In the field of electronics, charge pump voltage generators, hereinafter simply referred to as charge pumps, have several applications.
Charge pumps are particularly useful in integrated circuits. For example, in electrically programmable and electrically erasable and programmable semiconductor memories such as EPROMs, EEPROMs and Flash memories, integrated charge pumps are provided for generating the voltages required for carrying out the read, write and erase operations on the memory cells. In particular, the relatively high positive and negative voltages required for writing and erasing the memory cells, typically ranging from 12 to 15 V, are generated starting from supply voltages of 3 V or less.
A charge pump comprises a plurality of stages connected in cascade between a supply voltage rail (typically, the supply voltage VDD of the integrated circuit) and a charge pump output. Each stage is capable of receiving electric charge from a previous stage or from the supply voltage rail, storing the electric charge in a charge-storage element, typically a capacitor, and transferring the electric charge to a following stage. The output of the last stage forms the charge pump output. The operation of the charge pump stages is controlled by properly out-of-phase timing signals, generally referred to as phases, which are generated by a phase generator starting from a base clock signal, generated for example by an oscillator circuit, or received from outside the integrated circuit.
In some applications, the voltages generated by the charge pumps should be highly stable irrespective of the current sunk from the charge pump. This is for example the case of the write/erase voltages for electrically alterable semiconductor memories, wherein the current sunk from a charge pump may easily reach the tens or even hundreds of μA. In particular, it is extremely important that the charge pump output voltages do not exceed prescribed safety levels, not to cause permanent damages to the tiny oxides forming the gates of the transistors and the dielectric of integrated capacitors. Unfortunately, charge pumps are, as known, far from being ideal voltage generators, because, due to their relatively high output impedance, the charge pump output voltages are greatly affected by the amount of current sunk therefrom.
For these reasons, charge pumps are normally equipped with regulator circuits that control the charge pump operation in such a way as to ensure that the output voltages are kept stable irrespective of the current sunk.
A conventional charge pump regulator typically comprises a voltage comparator circuit, for comparing the charge pump output voltage to a precisely known and stable reference voltage, generated for example by a band-gap reference voltage generator. The voltage comparator drives a control circuit that is adapted to condition the transmission of the clock signal to the phase generator. If the comparison performed by the voltage comparator reveals that the charge pump output voltage exceeds (in absolute value) a target voltage, equal to or derived from the reference voltage, the control circuit blocks the transmission of the clock signal pulses to the phase generator, thereby causing the charge pump to temporarily stop working (i.e., stop transferring electric charge between stages); the charge pump output voltage thus tends to decrease (in absolute value), due to the current absorbed by the charge pump load and to inevitable leakages. On the contrary, when the charge pump output voltage falls below the target voltage, the voltage comparator switches and the control circuit allows again the transmission of clock signal pulses to the phase generator; the charge pump is restarted.
The feedback nature of this voltage regulation scheme allows reaching a dynamic equilibrium condition, in which the charge pump output voltage is kept substantially equal to the target voltage. Once the output voltage has reached the target voltage, the charge pump is substantially turned off; in this way, when the charge pump works in an unloaded condition, the power consumption is significantly reduced, because it is only necessary to compensate for the leakages associated with the charge pump output. When instead a load is connected to the charge pump output, the regulator causes the charge pump to work at a rate that is exactly equal to what is needed for compensating the current sunk by the load (and, of course, the leakage current, which is however several orders of magnitude lower than the current sunk by the load).
Typically, the voltage comparator included in the charge pump regulator comprises a resistive voltage divider, to which the charge pump output voltage is applied, for deriving therefrom a precisely known percentage thereof, i.e. a suitable down-scaled value that is comparable to the band-gap reference voltage. This down-scaled charge pump output voltage is fed to a comparator, together with the reference voltage. The precision of the voltage regulator largely depends on the capability of the intermediate tap of the resistive voltage divider of rapidly tracking the variations in the charge pump output voltage. In order to compensate for effects caused by parasitics associated with the integrated resistors making up the resistive divider, shunt capacitors are provided in parallel to the resistors.
The conventional charge pump regulators work satisfactorily well, but have a non-negligible current consumption (current consumption levels of the order of 5 -10 μA are typical. Such a current consumption does not pose problems in normal operating conditions when the charge pump has to deliver several tens or hundreds of μA, but it is unacceptably high in case a stand-by operation is envisaged.
In fact, in order to shorten the time necessary for an integrated circuit, for example a semiconductor memory, to return from a stand-by condition to the normal operating condition, the charge pump or pumps integrated therein are preferably kept in a working condition even when the integrated circuit is put in stand-by.
However, the power consumption requirements in stand-by are normally quite strict, and the relatively high current absorbed by the charge pump regulator cannot be tolerated. In order to reduce the current absorbed by the charge pump regulator, very large resistance values should be adopted in the resistive voltage divider, which means large silicon area and slow response to variations in the charge pump output voltage.
Summarizing, the conventional charge pump regulators, which perform satisfactorily in normal operating conditions, are rather power-consuming and are not well adapted to the operation in stand-by conditions.