Hi,
I'm creating a Bipolar supply to drive an ECL Phase Frequency Detector. The Positive Supply (LM317) provides +1.3V (From a +5V supply). The Negative Supply (LM2991S) provides -3.7V (from a -5V supply)(the LM2991S has excellent LOW DROP-OUT Voltage performance at up to 1-amp - that I haven't found an alternative part). Therefore the ECL PFD IC (MCH12140) provides BALANCED +/-400mV logic swings to diode bridge switch a current source into an integrator.
I'm getting DC offsets within the system and I'd like to jointly DAC control the fine trimming of the +1.3V and -3.7V supplies to zero the offset (possibly by +/- 25mV). If it was an LM317 and an LM337, I could OPAMP drive the two voltage level set resistors which "common" at the circuit ground potential. This is possible because the 1.23V ADJUST voltage is between the ADJUST pin and the OUTPUT pin on both regulators.
However, LM2991S has its 1.23V ADJUST voltage between the ADJUST pin and it's GND pin (The LM337 doesn't have a GND pin). In the LM2991S case, the output level set resistor is between ADJUST and the OUTPUT. Quite the opposite of the LM317/LM337.
I'm considering disconnecting the LM2991S GND pin from circuit ground and driving it from the opamp output (along with the LM317). But the schematic of its internals gives the impression that the GND pin is more important than just a reference return. It may actually pass significant current (mA's) or definitely need to have low impedance to GND.
In the LM2991S case, can I fine trim the GND pin to adjust the output, or must it definitely be connected to circuit GND potential?
Thanks
Peter