Voltage Reg Resistor Values
- Thread starter fatcat
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Andrew L Weekes
Reverse Engineer
The basic formula for calculating the o/p voltage is: -
Vout=Vref * (1+R2/R1)
Vref is always 1.25V, R1 is the resistor between o/p and adjust, R2 is the resistor between adjust and 0V.
To choose the resistors it's a simple matter of re-arranging the equation. We have two unknowns, R1 and R2, so to solve it we have to choose a starting value for one of them, in this case it's usually R1 (somewhere between 120 and 240R, normally): -
R2 = R1 * ((Vout / Vref) -1)
It's worth understanding how the regulator works, in order to understand the calculation more intuitively, hopefully this makes sense...
The regulator's sole job is to maintain 1.25V between the output and the adjust pin - it knows nothing about the actual voltage at it's output. By maintaining 1.25V across this resistor, we have, by definition, a constant current flowing through it, determined by Ohms law: -
V=I*R, therefore I=V/R
If R1=125R (to make the maths easy), then
I=1.25 / 125 = 0.01A, or 10mA
Now, of this 10mA there is a tiny current that flows into the adj pin (which for most practical examples, we can safely ignore), the rest flows to 0V, through the resistor between the adjust pin and 0V.
If then, for example, we make the lower resistor 1K, it will have a voltage drop across it, again determined by Ohms law: -
V=I*R = 0.01A * 1000R = 10V.
Since there's is 10V across the lower resistor, and 1.25V across the upper one, the output voltage is 11.25V! If you build the reg with R1=125R and R2=1K you will get 11.25V approx.
You can confirm this by the re-arranged formula above: -
R2 = R1 * ((Vout / Vref) -1)
R2 = 125 * ((11.25 / 1.25) -1)
R2 = 125 * (9 - 1)
R2 = 125 * 8 = 1000 (Ohms) or 1K
Obviously using standard resistor values the numbers will be different, but the principle is the same - there's a whole range of resistor combinations that will give a particular output voltage.
Choosing any particular combination is a process of finding standard values that result in the desired output voltage within the tolerances you require,
e.g. 130R / 390R = 5V, as does 120R / 360R, wheress using R1=240R, would make R2 = 720R, the closest preferred value being 750R.
Using 240R / 750R gives 5.15V, which may or may not be critical.
For critical applications you can use series / parallel combinations to make up the correct value.
Andy.
Vout=Vref * (1+R2/R1)
Vref is always 1.25V, R1 is the resistor between o/p and adjust, R2 is the resistor between adjust and 0V.
To choose the resistors it's a simple matter of re-arranging the equation. We have two unknowns, R1 and R2, so to solve it we have to choose a starting value for one of them, in this case it's usually R1 (somewhere between 120 and 240R, normally): -
R2 = R1 * ((Vout / Vref) -1)
It's worth understanding how the regulator works, in order to understand the calculation more intuitively, hopefully this makes sense...
The regulator's sole job is to maintain 1.25V between the output and the adjust pin - it knows nothing about the actual voltage at it's output. By maintaining 1.25V across this resistor, we have, by definition, a constant current flowing through it, determined by Ohms law: -
V=I*R, therefore I=V/R
If R1=125R (to make the maths easy), then
I=1.25 / 125 = 0.01A, or 10mA
Now, of this 10mA there is a tiny current that flows into the adj pin (which for most practical examples, we can safely ignore), the rest flows to 0V, through the resistor between the adjust pin and 0V.
If then, for example, we make the lower resistor 1K, it will have a voltage drop across it, again determined by Ohms law: -
V=I*R = 0.01A * 1000R = 10V.
Since there's is 10V across the lower resistor, and 1.25V across the upper one, the output voltage is 11.25V! If you build the reg with R1=125R and R2=1K you will get 11.25V approx.
You can confirm this by the re-arranged formula above: -
R2 = R1 * ((Vout / Vref) -1)
R2 = 125 * ((11.25 / 1.25) -1)
R2 = 125 * (9 - 1)
R2 = 125 * 8 = 1000 (Ohms) or 1K
Obviously using standard resistor values the numbers will be different, but the principle is the same - there's a whole range of resistor combinations that will give a particular output voltage.
Choosing any particular combination is a process of finding standard values that result in the desired output voltage within the tolerances you require,
e.g. 130R / 390R = 5V, as does 120R / 360R, wheress using R1=240R, would make R2 = 720R, the closest preferred value being 750R.
Using 240R / 750R gives 5.15V, which may or may not be critical.
For critical applications you can use series / parallel combinations to make up the correct value.
Andy.
fatcat
pfm Member
Andy
Thanks a lot, I now understand.
I was getting confused by the formula
Vout=1.25V * (1+R2/R1) + (-IADJ x R2)
I didnt understand the (-IADJ x R2). But I guess this part of the formula refers to the value you say we can safely ignore.
I am going to fit the Regs into my CD player. Are there any parts of a CD player you would regard as critical applications.
Thanks
Thanks a lot, I now understand.
I was getting confused by the formula
Vout=1.25V * (1+R2/R1) + (-IADJ x R2)
I didnt understand the (-IADJ x R2). But I guess this part of the formula refers to the value you say we can safely ignore.
I am going to fit the Regs into my CD player. Are there any parts of a CD player you would regard as critical applications.
Thanks