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By accident I stumbled upon this Linear Technology application note (LT1074/LT1076 Design Manual):

https://www.analog.com/media/en/technical-documentation/application-notes/an44fa.pdf

Just for fun I simulated the positive to negative converter in Fig. 16. exactly as it is.

Figure 16 from Linear Technology Application Note 44 - LT1074/LT1076 Design Manual

The authors state on page 26:

Positive to negative converters have a “right half plane zero” in the transfer function which makes them particularly hard to frequency stabilize, especially with low input voltage.

Why is this the case especially for low input voltage? I see that the compensation network R1, R2, C4 is very important for input voltage Uin=5V and that there is ringing oscillation at the output when it is omitted.

I would like to understand from a theoretical point of view, what this network does: Why is it especially required for low input voltages and not for higher? Most interestingly, at higher input voltages it can be removed and the circuit is stable. I wonder also, why it is connected to the switch side of the inductor and not at the output?

At least in LTSpice it is very critical to use the stated values; changing the capacitor by factor 2 or 0.5 leads again to ringing in the output.

What is the key to understand this sensitive design?

enter image description here

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    \$\begingroup\$ A right-half-plane-zero or RHPZ is inherent to a buck-boost or a boost converter operated in CCM and DCM. In CCM, its position varies with the duty ratio and the inductor value. At very low input voltage, this RHPZ can be of low value and it is recommended that crossover remains below 20-30% of the lowest RHPZ position. Difficult to obtain a fast loop in these conditions then. Finally, the LT1074 is internally compensated and intended to be operated from a positive rail with a positive FB. I believe the added resistive network cheats the part to make it regulate a negative rail. \$\endgroup\$
    – Verbal Kint
    Feb 20, 2021 at 21:26
  • \$\begingroup\$ Makes sense. I'm going to evaluate the transfer function of the control loop. \$\endgroup\$
    – MichaelW
    Feb 21, 2021 at 7:09

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