Help with Analog Devices Regulator (ADP7105ACPZ-R7) Soft Start Function

Hello,

This question is about the ADP7105ACPZ-R7, but may relate to other AD regulators with soft start functionality.

I am aware that the Soft Start pin of the IC can be connected to a capacitor to GND in order to program a soft start feature. However, I am unclear whether it is necessary to add this capacitor. Can this pin be left floating? I would presume not because the Soft Start pin seems to be driven internally by a current source.

Thank you!

Hello anuj,

Thank you for your inquiry. The soft start capacitor is not needed, it is only needed when soft start ferature is required. This can be left floating as it has an internal 5k pull-down resistor.

Hi Ryan,

Thanks for your response. Am I understanding the internal architecture properly? Here’s my current understanding:

So when using the Soft Start capacitor:

  1. We bring EN high and the switch closes
  2. The capacitor acts as a short immediately and then charges until it acts as an open circuit. Through that charging process, the voltage across the resistor slowly (linearly?) increases.
  3. The voltage at SS is held at (1uA * 5kOhm) = 5mV

And when not using the Soft Start capacitor:

  1. We bring EN high and the switch closes
  2. The voltage at SS is held at (1uA * 5kOhm) = 5mV

Is this accurate?

Thanks,
Anuj

Hello anuj,

Thank you for your reply. I am checking with our product manager on this to see if they can be of further help.

Hi anuj,

I believe Ryan is correct in that a soft start (SS) capacitor is not required. Evidence of this would include the following information from the datasheet:

Note that both in the specification table on page 3 and the graph, Fig.67 on page 19, show characteristics for the regulator under zero SS capacitance conditions.

I believe the internal circuit is a little more complex than what you have drawn. Just above the Fig. 67 graph on page 19, it gives the formula for soft start time as:

image

Note that Vref is 1.22V rather than the 5mV you came up with above. This implies that the resistor you drew is not connected to the circuit when Enable is high. I believe there is a separate switch between the 5k resistor and the SS cap which is only closed when Enable is low, and its only purpose is to quickly discharge the SS cap.

In conclusion, the soft start capacitor is not required for proper functionality of the regulator, and is only necessary if you wish to limit the inrush current of the regulator and/or control the rise time of the output voltage.

Hello David,

Thank you for the detailed response. Initially I was confused whether the 0 nF capacitor referred to there indicated a direct short to ground, but in retrospect a 0 nF capacitor is essentially an open circuit, so I can see how the datasheet indicates no capacitor is needed.

I still don’t quite understand where the Vref comes into play if the pin is truly driven by a current source. Do you have a suggestion of how the internal circuit actually looks?

Thanks again for the help,
Anuj

It’s been a while since I designed any current sources, but IIRC most practical designs require a voltage reference.

anuj,
If you are able to let us know what the reason is for wanting to understand the circuit to this depth, that may be able to help us. Usually this kind of question is regarding quiescent current, which this circuit has very little at 900uA. I am also seeing if there is a more detailed schematic available.

Hi anuj,

After reading through some datasheets of other regulators with soft start pins, it looks like the common method is to use the voltage at this pin to control the reference voltage which is used to set the output voltage via the resistor divider on the Adjust pin. Once the voltage reaches the voltage clamp level (1.22V for this part), the circuitry switches over to using that fixed internal reference to control the output.

Below is a somewhat more detailed, but still not complete, internal schematic from another vendor’s part, but it gives you an idea of the general architecture and complexity of the circuit.

image

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Hi all,

Thanks so much for the detailed help on this.

The impetus for this question was that we designed a PCBA using several ADP7104ACPZ power regulators, but those regulators went out of stock after getting the PCBs made. We were trying to figure out whether we could simply drop in ADP7105ACPZ regulators instead during assembly, given that the Soft Start pin on the ADP7105ACPZ is an Unconnected pin on the ADP7104ACPZ. All in all, from these answers it sounds like we should be able to do so!

Thanks!
Anuj

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That should work just fine.
Best of luck!

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