Using the SS pin on a switchmode controller to vary the output?

[dmills]

Hi all,
I am playing with the design of a simple drain modulator for a transmitter, and got to wondering, lots of the modern switchmode controller chips are sort of ideal (And have all sorts of details taken care of for you), but get a bit complicated to control if you diddle the feedback input as they often do funny things if you increase the feedback voltage too rapidly (to reduce the output voltage).

there is however another input, the soft start pin which typically charges a cap and serves as a limit on the output voltage until the pin exceeds (typically) the internal reference.

Has anyone tried applying a voltage to this pin and driving it at audio rates to try to get the supply to follow, obviously with the feedback divider setup to limit the voltage to whatever desired max is?

I am thinking of current mode parts like the LTC3895 which look like they should easily stretch to being a multi phase design.

I can feel an experiment coming on.

Regards, Dan.

[T3sl4co1l]

Why not tie a resistor from the modulation input to the voltage sense divider and control it directly, no hacks?

Also on the ones with external compensation, the pin is usually a transconductance amplifier output, with the voltage of the pin corresponding to the average or peak current or PWM% (usually a combination thereof, due to slope compensation).  That's usually not exactly the output current, or input current or whatever you're ultimately controlling, so not as helpful, but important to note.

Tim

[dmills]

I am trying to work around the aggravating tendency of these things to have (in among all the really nice features) such annoyances as over voltage crowbars that turn on the sync fet if the output ever goes 10% above regulation.

Build it and see I guess, it is not the sort of thing I would expect a spice model to do a good job of modelling.

And yep, that external compensation pin has its uses (makes a lovely current share bus).

Regards, Dan.

[T3sl4co1l]

Which ones have a crowbar?

Tim

[mark03]

I can vouch for the fact that the SS pin is handy for unintended uses, though not necessarily what you have in mind.  On my bike light I drive a switcher's SS pin to reduce the feedback voltage from 0.6V to some tens of mV.  That way I reduce the power losses in my current-sense resistor.  It's a hack, but it works.

[ogden]

get a bit complicated to control if you diddle the feedback input as they often do funny things if you increase the feedback voltage too rapidly (to reduce the output voltage).

- Then don't increase feedback voltage too rapidly!

I think using SS (soft start) pin to control output voltage is bad idea. SS  is implementation-specific and when you use SS for regulation, you lose it's original purpose. Meddling with feedback (pin) is simple - stay within  loop bandwidth of regulator you want to "regulate" and you will be fine. Sorry, but wheel is already invented. One of many ways to manipulate feedback voltage:

http://www.analog.com/en/technical-articles/linear-adjustment-of-switched-mode-power-supplies.html

https://training.ti.com/voltage-regulator-margining-precision-dacs

https://www.maximintegrated.com/en/app-notes/index.mvp/id/818

[T3sl4co1l]

As for what you can do with an SS pin -- depends on the function.  There are many ways it's used.  Also, enable or UVLO and similar pins.

SS is usually a capacitor externally, to a current source and switch internally.  The capacitor is discharged in reset (may include output fault conditions), and the current source charges the capacitor otherwise.  As the voltage rises, either the output voltage or an internal parameter like peak current or PWM%, is limited to the value on the capacitor.

Example, TL494's SS pin sets PWM%.  It's only coincidence that it "soft starts" the load at all -- a shorted load, or an overly large load capacitance, defeats it easily!

The ones which describe supply sequencing, will have V(SS) in terms of V(FB), and SS can be used as a second setpoint or limit input.  Particularly handy if you want to add your own custom current limit, say.  Assuming you don't need to worry about other features attached to the pin like fault logic or anything.

Tim