Convert negative inputs to positive outputs

Shane Chang and Budge Ing

You can obtain a precise, positive-output voltage from a negative-voltage supply with a boost converter and a linear regulator. The input and output capabilities of the circuit in Figure 1 depend on the allowable I/O voltages of IC₁ and IC₂. In this case, IC₁ and IC₂ convert a –5V input voltage to a 3.3V output voltage.

 
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Figure 1 A two-IC circuit converts a –5V input to a 3.3V output.

IC₁ is a boost converter that accepts –5V when its V_CC pin connects to common ground – that is, the ground of the negative-power-supply input. Voltage divider R₁/R₂ at IC₁’s output provides feedback that sets the output voltage 10.5V above IC₁’s ground pin. With the feedback-threshold voltage factory-set to 1.226V, you can choose values for R₁ and R₂ using this equation:

(1.226V/R₂)×(R₁+R₂)=10.5V.

Current through R₁ and R₂ should be at least 2 µA. The IC₁ output, which is IC₂’s input, is 10.5V higher than –5V, which is 5.5V with respect to common ground.

Figure 2 The circuit's output voltage drops as current increases. Plots indicate source voltages of -4.5, -5, and -5.5V.

IC₂, a linear regulator whose ground pin connects to the common ground, accepts input voltages as high as 6.5V. Its output is factory-set at 3.3V. Figure 2 shows the output voltage versus the output current for the circuit in Figure 1 with input voltages of –4.5, –5, and –5.5V.