Voltage regulator goes digital

Texas Instruments LP2951

Chuck Wojslaw

EDN

Power consumption and failing batteries are key issues in battery-operated applications. Features in the new generation of series voltage regulators address these concerns. For example, LP2951 micropower voltage regulator provides a logic-level output signal indicating a low regulated output voltage; the IC also has a logic-level input to shut down the regulator to conserve power. These signals are digital; therefore, they are compatible with µP-based systems.

A digital potentiometer adds programmability to an ordinary series voltage regulator.
Figure 1. A digital potentiometer adds programmability to an ordinary series voltage regulator.

But what about the programming or controlling the regulator's output voltage? A mechanical potentiometer or selected resistors are not elegant solutions. You can complete the so-called computerization or digitizing of the voltage regulator by using a digitally controlled potentiometer to program the regulator's output voltage. The circuit in Figure 1 is a wide-range, computer-controlled voltage regulator with a nominal output voltage that varies from 1.235 to 14.8 V. The regulator uses a Xicor XDCP X9312W digitally controlled potentiometer that, with its 100 steps, can program the regulator with a resolution of 0.136 V per step. The output voltage is

where k is a number from 0 to 1 and reflects the proportionate position of the wiper from one end of the pot (0) to the other end (1). R is the end-to-end resistance of the potentiometer.

The XDCP's programming uses a three-wire bus. The potentiometer is configured as a two-terminal variable resistor. The regulator's ERROR output signal warns of a low output voltage; you can use it as a power-on reset. The logic-compatible SHUTDOWN input signal allows you to switch the regulator on and off to conserve power. These signals, along with those you require to program the XDCP, typically connect to the I/O port or the µP or µC. The potentiometer adds variability to the regulator circuit; its digital controls, which are attached to a computer-controlled bus, provide programmability. For example, an automated closed-loop calibration procedure to program the regulator saves manufacturing test time. You can use the circuit as a bias supply; a voltage reference; or a programmable, high-output-current voltage source in test-and-measurement applications.

Materials on the topic

  1. Datasheet Texas Instruments LP2951
  2. Datasheet Intersil X9312W

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