Current Sensing on positive and negative voltage rails

Question

I'm working on a design that uses two separate buck regulators to generate a positive and negative voltage, and I'm trying to implement current limiting with a current sensor. However, I am running into issues with the way the sense resistor is integrated into the feedback loop as shown in my block diagram below:

^{simulate this circuit – Schematic created using CircuitLab}

With the way the regulators are set up, the current sense resistors have to be included in the appropriate feedback loops in order to maintain the correct output voltage. Otherwise, the regulator won't take into account the voltage drop, and the output voltages will be less than what is expected.

The issue is with the way the two regulators are configured. When you add the sense resistor to the negative voltage circuit, it seems to create a different 'return node' for the negative voltage. If it wasn't there, both regulators can be tied to the ground node at their returns. What other methods of current sensing can be used for the negative voltage circuit such that the appropriate returns can be tied together?

EDIT: As Hacktastical said, there is a way to have a buck regulator output a negative voltage. I based the reference circuit off of an app note from TI:

Positive to Negative Buck-Boost Converter

Answer 1

The schematic you show will most certainly NOT work in the way you want.

You are adding a series resistor into the output and expecting the feedback loop to offset the effect of it. The chip does not have enough response to be able to do this.

The LM2673S already has short circuit protection built in, please read the datasheet.

The current protection is NOT in the output circuit, it limits the current through the switch and therefore the peak current that flow in the inductor. This can be set to provide just enough energy into (c1,C4) and (C2,C3) to allow the PWM to maintain your required output voltage. If your output current exceed a level that can be maintained by the maximum PWM on time, then the voltage will begin to fall.

You must also remember that in the case of a fault current (ie a sudden short circuit on the output) the current is defined by the output capacitors. They will deliver a large pulse of current under these conditions.

Note: You should also include a capacitor from the Vin to -Vout as shown in the application note on using the LM2673 for negative supplies. Without this capacitor, startup will be distorted.

Answer 2

Without going into too much detail, your approach violates the ESR requirements with an external sense R and also that is redundant with the internal Rs. There is a separate port to provide gain Ri for the OCP threshold with internal current sensing.

If you want to sense current on your own, use a 2~ 10 mOhm max ground sense resistor and high gain balanced Diff Amp with low ESL and a good layout to reject CM noise.

Total ESR on the load cap plus Rsense must be between 10 and 130 mOhm (?) from memory. So your 1 Ohm will result in instability and poor load regulation.

Answer 3

I think what you've shown is fine. What will happen is, when there is more IR drop on NEG_SENSE, this will cause Vf to shift down, making the regulator try to compensate by increasing its output voltage. NEG_V increases as a result.