Efficient Negative Voltage Regulation

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-voltage 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. In the circuit, two programming resistors, Rl 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 CI improves ripple rejection, and protection diode Dl eliminates common-load problems. Because a secondary winding is galvanically isolated, a regulator"s 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 ground, and the -12-V output comes off the anode of Dl. The V-m 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.

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