Many applications require highly efficient negative-voltage post regulators with low dropout voltage in switch-mode supplies.

A way to provide good negative-voltage regulation is with a low-dropout positive-negative regulator operating from a well-isolated secondary winding of the switch-mode transformer. The technique works with any positive-voltage regulator, although highest efficiency occurs with low-dropout types.

Click on Picture for Full Size

In the circuit, two programming resistors, R1 and R2, set the output voltage to 12 v, and the LT1086s servo the voltage between the output and its adjusting (ADJ) terminals to 1.25 V. Capacitor C1 improves ripple rejection, and protection diode D1 eliminates common-load problems.

Because a secondary winding is galvanically isolated, a regulators 12-V output can be referenced to ground. Therefore, in the case of a negative-voltage output, the positive-voltage terminal of the regulator connects to the ground, and the -12-V output comes off the anode of D1. The Vin terminal floats at 1.5 V or more above ground. This arrangement is the equivalent of connecting the positive terminal of a battery to ground and taking the output from the negative terminal.