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

-April 24, 2003

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 –48V. 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.

Figure 1
This simple circuit provides a 10V power source from −48V telecom power rails.

The LMV431 voltage reference, along with the voltage-setting resistors sets the output voltage to approximately (1+1 kΩ/280Ω)×1.24V~–5.7V. This output voltage then goes to the base of Q1, the 2N2222 transistor. The configuration of the transistor causes a VBE drop of approximately 0.7V, resulting in a net voltage of –5V 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 –5V reference (GND pin). Small capacitors C1 and C2 enable the pumping and inverting action required to convert the –5V to 10V. 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.

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