Below is a question from a customer if the SS input of LMR24220 can be used as a tracking input. Could you please help the customer out?
LMR24210/LMR24220 have a soft start pin that is internally connected to one of the two positive inputs of an internal error amplifier.
My question is – can I use the SS input on the LMR24220 as a tracking input (0…0.8V) for controlling the output voltage? My 1 kohm shunt resistor (in red in the attached block diagram) is so low that the SS-current is small enough not to cause significant error.
It is clear that the error amplifier will track the SS capacitor voltage during soft start, but the data sheet does not explicitly tell if the input range extends all the way to GND and if the error amplifier will work properly and closing the loop in such use. In other controllers (as in LM34937 and TPS40170) this works fine.
The SS-current flowing out from the pin causes a small error and can be easily overcome using low impedance tracking divider resistors (in red). Independently, this would be a neat way of controlling the output voltage in the Simple Switcher family without interfering with the feedback circuitry.
The SS pin of LMR24220 is not evaluated as tracking input. Vout will track during soft start. If the tracking voltage can be removed after soft start is finished, the part will work fine. If the tracking voltage is still there in steady state operation, it is not clear how much it will affect Vout regulation.
Would you let us know your VIN range, Vout and Load range? We have similar parts may work in your application.
I have not tried this on bench, but I don't think that it is going to work well. The block diagram does not show some details on the regulation circuit.
After startup is completed, the softstart capacitor is being used as an integrator capacitor and holds the reference voltage. The softstart voltage is not ignored after startup is completed and because of this I don't think it will be suitable for tracking.
I hope this helps.
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