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LM317/LM338 Current Limit Mode

flaredelectronics

This should be pretty simple, but I want a clear answer before I assume and continue.

 

LM317/LM338 has two popular modes, Voltage Regulation (Positive), and Current Regulation(Negative)

317 = ~1.5A 338 = 5A

 

My question is, when in current limit mode, is the current sourced from the Middle Pin or the Adjust Pin.

 

The reason to ask is, between the middle pin and sense pin you put the resistance, but I want to know if all the current is sourced from the middle pin.

Specifically, if you want to source 5A from the 338, do you need a resistor capable of 5A. (Example 5v 5v R1=25W)

 

I am curious if it is feedback only or if you really must have a resistor capable of passing 100% current.

 

 

 

 

 

 

Extra Information:

Once I know, I am going to use a PTC/NTC or IRCL in place of the resistor. (ICL = 2.2ohm 3.7A max)

  • LM317 in current-source mode the sense resistor has 1.2 V across it, power = V * I, power comes from the output terminal

  • in reply to D_Hersey

    So just to confirm:

     

    317/338:

     

    Pin 1 Sense

    Pin 2/tab Output

    Pin 3 Input

     

    (the circuit, don't need to confirm the pinout)

     

    P3=VIN 5.000v

    P2----R1-----P1

    P2----R1-----Output

     

     

     

    R1: 1 Ohm (to make it easy, the concept is what I am after)

     

     

     

    Output:

    5V | 5A ~25watt (25 watt resistor required)

    ---or---

    3.8v | 3.8A ~12watt (12watt resistor required)

     

     

     

    I have a good understanding of electronics, but the datasheets I have read (338) have confused me a bit in this mode of operation.

  • 5 A * 1.2V = 6W

     

    I think that your power dissipation may be too high

  • Hi Keith,

     

    Yes your sense resistor must be able to handle the current which is supplied by the center output pin. As Don   stated the LM 317 has a 1.25 Volt difference between Vout and Adjust. If you use a 1.25 Ohm resistor for the sense your 317 will be a 1 Amp current source. (1.25 V / 1.25 Ohms = 1 Amp)  If the Sense resistor is 5 Ohms your current source will be regulating at 250 mA. (1.25 V / 5 Ohms = 0.25A). Since the sense resistor will always have 1.25 volts across it you will need a wattage rating that matches or exceeds the current you are regulating times 1.25 Volts. Don indicates that this would be 6 watts in the case of a 5 A current source and your sense resistor in this case would have to be 0.25 Ohms. ( 1.25 / 0.25 Ohms = 5 A)

     

    John

  • in reply to jw0752

    If you put a protection diode on each line, probably a shottky, you can parallel VRs in current source mode, IIRC.