# Does Max. Output current in 7815 depend on Vin and Vout difference ?

Thread Starter

#### gargrahul277

Joined Apr 5, 2016
15
Hi All.

Im building a 15 V DC power supply and was referring 7815 datasheet.
I came across this particular graph in datasheet (refer attachment) showing dependency of peak output current on input output voltage difference.

im trying to understand the graph.
So correct me if im wrong.

the graph shows the voltage difference between input and output that one should maintain to acheive particular current limit.
example :-
if i want my regulator to withstand a current of 2.5 A (let Tj=0 degree) then i have to maintain Voltage difference between 5 to 15
if the voltage difference goes below 5 or above 15 it wont be to handle 2.5 A through it.

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#### Ramussons

Joined May 3, 2013
1,417
You got it wrong. The voltage difference decides the allowable current - not the other way.

Below 5 Volts, the regulator "losses" are such that the available voltage cannot drive the load. (I=V/Load R).
Above 5 Volts, the current is Limited by the Permitted Power Dissipation - as the chart shows.

#### Dr.killjoy

Joined Apr 28, 2013
1,196
You are on the right track there..
Stop

Don't listen to me lololol..

Last edited:

#### Dr.killjoy

Joined Apr 28, 2013
1,196
You got it wrong. The voltage difference decides the allowable current - not the other way.

Below 5 Volts, the regulator "losses" are such that the available voltage cannot drive the load. (I=V/Load R).
Above 5 Volts, the current is Limited by the Permitted Power Dissipation - as the chart shows.
Whats wrong with what he said ??

Thread Starter

#### gargrahul277

Joined Apr 5, 2016
15
You got it wrong. The voltage difference decides the allowable current - not the other way.

Below 5 Volts, the regulator "losses" are such that the available voltage cannot drive the load. (I=V/Load R).
Above 5 Volts, the current is Limited by the Permitted Power Dissipation - as the chart shows.
thank you for the reply
my view was very application oriented : )
but now i have all my doubts cleared

i knew current was limited by permitted power dissipation
but was wondering why allowable current at ,say , 3 volt is less than that at 5 volt
its because 3 volt difference is not sufficient to drive load because of losses in regulator

#### #12

Joined Nov 30, 2010
18,224
Whats wrong with what he said ??
Thread Starter was talking about minimum drop-out voltage and showing the graph of the maximum voltage before the chip goes into safety shut-down. These are two different things.

#### Ramussons

Joined May 3, 2013
1,417
Whats wrong with what he said ??
In any graph, X axis the the "independent" variable and Y axis , the dependent.
The current limits are decided by the voltage differences and NOT the other way round.

#### AnalogKid

Joined Aug 1, 2013
11,160
the graph shows the voltage difference between input and output that one should maintain to acheive particular current limit.
Nope. Actually, maybe, depending on how one interprets your statement.

For three different junction temperatures (the temperature of the silicon inside the device package), the graph shows the maximum current allowed before the part burns up, as a function of the differential voltage. Basically, what the chart is saying is that while the transistors on the chip are able to pass 1.5 A, the package cannot move enough heat fast enough to allow that current level at high differential voltages. This can be improved with forced air cooling. To work out how, there is another number on the datasheet somewhere, the thermal impedance from junction to case or junction to ambient

ak

#### Dr.killjoy

Joined Apr 28, 2013
1,196
Thread Starter was talking about minimum drop-out voltage and showing the graph of the maximum voltage before the chip goes into safety shut-down. These are two different things.
In any graph, X axis the the "independent" variable and Y axis , the dependent.
The current limits are decided by the voltage differences and NOT the other way round.
Thanks for correcting my mistake..