Triple Capacitive Voltage Inverter with the MAX871

By:  Len Sherman

Abstract: A -3X voltage inverter is made with a SOT charge-pump IC and diode-capacitor network. Up to -15V at 10mA can be generated from 5V.

This design idea is based on a customer request for a cheap, and not very accurate, negative voltage supply of about -12V to -15V with a low output current of < 5mA and an input voltage of 5V. This application might be used for negative op-amp supply. Figure 1 shows the typical operation circuit with the flying capacitor C1 and C2. The output voltage on pin 1 is -VIN. Four additional capacitors (C3 to C6) and four additional diodes (D1 to D4) triple the negative output voltage at the OUT pin from -VIN to -3VIN. C5 and C6 decreases the voltage with every step by -VIN. Without any diode voltage drop C3 got -2VIN and C4 charges ideally to -3VIN. It is possible to add more of these cascades but with every step the voltage drop of the diodes will reduce the effort.

Figure 1. Triple voltage inverter with MAX871.
Figure 1. Triple voltage inverter with MAX871.

The schematic of the design idea has been tested with 4.5V and 5V input voltage. Figure 2 shows load current vs. output voltage. The general voltage drop on VOUT is caused by the forward voltage of the external diodes of typically 0.3V to 0.4V per diode, depending on the load current. VOUT is:

VOUT = -3 × VIN + 4 × VD

VD: typically 0.3V to 0.4V

VIN is the input voltage on pin 2. VD is the forward voltage drop of diode D1 to D4. If Schottky diodes are used VD is typically 0.3V to 0.4V.

Figure 2. Load current vs. output voltage.
Figure 2. Load current vs. output voltage.

Component List
C1 - C6 470nF ceramic
C input 10µF
D1 - D4 BAT41

A similar version of this article appeared in the October 25, 2001 issue of Electronic World and Wireless World magazine.

Related Parts
MAX871 Switched-Capacitor Voltage Inverters Samples  

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APP 1875: Feb 03, 2003
APPLICATION NOTE 1875, AN1875, AN 1875, APP1875, Appnote1875, Appnote 1875