Switched-capacitor IC and reference form elegant -48 to +10V converter

Texas Instruments LM2682 LMH6672 LMV431

Wallace Ly


A system designer must almost always face a trade-off in choosing the right part for an application. The trade-off usually involves performance, price, and function. An example is the issue of powering circuits from a telecom-voltage source. Telecom systems almost exclusively use high-potential negative rails, such as –48 V. Digital circuits typically in use in such applications usually operate from a "brick"-type power supply. However, analog circuits rarely require enough power to justify using a costly brick. At the heart of these bricks is nothing more than a specialized switching converter in tandem with an isolated flyback-transformer coil. But some applications neither require nor can tolerate the use of a coil-based approach. Figure 1 depicts a way to address the problem. The circuit provides a small amount of power to analog/digital circuits, such as the LMH6672 DSL op amp.

Switched-capacitor IC and reference form elegant -48 to +10V converter
Figure 1. This simple circuit provides a 10 V power source
from −48 V telecom power rails.

The LMV431 voltage reference, along with the voltage-setting resistors sets the output voltage to approximately

This output voltage then goes to the base of Q1, the 2N2222 transistor. The configuration of the transistor causes a VBE drop of approximately 0.7 V, resulting in a net voltage of –5 V for the next stage. The purpose of the transistor is to provide additional current to the LM2682 switched-capacitor converter. Note that the converter has a –5 V reference (GND pin). Small capacitors C1 and C2 enable the pumping and inverting action required to convert the –5 V to 10 V. Furthermore, the MSO-8 package of the LM2682 and the SOT-23-3 package of the LMV431 allow the circuit to consume little board space. In roughly the size of a small transformer, the proposed circuit does an elegant job of powering low-power circuits from a negative high-voltage source.

Materials on the topic

  1. Datasheet Texas Instruments LM2682
  2. Datasheet Texas Instruments LMH6672
  3. Datasheet Texas Instruments LMV431


You may have to register before you can post comments and get full access to forum.
User Name