Charge pump circuit for driving N-channel MOS transistors

This charge pump circuit comprises a capacitor connected with a first terminal thereof to a reference voltage point through a first switch element and with a second terminal thereof to a switching section. The switching section, which is arranged between a positive supply voltage line and the ground, is controlled so as to alternately and selectively connect the second terminal of the capacitor to the positive supply and to ground. The first terminal of the capacitor is further connected to the gate of the MOS transistor to be driven. During operation the switch section is controlled so as to alternately charge the capacitor and allow transfer of the charge of the capacitor to the MOS transistor gate, thereby achieving a fast charging of the MOS transistor and a low circuit dissipation in the DC mode.

BACKGROUND OF THE INVENTION 
The present invention relates to a charge pump circuit for driving 
N-channel MOS transistors. 
As is known, for driving a load in the motor-car or in general in the 
industrial field, it is necessary to have driving components which may be 
power elements or not and should operate in an efficient manner. Nowadays, 
as driving elements, N-channel MOS transistors are preferred, as they 
require a lower integration area than analogous P-channel devices. As is 
also known, these N-channel MOS transistors are designed so as to have 
their drain connected to the positive supply and their source connected to 
the load, while their gate should be maintained at a higher voltage than 
the supply one, so as to assure an efficient and low-dissipation operation 
of the MOS transistor. 
Furthermore for the DC mode, a fast charging of the gate of the MOS 
transistor is required, so as to assure fast responses of the system (i.e. 
fast turn-on of the MOS transistor). To this aim, charge pumps are already 
known which are ground referred to obtain a fast charging of the 
capacitance of the MOS transistor gate. However, these charge pumps have 
shown to be inadequte to simultaneous satisfying both requirements. 
SUMMARY OF THE INVENTION 
Accordingly, the aim of the present invention consists in providing a 
charge pump circuit capable of solving the disadvantages shown by the 
prior art. 
A particular object of the present invention is to provide a charge pump 
circuit capable of adequately supplying a load in the DC mode and having a 
low dissipation during operation. 
Another object of the present invention is to provide a charge pump circuit 
allowing a fast charging of the gate capacitance of the MOS transistor and 
thus a fast activation time of the circuit. 
A further object of the present invention is to provide a charge pump 
circuit comprising conceptually simple elements, which can be integrated 
in a single structure according to known technologies, so as to have low 
manufacturing costs. 
The above aim and objects as well others which will be better appreciated 
hereinafter are achieved by a charge pump circuit for driving N-channel 
MOS transistors, according to the invention, for connection to a N-channel 
MOS transistor having a drain electrode connected to a supply voltage 
line, a source electrode connected to a load and a gate electrode, 
characterized in that it comprises a capacitor connected with a first 
terminal thereof to a reference voltage line through a first switch 
element and with a second terminal thereof to a switching section arranged 
between said supply voltage line and a ground line and being controlled so 
as to alternately and selectively connecting said second terminal of said 
capacitor to said supply voltage line and said ground line, said first 
terminal of said capacitor being further connected to said gate electrode 
of said N-channel MOS transistor.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIG. 1, the equivalent capacitance of the MOS transistor 
gate is indicated at 27, such capacitance being the element for which a 
fast charging is desired and which then should be held at a higher voltage 
than the supply one, so as to shorten the turn-on time of the associated 
MOS transistor and to obtain an efficient operation thereof. 
In detail, the charge pump circuit according to the invention is composed 
of a pair of switches 20 and 21 placed between the DC supply V.sub.CC and 
the ground 22, and controlled as to opening and closing by an oscillating 
signal (for example, with a frequency of 500 kHz) supplied on the terminal 
23. The control signal applied to the switches 20 and 21 is suitably 
phase-shifted or 180.degree. so that, when one switch is open, the other 
is closed and vice versa. Such a phase opposition is obtained, for 
example, by means of a logical inverter 28 or the switches may be 
implemented by devices switched-on by opposite phases of the oscillating 
signal. The circuit furthermore comprises a capacitor 24 connected with a 
terminal to a point intermediate to the two switches 20 and 21, and with 
its other terminal to a DC supply voltage (e.g. at 12 V) through a diode 
25. A further diode 26 is connected with its anode to the connection point 
between the cathode of the diode 25 and the capacitor 24 and with its 
cathode to the capacitor 27 (representing the capacitance of the MOS 
transistor) and to the cathode of a diode 10, the anode whereof is 
connected to the supply voltage V.sub.CC. 
The charge pump circuit according to FIG. 1 operates as follows. In a 
steady state of the circuit the voltage drop on the capacitor 27 is 
maintained approximately at 12 v (i.e. the voltage on line 29), by virtue 
of the capacitor 24 which, through the pair of switches 20 and 21 and the 
diode 25, continually charged to a 12 V voltage at a frequency of 500 kHz. 
FIG. 2 shows an example of the application of the charge pump, according to 
the invention, to a MOS transistor for driving a load. In this drawing 
figure, the components of the charge pump have been indicated at the same 
reference numerals of FIG. 1, except the capacitor 27 which has been here 
neglected, since it is inherent to the gate of the MOS transistor 2. Thus 
the circuit comprises the drive transistor 2, having its drain connected 
to the supply voltage V.sub.CC, its gate connected to the cathode of diode 
26 and its source connected to a circuit which has the function of 
assuring the connection to the positive supply, such circuit comprising a 
diode 10 having its anode connected to the supply voltage and its cathode 
connected to the gate of MOS transistor 2 through a transistor 6, a 
current source 7 and a diode 8. The circuit further comprises a MOS 
transistor 35 arranged between the anode of diode 25 and the reference 
voltage line 29, at 12 V. In the figure the parasitic diode 37 is also 
shown, which is formed between the source and the drain of the MOS 
transistor 35. The gate of the transistor 35 is furthermore fed with a 
control signal V.sub.IN for controlling turning on or off of the charge 
pump circuit. 
The circuit according to FIG. 2 operates as follows. By virtue of the diode 
10 being connected to the supply voltage, the gate of the MOS transistor 2 
is quickly charged at the supply voltage, then (when the gate of the 
transistor 35 is fed with the control signal V.sub.IN of such a level as 
to cause turning-on of the same transistor 35 thus allowing connection of 
diode 25 to the reference voltage line 29) the charge pump is switched on 
and in turn provides for raising of the gate voltage of the transistor 2 
thereby shortening the turn-on time and allowing proper operation of the 
transistor 2 itself. 
Therefore, as explained, the charge pump circuit according to the invention 
allows for an efficient operation and a quick turning-on of the MOS 
transistor for driving loads, in particular inductive loads, through 
simple elements, which are easily integrated and highly reliable. 
The invention thus conceived is susceptible of several modifications and 
variations, without departing from the inventive scope. In particular all 
the elements may be replaced by other technically equivalent ones.