Power saving DC-DC converter for radio pagers

Abstract: In a radio pager, the front end (2) of the pager is powered by a battery (3). A decoder (4) processes an incoming signal received by the front end and generates a first or a second signal when it is in an active state or an inactive state, respectively. A high duty-ratio pulse signal or a low duty-ratio pulse signal is generated in response to the first and second signals from the decoder, respectively. A voltage inducing circuit is connected between the battery (3) and a ground potential to respond to the high duty-ratio and low duty-ratio pulse signals by allowing a current to periodically pass through an inductor (15) for a varying duration proportional to the duty ratio of either of the pulse signals to cause a voltage higher than the battery voltage to be induced in the inductor. A capacitor (18) coupled to the inductor stores DC energy proportional to the duration of the current and supplies power to the decoder. A power savings circuit is provided for comparing the voltage developed in the capacitor with a reference voltage for ceasing the voltage in the capacitor to increase when it is higher than the reference voltage.

Description: The present invention relates to a DC-DC converter for hand-held radio pagers. Hand-held radio pagers are equipped with a built-in DC-DC converter for powering a decoder with a rated voltage. The DC-DC converter comprises a voltage inducing circuit coupled to a battery from which the front end of the pager receives its operating energy. The voltage inducing circuit is formed by a series of an inductor and a switching transistor which is turned on and off in response to gating pulses to charge a capacitor from which the decoder receives its operating energy. One example is to be found in "Transistor Technology" Journal, page 520; September 1989. Other examples are to be found in the documents GB-A-2 061 066 by the same applicant and US-A-3 953 785 (Bell). The gating pulse has a constant duty ratio that is predetermined so that the amount of DC energy stored in the capacitor meets the decoder's maximum energy requirement. However, the inactive state of the decoder accounts for a greater period of the pager operation, and therefore, the prior art DC-DC converter for radio pagers wastes most of the limited amount of energy. It is therefore an object of the present invention to provide a power saving DC-DC converter for radio pagers. According to the present invention, there is provided a DC-DC converter for a radio pager having a front end and a decoder for processing an incoming signal received by the front end, wherein the front end is powered by a battery and the decoder generates a first signal indicating a first, or active state of the decoder and a second signal indicating a second, or inactive state of the decoder. A first pulse signal having a first duty ratio or a second pulse signal having a second duty ratio is generated in response to the first or second signals from the decoder, respectively. A voltage inducing circuit is connected between the battery and a ground potential. The circuit includes an inductor and a gate-controlled switching element coupled thereto. The switching element is responsive to the first and second pulse signals for allowing a current to periodically pass through the inductor for a varying duration proportional to the duty of either of the pulse signals to cause a voltage higher than the voltage of the battery to be induced in the inductor. A capacitor is coupled to the inductor for storing DC energy proportional to the duration of the current and powering the decoder with a voltage developed therein. A power saving circuit is provided for comparing the voltage developed in the capacitor with a reference voltage. When the voltage in the capacitor is higher than the reference voltage, the capacitor voltage ceases to increase for power saving purposes. The present invention will be described in further detail with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a radio display pager incorporating a DC-DC converter of the present invention; and Fig. 2 is a waveform diagram showing waveforms of the pulses generated in the circuit of Fig. 1. In a radio display pager shown in Fig 1, radio-frequency paging signals detected by antenna 1 are fed into a receiver, or front end 2 in which the signals are converted into a form that can be digitally processed and applied to a decoder 4 of known circuitry. In a known manner, decoder 4 drives a display 5 for providing a display of received information and a loudspeaker 6 for alerting the user. Front end 2 is powered by a battery 3 and decoder 4, whose energy consumption varies significantly between active and inactive times, is powered by a DC-DC converter 7. DC-DC converter 7 is essentially a switching regulator and includes a pulse generator 10 that generates square-wave pulses "a" as shown in Fig. 2. This pulse signal is delayed by a predetermined amount by a delay circuit 11, producing a delayed pulse signal "b". These pulse signals are combined by an AND gate 12 to produce low duty-ratio pulses "c" in comparison with pulses "a". The high duty-ratio pulse signal from pulse generator 10 and the low duty-ratio pulse signal from AND gate 12 are supplied to the upper and lower positions of a switch 13, respectively, and one of the pulse signals is selectively applied to an AND gate 14 in accordance with the binary state of a load indicating signal which is supplied on lead 21 from decoder 4. The output of AND gate 14 is coupled to the base of a switching transistor 16 having an emitter-collector path connected between ground and one end of an inductor 15, the other end of which is coupled to battery 3. When AND gate 14 is enabled, switching transistor 16 is gated on and off according to the duty cycle of the pulses from AND gate 14 to induce a damped oscillatory voltage across inductor 15 higher than the voltage from battery 3. This voltage is rectified by a diode 17 and a capacitor 18, coupled to the cathode of diode 17, is charged to store DC energy of a varying level according to the load. As will be described, decoder 4 is sufficiently powered by this DC energy when processing the incoming signal and allows DC-DC converter 7 to decrease the amount of energy stored in capacitor 18 in the absence of incoming signals. To the cathode of diode 17 is coupled the negative input of a comparator 19. The positive input of comparator 19 is connected to a constant voltage source 20. By comparator 19 the voltage developed across capacitor 18 is compared with a prescribed reference voltage set by constant voltage source 20. A logic-0 output is produced by comparator 19 when the capacitor voltage is higher than the reference voltage, otherwise a logic-1 output is produced. The output of comparator 19 is applied to AND gate 14. Therefore, AND gate 14 is enabled when the voltage developed in capacitor 18 is lower than the reference voltage to supply DC energy thereto, and is advantageously disabled for power saving purposes when it is higher than the reference voltage. Decoder 4 generates a load indicating signal having a logic-1 state indicating that it is processing an incoming signal and a logic-0 state indicating that it is not processing signals. When the load-indicating signal is at logic-1 state, the contact of switch 13 is switched to the upper position for coupling the high duty-ratio pulse signal to AND gate 14. Otherwise, the switch is moved to the lower position for coupling the low duty-ratio pulse signal to AND gate 14. Since the amount of energy stored in capacitor 18 is proportional to the amount of current flowing through transistor 16, the DC energy stored in capacitor 18 increases when transistor 16 is gated with the high duty-ratio pulses and decreases when it is gated with the low duty-ratio pulses. Therefore, the stored energy increases in the presence of an incoming signal and decreases in the absence of an incoming signal. The foregoing description shows only one preferred embodiment of the present invention. Various modifications are apparent to those skilled in the art without departing from the scope of the present invention which is only limited by the appended claims. Therefore, the embodiment shown and described is only illustrative, not restrictive.

