Assuming the below circuit is functional, can you tell me how soft-start part works?

  • Would the same setup be valid for Vin=~3V and Vo =~1.3V, or would I need to change the C1 and R3?
  • How can I increase the soft-starting time?

Edit: The image is from the onsemi LM317 datasheet. I found no explanations.

enter image description here


2 Answers 2


An LM317 is not really a good choice for a 1.3V regulator, and even assuming you do get it working that low, this soft-start part of the circuit wouldn't work.

LM317's Vref "reference" voltage is 1.25V and this is what it maintains between its Vout and Adj pins.
The 240R resistor between Vout and Adj then sets the "reference" current through itself and R2 in your circuit. Within limits, this current doesn't change - the 1.25V is constant and the resistance is constant, so the current is constant.
So since you have a constant current flowing down through R2, adjusting the value of R2 adjusts the voltage across R2, and since we already determined that this current is (reasonably) constant, the voltage across R2 is constant (once you've stopped adjusting it).
So Your output is determined by the voltage across R2 + Vref.
Unfortunately there is another source of current flowing through R2, and this is the LM317's own operating current, which flows out of the Adj pin. If you choose the value for the resistor between Vout and Adj to be reasonably low (like the 240R in your circuit), the "constant" current will be a few orders of magnitude higher than the LM317's operating current and it's usually forgivable to ignore it.

So firstly - using an LM317 as a 1.3V regulator from a 3V source.
Since the LM317's Vref is a constant 1.25V, for a 1.3V output that only leaves 0.05V (50mV) across R2. It will be a little tricky (although certainly not impossible) to choose a sensible combination of resistors to use in this circuit. But you'll find that this choice of resistors will almost certainly result in a much higher value than 240R between Vout and Adj, which in turn means a proportionally lower "constant" current. This means that you can no longer ignore the LM317's operatin current flowing out of Adj becasue it will raise the voltage across R2 and will affect your output voltage.
Also, an LM317 needs at least 3V between its Vin and Vout to operate, so for a 1.3V output you'd need 4.3V in - there's no way around this.

Secondly - the soft-stat circuit.
Assuming you've relaxed the 3V input requirement and have at least 4.5V coming in, and that you've managed to tweak your resistor values to get 1.3V out.
This soft-start circuit will never power-up with the output slowly increasing from 0V.
If you paid attention to part-one up above you'll see that, even with Adj shorted directly to ground, the output will never be lower then 1.25V - so even assuming the soft-start circuit would function in this case (it won't though) the circuit would power-up with a 1.25V output and then 'soft-start' up the remaining 50mV to 1.3V ... not much of a "soft start".
There are other issues which will cause the soft-start circuit to not work, but since we can see that even of it did "work" it would be ineffective, there's probably not much point in making this answer much longer ...

  1. The LM317 requires a minimum difference of 3V between input and output. It can't provide any kind of regulated ouput when the input is only 3V.

  2. If the input were higher, you would need a negative voltage source to get down to 1.25 V. The LM317 uses a reference voltage of 1.25 V. The ouput voltage must be above 1.25V, else the regulator can't operate. The datasheet gives a circuit using a negative voltage supply to change the operating conditions so that you can regulate the output right down to zero volts.

Probably simpler and cheaper to get a modern part that is intended to work for your case than to try to make the ancient and venerable (and decrepit) LM317 do the job.

All of the things I've pointed out are mentioned in the datasheet of the LM317.

That's the reason you will see the often repeated mantra: "Read the datasheet."

The softstart works by pulling the adjust pin to ground through the PNP transistor.

A grounded adjust pin forces the LM317 to output its minimum voltage.

The base of the transistor is held low by the (discharged) 10uF capacitor.

When a little voltage appears at the output of the 317, the capacitor starts to charge. The voltage rises, and the transistor slowly starts to shut off.

This allows the voltage on the adjust pin to rise - the output starts to head towards the set output voltage.

At some point, the transistor shuts off completely, and the output is at the set voltage.

The length of the soft start is determined by the 10uF capacitor and the 50k resistor.

A larger resistor and/or capacitor will lengthen the softstart.

Smaller values will shorten the softstart.

It is actually the product of the resistance and capacitance that control the time, so you could increase one and decrease the other and come out with the same time.

You also aren't completely free to choose just any values you like. There are limits to what values you can use, but I'm don't know how to quantify it (and don't have much interest in figuring it out.)

The soft start would probably work if you had a supply voltage high enough (at least 4.3V) and you were using the negative voltage as described in the datasheet.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.