LM1084 Output current

That would be a Vin-Vout of 1.7V, just above the 1.5V minimum.

I would estimate the full load current to be about 6A.+/- 1A. It would not take too much variation in either the slope, or the differential voltage, to make a dramatic change in the available output current.

Hello Mr.Donald

Thank you for quick reply.

Then how much is minimum load current? 5mA?

I am looking forward to your reply.

Best regards.

Minimum load current depends on the version

For the -ADJ version : 5mA typical, 10mA maximum. The 10mA requirement is typically met by using 121 ohms for R1, then setting R2 as needed. (See Figure 4 in the datasheet) This allows any additional load to be as low as 0mA.

For the -3.3 version : 0mA

Hello. Mr.Donald

Thank you for quick reply.

In this case,used device is LM1084-ADJ.

How is a relation between Vin,Vout and minimum load current  ?

By any chance,external resistance is related ?

I am looking forward to your reply.

Best regards.

There is no signifigant relationship of 'Vin-Vout' to 'minimum load current' requirement.

Current through R1 & R2 string counts towards load current, and is defined as 'Vref / R1'.

In figure 4 the current through R1 (and R2) = (1.250V / 121 ohms) = 10.3mA, and that alone meets the minimum load current.

Hello,Mr.Donald

Thank you for reply.

This question may be elementary,however what is minimum load current ?

Mininumu consumption current ? The current when there is no load ?

Customer had a problem that output volatage wasn't steady,however they seem to change R1:121Ω to R1:220Ω and solved the problem.If you undesrstand this reason,would you teach me?

And you posted  "there is no signifigant relationship of 'Vin-Vout' to 'minimum load current' requirement." before,however when customer changed  Vin:5V to Vin:24V at R1:121Ω,output voltage seem to be steady.Could you teach me this reason?

I am looking forward to your reply.

Best regards.

Taichi

Q1 : "... what is minimum load current ..."

A1 : The LM1084 datasheet defines "Minimum Load Current" as : The minimum output current required to maintain regulation.

In other words : The minimum current from the output pin required to maintain voltage regulation.

If you look at the LM1084 Simplified Schematic (Figure 5, page 2), the output pin (aka Vout) is the internal common ground and all the bias current to get the bandgap and control loop into the linear range (up to 10mA) must flow out the output pin. This is the equivalent of the ground pin current in more traditional regulators.

This current MUST have a path or the LM1084 will not be properly biased internally. This 10mA current does not care where it goes, though R1/R2, through the Load, or through both.

Q2 : "... at R1:121Ω,output voltage seem to be steady.Could you teach me this reason?..."

A2: This was answered in my earlier post on 28-Feb.

The current rhough R1 and R2 = (Vref / R1) = (1.250V / 121Ω) = 10mA

Hello,Mr.Donald

Thank you for quick reply.

Maybe my question was wrong.

Q1: A diffrence of  external resistor

Output voltage isn't steable when the current  = 1.25V/121Ω ≒ 10mA.

Output voltage is steable when the current  = 1.25V/220Ω  ≒5mA.

Why is output volatage stabilized when load current down ?

Q2:A diffrence of input volatage

Output voltage isn't steable when Vin = 5V,R1 = 121Ω.

Output voltage is steable when Vin = 24V,R1 = 121Ω.

Why is output volatage stabilized when load current down ?

I am looking forwward to your reply.

Best regards.

Something is not right. The output should be stable with 10mA.

What is R2 value when R1= 121Ω, and R1= 220Ω? What is Cout type and value?

Please define/describe what is meant by "... Output voltage isn't stable ...". Is the output voltage drifting, is the output voltage oscillating?

Hello,Mr.Donald

Thank you for quick reply. 

I add an information about R2 ,Cout and output volatage.

Output voltage seem to be oscillating.

Q1: A diffrence of  external resistor

Output voltage isn't steable when the current  = 1.25V/120Ω ≒ 10mA.(then R2 = 200Ω,c = 10u)

Output voltage is steable when the current  = 1.25V/220Ω  ≒5mA. (then R2 = 360Ω,c = 10u)

Why is output volatage stabilized when load current down ?

Q2:A diffrence of input volatage

Output voltage isn't steable when Vin = 5V,R1 = 120Ω,R2 = 200Ω,c = 10u.

Output voltage is steable when Vin = 24V,R1 = 120Ω,R2 = 200Ω,c = 10u.

Why is output volatage stabilized when load current down ?

I am looking forwward to your reply.

Best regards.

 " ... What is Cout type ...?"

LM1084 has specific requirements for the output capacitance. If the requirements are not met the control loop will not be stable.

Hello,Mr.Jones

I think that there is a difference with same Cout and different resistors  so  Cout  type isn't  related.

How do you think about my opinion ?

I am looking forward to your reply.

Best regards.

Output voltage oscillation of  is typically due to low phase margin.