Precision reference bans precision resistors
By Budge Ing, Maxim Integrated Products, Sunnyvale, CA -- EDN, June 10, 1999
Combining a switched-capacitor charge pump with a precision reference yields an inverted reference from a positive power supply (Figure 1). Unlike the more typical combination of a positive three-terminal reference and an op-amp inverter, this circuit performs accurate inversions without the need for precision resistors and a negative supply. The compact circuit requires only three surface-mount capacitors, and the ICs occupy tiny SOT-23 packages.
The charge-pump inverter, IC2, delivers -5V by inverting the output of a 5V precision reference, IC1. IC1 has an input range of 5.2 to 12.5V. Replacing IC1 with a 2.5V reference that accepts 2.7 to 12.5V inputs produces a -2.5V output.
Output-voltage accuracy depends partly on the initial accuracy of IC1, which in this case is 1%. To determine the overall accuracy, you must add the error from the dropout voltage, which is less than 2 mV for 90 µA of load current (Figure 2a). For a -2.5V output, the circuit draws quiescent current that ranges from 86 µA for a 2.7V input to 105 µA for a 12.5V input. For -5V outputs, the circuit draws 127 µA for 5.2V inputs and 140 µA for 12.5V inputs (Figure 2b). (DI #2368)
