Voltage regulator circuit

Abstract: A voltage regulator circuit includes a regulator chip (12) having a start pin thereon and including an internal transistor which switches on and off to cause the provision of an increasing negative voltage at the voltage regular circuit output terminal. In response to a Start signal at the start pin, a start circuit (10-2) is effective to provide a forcing current at the output terminal from a voltage source of negative voltage. When the negative voltage at the output terminal increases in magnitude to reach the voltage level of the voltage source, the start circuit (10-2) is turned off.

Description: This invention relates to voltage regulator circuits of the kind adapted to supply a regulated negative voltage. The invention also relates to a method of operating such a voltage regulator circuit. A switching voltage regulator chip, such as the TL497 chip, which is manufactured by Texas Instruments, may be used in a circuit which has an output terminal from which a regulated negative voltage is obtained. The regulator chip includes a start terminal and an internal switching transistor which switches "on" and "off" to provide current to other elements included in the circuit. As soon as an energizing signal is placed on the start terminal, the switching transistor starts to switch on and off, and during this start-up time, the voltage on the output terminal of the circuit begins to increase in a negative direction. One of the problems with a typical prior art negative switching voltage regulator of the type mentioned is that the negative output voltage typically can take an unduly long time, for example up to 10 seconds to reach its predetermined level. If, for example, a liquid crystal display (LCD) is operated from the negative voltage, during the 10 seconds that the voltage is increasing negatively to the predetermined level, the LCD and the terminal in which the LCD is located may be considered inoperative. It is an object of the present invention to provide a voltage regulator circuit of the kind specified which starts up rapidly in a reliable manner, yet is of simple construction and inexpensive to manufacture. Therefore, according to one aspect of the present invention, there is provided a voltage regulator circuit having an output terminal and including a switching regulator chip having a start input terminal, with the voltage at said output terminal increasing in magnitude towards a regulated predetermined negative voltage when an energizing signal is placed on said start input, characterized by a start circuit having a voltage terminal adapted to be connected to a source of negative voltage, said start circuit being adapted to couple said voltage terminal to said output terminal as soon as a said energizing signal is placed on said start input terminal and to uncouple said voltage terminal from said output terminal when the voltage coming from the switching regulator circuit is greater in magnitude than the voltage at said voltage terminal, but less in magnitude than said predetermined voltage. According to another aspect of the present invention, there is provided a method of operating a voltage regulator circuit having an output terminal and including a switching regulator chip having a start terminal, characterized by the steps of placing an energizing signal on said start terminal causing said switching regulator chip to effect the provision of a negative voltage which increases in magnitude towards a predetermined negative voltage at said output terminal; coupling a source of negative voltage to said output terminal in response to said energizing signal, with the voltage of said second source being less in magnitude than said predetermined negative voltage; and uncoupling said source of negative voltage from said output terminal when the increasing negative voltage reaches the voltage of said second source of negative voltage. One embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Figs. 1A and 1B, taken together, show a circuit which is a preferred embodiment of this invention. Referring to Figs. 1A and 1B, there is shown a voltage regulator circuit 10 which includes a first circuit portion 10-1 shown in Fig. 1B, and a second circuit portion 10-2 shown in Fig. 1A. The first circuit portion 10-1 (Fig. 1B) represents a negative switching voltage regulator circuit which includes a regulator chip 12, like chip TL497, which is manufactured by Texas Instruments, for example. The first circuit portion 10-1 also includes: resistors R1, R4, R5, R6, R9, and R18; capacitors C1, C2, C3, C4, C5, and C6; and inductor L1 which are all interconnected, as shown, to provide a negative switching voltage regulator. Typical sizes of the components used in Fig. 1B are shown thereon. The first circuit portion 10-1 has an output VLCD which is connected to an output conductor 14. The regulator chip 12 has an internal switching transistor (not shown) coupled to pins 8 and 10 of the regulator chip 12; this transistor switches on and off to provide a current through the inductor L1. This internal switching transistor starts the switching in response to a Start signal placed on conductor 16. The Start signal changes the voltage level on conductor 16 from a high level to a low level when the Start signal is applied. The Start signal may come from a controller (not shown) which is not important to an understanding of this invention. In normal operation, the voltage level at the output VLCD of the first circuit portion 10-1 begins to increase in the negative direction as soon as the internal transistor starts switching on and off in response to the Start signal falling to a low level. In a typical application, the voltage level at the output VLCD may reach a predetermined level, like about a negative 20 volts, after a period of up to ten seconds as stated earlier herein. In some situations, the voltage at the output VLCD may not come up to specifications at all. A feature of the present invention is that it provides a method and circuit to quickly and reliably start the negative switching voltage regulator circuit shown as first circuit portion 10-1 shown in Fig. 1B. This quick starting is effected through providing a forcing current onto the output VLCD. The second circuit portion 10-2 shown in Fig. 1A provides the forcing current. Generally stated, the second circuit portion 10-2 is a start circuit which is coupled to a second source of negative voltage (-12 Volts) to couple the second source of negative voltage to the output terminal VLCD as soon as the Start signal becomes active. The second circuit portion 10-2 also uncouples the second source of negative voltage (-12 Volts) from the output terminal VLCD when the voltage coming from the first circuit portion 10-1, itself, is greater than the voltage at the second source of negative voltage, but is less than the predetermined voltage (-20 Volts) of the first circuit portion 10-1. The operation of the second circuit portion 10-2 will now be described with reference to Fig. 1A. When the Start signal is placed on conductor 16, the low level signal is also placed on conductor 18 which is fed into the base of a PNP transistor Q2 through a resistor R8 to cause transistor Q2 to conduct. The emitter of transistor Q2 is coupled through a diode to a source of +5 Volts. When the transistor Q2 conducts, a positive voltage from the collector of transistor Q2 is placed on the base of an NPN transistor Q1 through a resistor R7, causing the transistor Q1 to conduct. The emitter of transistor Q1 is coupled to the second source of negative voltage (-12 Volts) via conductor 20. When the transistor Q1 conducts, the collector of Q1 and the conductor 22 will have a nominal -12 Volts thereon. Because the conductor 22 has a -12 Volts thereon, it pulls conductor 14 to the nominal -12 Volts through a diode CR3. Due to the voltage drop across the transistor Q1 and the diode CR3, the actual voltage on conductor 14 and the output VLCD is about -11.2 or -11.3 Volts. To review, as soon as the Start signal is placed on the regulator chip 12, the second circuit portion 10-2 causes a forcing current (the second source of negative voltage -12 Volts) to be applied to the output VLCD of the circuit 10. The first circuit portion 10-1 continues to provide an increasing negative voltage to the output VLCD of the circuit 10 until the output of the first circuit portion 10-1 reaches the predetermined amount; in the embodiment described, this predetermined amount is about -20 Volts. The diode CR3 continues to conduct until the voltage coming from the first circuit portion 10-1 and appearing at the output VLCD reaches the nominal -12 Volts. When the output VLCD reaches -12 Volts, the diode CR3 will no longer conduct; this will cause the second circuit portion 10-2 to drop out of the circuit 10. This is an important feature for lap top terminals because it saves battery power. The first circuit portion 10-1 will continue to increase the negative voltage at the output VLCD until the predetermined voltage level is reached. The output voltage VLCD of the circuit 10 may be coupled to an LCD as shown in Fig. 1B. A discussion of some miscellaneous points appears appropriate. When the internal switching transistor in the regulator chip 12 switches "off", the inductor L1 wants to maintain the same current and current direction. In fact, the inductor L1 wants to pull the current towards ground level; the additional current comes from output VLCD and passes through diode CR1. Diode CR2 (Fig. 1A) is there to make sure that the transistor Q2 is turned on when the Start signal goes to a low or active level; it also makes sure that the transistor Q2 is turned off when the signal level on the Start pin of the regulator chip 12 is at a high level. The diode CR3 is also present to make sure that switching voltage generated by the first circuit portion 10-1 can increase to a more negative level than the voltage level of the second source of negative voltage which is -12 Volts in the embodiment described. Naturally, the voltage levels discussed herein can be changed to accommodate different applications. A feature of this invention is that there is a definite start up point and turn off point for the second circuit portion 10-2.

