transistor.

 

Hello,

Do you intend to use a regulator as constant current source?
The LM317 datasheet gives this schematic:



The voltage drop will be the voltage drop of the regulator itself + 1.25 Volts.

Bertus

 

Thanks.
Yes, constant current source of 300mA from a supply voltage that will vary from ~4.2VDC to ~2.5VDC from a 18650 discharging.
Good to know, Bertus. 1.25V overhead. That should work.

 

1.25V overhead. That should work.

It won't work. The drop out voltage will be 1.25V plus several volts for the regulator.

At room temp and 300mA, you're looking at 3V overhead:

 

Hello,

You still will have to take the voltage drop of the regulator itself in account.
At 300 mA that will be about 1.7 Volts.



That and the 1.25 Volts accross the resistor, will give you a total drop of about 2.95 Volts.

Bertus

 

Hi.
Does it apply also when used as a current regulator ?
Am after a 300mA constant current IC capable of doing its work at 2V supply.
Which regulator should be used ?

What's your tolerance on that 300 mA?

How much temperature variation will there be for this circuit (other than self-heating)?

Do have another supply available to power the circuit, or does everything have to come from the 2 V supply?

 

An LM134/234/334 makes a handy constant-current source capable of operating with as little as 1.0 volt headroom, but by itself it only goes up to 10 mA. However, the data sheet shows how that can be increased by using an external pass transistor and a couple of other parts. See Figure 20 on page 11. This should work down to about 1.7 volts, depending on the Vbe of the transistor.

 

Thanks again, gentlemen, good assistance.
Tolerance of +/-10 % should be fine. Operating temperature will be ambient outdoor... like -25C to +50C. A single 18650 cell as supply.
The supply will never get as low as 2V, as it is under the discharge limit of the cell ~2.5V, but I was better looking for an IC capable of performing under that value (2V)

Trying now to refine the project, am looking to constant current regulate 300mA with a constant current regulator IC capable of ~1A as safety margin, with a shut-off pin,
'2V' input capable, perhaps LT3083 until I find something more promising.

Li-ion(+)-------------------------Current regulator-----------------------300mA device---------------------(-)
                                         |
                                         |
                                      on/off 
                                    control pin

 

perhaps LT3083

Below is the LTspice simulation of the LT3083 configured as a CC source.
With a 1Ω load, it generates a CC of 300mA down to 1.75V input with a 1.44V input-output voltage drop.

M1 provides a shut-off input (shown at 1 second).

 

Thanks, crutschow.
Ordered a few of these that are intended for LEDs instead for first evaluation before trying 3083's ; not down to 2.5V as wished but it is a near decent value of 2.8V. Load is 10 Ω. And $1 instead of $10 for LT3083. Will see...
----> http://www.consonance-elec.com/pdf/datasheet/DSE-CN5711.pdf

 

With a 10Ω load you need 3V to get 300mA! After the dropout voltage! So you will have trouble getting that with the battery fully charged.

Bob