Claims: A DC-DC converter for a radio pager having a front end (2) powered by a battery (3), a decoder (4) for processing an incoming signal received by the front end, an oscillator (10-13) for generating oscillator pulses, an inductor (15) connected to the battery (3), a switching element (16) coupled thereto for interrupting a current through said inductor in response to the oscillator pulses to induce an oscillating voltage in said inductor, a capacitor (18) and a diode (17) for rectifying the oscillating voltage and charging the capadtor (18) with the rectified voltage, characterized: in that the decoder is powered by the voltage developed by said capacitor (18) which is coupled to the inductor for storing DC energy proportional to the duration of the current and generates a first signal indicating an active state of the decoder and a second signal indicating an inactive state of the decoder, the switching element being responsive to each of the first and second pulse signals for allowing a voltage to develop in the inductor, in that the oscillator (10-13) is a variable duty pulse generator means which generates a first pulse signal of a high duty ratio in response to said first signal and a second pulse signal of a low duty ratio in response to said second signal, and in that power saving means (14, 17, 19, 20) is provided for comparing the voltage developed by the capacitor (18) with a reference voltage and preventing the voltage in said capacitor from exceeding beyond the reference voltage. A DC-DC converter as claimed in claim 1, characterized in that the power saving means comprises: gate means (14) coupled between the variable duty pulse generator means (10-13) and the switching element (16) of the voltage inducing circuit; and a comparator (19) for comparing the voltage developed by said capacitor with the reference voltage and causing the gate means (14) to apply either of said first and second pulse signals to the switching element when the stored voltage is lower than the reference voltage and causing the gate means to cease to apply said first and second pulse signals to the switching element when the stored voltage is higher than the reference voltage. A DC-DC converter as claimed in claim 1 or 2, characterized in that the first pulse signal and the second pulse signal are of the same pulse repetition rate. A DC-DC converter as claimed in claim 1, 2 or 3, characterized in that said variable duty pulse generator means comprises: a pulse generator for generating said first pulse signal; delay means for delaying said first pulse signal; and gate means for combining the first pulse signal and the delayed pulse signal to produce said second pulse signal.

Inventors: NOBUSAWA HIDEAKI, NOBUSAWA, HIDEAKI, Nobusawa, Hideaki, c/o NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1991
Main classification: H04Q7
IPC codes: G05F1, G08B3, H02M3