Claims: A voltage regulator circuit having an output terminal and including a switching regulator chip (12) having a start input terminal, with the voltage at said output terminal increasing in magnitude towards a regulated predetermined negative voltage when an energizing signal is placed on said start input, characterized by a start circuit (10-2) having a voltage terminal adapted to be connected to a source of negative voltage, said start circuit (10-2) being adapted to couple said voltage terminal to said output terminal as soon as a said energizing signal is placed on said start input terminal and to uncouple said voltage terminal from said output terminal when the voltage coming from the switching regulator circuit is greater in magnitude than the voltage at said voltage terminal, but less in magnitude than said predetermined voltage. A voltage regulator circuit according to claim 1, characterized in that said start circuit (10-2) includes coupling means (Q1, Q2, CR2, CR3) coupled to said voltage source, to said start input terminal of said switching regulator chip (12) and to said output terminal. A voltage regulator circuit according to claim 2, characterized in that said coupling means (Q1, Q2, CR2, CR3) includes a diode (CR3) coupled between said output terminal and a first transistor (Q1) coupled to said voltage terminal. A voltage regulator circuit according to claim 3, characterized in that said coupling means (Q1, Q2, CR2, CR3) includes a second transistor (Q2) coupled between said start input and said first transistor (Q1), and adapted to condition said first transistor (Q1) to couple the cathode of said diode (CR3) to said voltage terminal when a said energizing signal is applied to said start input terminal. A voltage regulator circuit according to claim 4, characterized in that said first transistor (Q1) is an NPN transistor and said second transistor (Q2) is a PNP transistor, and in that a further diode (CR2) is coupled between the emitter of said second transistor (Q2) and a source of positive voltage. A method of operating a voltage regulator circuit according to claim 1 characterized by the steps of placing an energizing signal on said start terminal causing said switching regulator chip (12) to effect the provision of a negative voltage which increases in magnitude towards a predetermined negative voltage at said output terminal; coupling a source of negative voltage to said output terminal in response to said energizing signal, with the voltage of said second source being less in magnitude than said predetermined negative voltage; and uncoupling said source of negative voltage from said output terminal when the increasing negative voltage reaches the voltage of said second source of negative voltage. A method according to claim 6, characterized in that said coupling step and said uncoupling step are effected through using a single diode (CR3).

Inventors: NEWMAN LEE A, NEWMAN, LEE A.
Applicants: AT & T GLOBAL INF SOLUTION, AT&T GLOBAL INFORMATION SOLUTIONS INTERNATIONAL INC.
Application year: 1992
Main classification: H02M3
IPC codes: H02M1, H03K17, H02M3