Dual-supply op-amp LDO does not work, but single-supply does


I'm designing an LDO with dual outputs (Vout and -Vout), and only the positive output is shown in the pictures below. I'm having large trouble using dual supply op-amps in the design, and my single-supply one works fine and is shown in the first image.

Single supply that works

But my LDO using a dual-supply op-amp does not work at all in the same configuration, and I've tried a few different dual-rail op-amps. Am I missing something in the dual-supply design?

Dual-supply design that does not work

The design with a dual-supply op-amp just saturates to the positive rail.

The problem was that the dual supply could not get close enough to the rail.

Best Answer

Dual supply opamps are not known for their ability to operate near the rails. So, most likely is that the opamp can't get close enough to the \$V_{\text{dd}}\$ to control the FET. FDS4465 has a \$V_{\text{th}}\$ of -0.2V to -1.8V and the LT1007 for example can only get to about -1.5V of \$V_{\text{dd}}\$. If the FET you have has a nominal or less \$V_{\text{th}}\$, then try as it might, the opamp will not be able to reach a high enough output voltage to modulate FET conductance or turn FET off. A high \$V_{\text{th}}\$ FET, and/or a drive circuit that can reach \$V_{\text{dd}}\$ should be used.