In recent years there has been a considerable increase in equipment that uses a low supply voltage, such as communication equipment, portable units, etc. This equipment is provided with several internal devices that require circuits suitable for elevating a direct voltage in certain periods of time; among these voltage elevator circuits charge pump devices are widely used.
A charge pump structure is present in U.S. Pat. No. 5,874,850. The structure comprises generically a single stage circuit or charge section Si or n stages Si connected in series S1 . . . SN by means of the input terminals C1 . . . CN and the output terminals D1 . . . DN as shown in FIG. 1; in this case the respective side terminals A1 . . . AN of the single stages Si are all connected to a first output terminal O1 of an oscillator OSC while the terminals B1 . . . BN are connected to a second output terminal O2 of the oscillator OSC. The input terminal Ci of a single stage is connected to the output terminal Di+1 of the successive stage; the terminal CN of the last stage is connected to the supply voltage VS while the terminal D1 of the first stage, that constitutes the output terminal OUT of the charge pump circuit structure, is connected to a load storage capacitor SC which has the other terminal connected to ground. The signal generated by the oscillator OSC varies between the reference potential GND, in particular the ground, and the potential of the voltage VS. The voltage on the terminal OUT is equal to N+1 times the voltage VS.
A single stage circuit or charge section Si is shown in FIG. 2; said circuit stage Si comprises a first charge transfer capacitor TC1 and a second charge transfer capacitor TC2 which have first terminals connected to respective side terminals Ai and Bi, two inverters connected together in loop connection so as to form a flip-flop that has inputs Ji, Ki connected respectively to second terminals of the first charge transfer capacitor TC1 and of the second charge transfer capacitor TC2, negative supply terminals connected together with the input supply terminal Ci and positive supply terminals connected together with the output charge terminal Di.
The first inverter is constituted of MOS transistors M1 and M2 while the second inverter is constituted of MOS transistors M3 and M4. In addition FIG. 2 shows the bulk diodes Db1, Db2, Db3, Db4 of the respective transistor M1–M4; the diodes are positioned so that the cathodes of the diodes Db1 and Db3 are connected to the terminal Di while the anodes of the diodes Db2 and Db4 are connected to the terminal Ci.
If we consider that the charge pump structure is produced with a single stage circuit Si, when a supply voltage VS is applied the capacitor SC is discharged and the voltage in output is initially VS−2VD, where VD is the voltage at the ends of each diode, approximately 0.7V; the capacitor SC is charged through the bulk diodes. When the voltage difference between the output voltage and the supply voltage VS becomes higher than the threshold voltage of the MOS transistors, the same transistors start to conduct and to load the capacitor SC, replacing the bulk diodes in said operation. The capacitors TC1 and TC2 supply the capacitor SC according to whether respectively the first output terminal of OSC is low and the second output terminal of OSC is high or vice versa.
It is possible that in off conditions of the charge pump circuit above described it could happen that the difference of the values of the voltages on the terminals of the capacitors TC1 or TC2 associated with the terminals of the respective inverter is very high and leads to the breakdown of the MOS transistors that constitute the inverters. This can occur if for example one of the capacitors TC1 and TC2 discharges much more quickly than the other by means of the leakage currents or if the voltage on the output terminal increases causing the breakdown of the transistors.