1. Field of the Invention
The invention relates to power supply and, in particular, to power supply for real-time clock generation.
2. Description of the Related Art
Most modern electronic systems are provided with real-time clocks that keep track of time even when an electronic system is turned off. Typically, real-time clocks run on a special battery not connected to a normal power supply.
In the conventional power supplies for real-time clock generation, voltage of the system power VSYS or VCC is typically higher or even the highest in the system. In advanced semiconductor process technology, RTC circuits, however, are typically implemented with core devices having lower voltage endurance. Therefore, there is a need to have a new power supply which can provide sufficient power to an RTC circuit without exceeding the low voltage endurance.
An embodiment of a power supply provides power to a real-time clock generator when a system power is not available and comprises first and second regulators, an energy storage device, and a switch. The first regulator receives a system power and generates a first regulated voltage when the system power is available. The energy storage device is coupled to a node. The second regulator comprises an input coupled to the node and provides a second regulated voltage to a real-time clock generator. The switch is coupled between the first regulator and the node. The switch is turned on when the system power is available and turned off when the system power is not available.
Another embodiment of a power supply provides power to a real-time clock generator when system power is not available and comprises an energy storage device, a regulator, and a switch. The energy storage device is coupled to a node. The regulator comprises an input coupled to the node and provides a regulated voltage to a real-time clock generator. The switch is coupled between the system power and the node. The switch is turned on when the system power is available and is turned off when the system power is not available.
The invention provides a power supply for real-time clock generation. In the power supply of the invention, a rechargeable battery is recharged by system power and used as a redundant power supply when the system power is not available. In addition, the power supply of the invention sustains longer when the system power is not available and the improvement becomes more significant in advanced semiconductor process technologies.
A detailed description is given in the following embodiments with reference to the accompanying drawings.
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
In the disclosure of the invention, a cellular phone is used as an example of an electronic system having an RTC generator. Voltage of system power, i.e. battery power, in the cellular phone typically ranges from 3.3V to 4.2V. Operating voltage of the RTC generator lowers in advanced semiconductor process technologies, typically 1.2V in state of the art technology. As a result, voltage of the system power is much higher than voltage limits of devices in the RTC generator and a voltage regulator is thus required to down-convert the system power to the operating voltage of the RTC generator.
The linear regulator 310 converts the battery power Vbat to the operating voltage Vrtc of the RTC generator RTC and supplies electrical energy to the energy storage device 320 when battery power Vbat is available. The energy storage device 320 includes Cbat, which is a large capacitor or a small rechargeable battery. When battery power is interrupted, the linear regulator 310 cannot work and supply power to the RTC generator RTC. Meanwhile, the energy storage device 320 keeps supplying power to the RTC generator RTC until the operating voltage Vrtc is lower than the lower limit thereof.
When the battery is removed from the cellular phone, power of the RTC generator RTC is supplied by the energy storage device 320. The voltage Vrtc decreases when a current Irtc supplies to the RTC generator RTC. After a time period T, Vrtc will reach Vrtc_min, which is a minimum requirement for the RTC generator RTC to operate. The time period T can be calculated by T=(Vrtc−Vrtc_min)×Cbat/Irtc, wherein Cbat is capacitance of the energy storage device 320, and Irtc is a quiescent current of the RTC generator RTC. To increase the time period T, Vrtc−Vrtc_min or Cbat needs to be increased or Irtc needs to be reduced. However, in advanced semiconductor process technologies, Vrtc−Vrtc_min becomes smaller and it is difficult to reduce the quiescent current Irtc of the RTC generator RTC. Increase of the capacitance Cbat of the energy storage device 320 will increase chip area and cost.
When voltage of the battery power Vbat exceeds a predetermined value, the switch SW is turned on. Meanwhile, the first regulator 410 down-converts the battery power Vbat to a first regulated voltage Vreg. Since the switch SW is turned on, the first regulated voltage Vreg is transferred to the node N. The second regulator 420 receives the first regulated voltage Vreg and generates the second regulated voltage Vrtc. When voltage of the battery power Vbat is lower than the predetermined value, the switch SW is turned off. Since the switch SW is turned off, energy stored in the energy storage device 430 does not flow back to the first regulator 410. The energy storage device 430 provides energy stored therein to the second regulator 420 and the second regulator 420 keeps providing the second regulated voltage Vrtc to the RTC generator RTC until the energy stored in the energy storage device 430 is insufficient.
When the battery is removed from the cellular phone, power of the RTC generator RTC is supplied by the energy storage device 430. The voltage Vreg decreases when a current (Irtc+Ireg) supplies to the second regulator 420. After a time period T′, Vrtc will reach Vrtc_min, which is a minimum requirement for the RTC generator RTC to operate. The time period T′ can be calculated by T′=(Vreg−Vrtc_min−Vdrop_out)×Cbat/(Irtc+Ireg), wherein Vdrop_out is a voltage drop across the second regulator 420, Cbat is capacitance of the energy storage device 430, Irtc is a quiescent current of the RTC generator RTC, and Ireg is a quiescent current of the second regulator 420. Since the first regulated voltage Vreg is not directly provided to the RTC regulator RTC, the first regulated voltage Vreg is much higher than the normal operating voltage, i.e. the second regulated voltage Vrtc herein, of the RTC regulator RTC and even up to the voltage level of the battery power Vbat. Thus, (Vreg−Vrtc_min−Vdrop_out) in the power supply of the invention is much higher than (Vrtc−Vrtc_min) in the previously disclosed power supply. As a result, if the quiescent current Ireg of the second regulator 420 is small enough, the power supply can provide power to the RTC generator with longer time.
In
The power supply for real-time clock generation can further comprise a control bit latch 440. The control bit latch 440 is coupled to the second regulator 420. A control input CK and a data input D of the control bit latch 440 respectively receive the enable signal en and a control signal Sc from the system. When voltage of the battery power Vbat exceeds a predetermined value, the enable signal en is at a logic state “high” and the control bit latch 440 receives and directly outputs the control signal Sc to the second regulator 420. The second regulator 420 is reconfigured according to the control signal Sc and the second regulated voltage Vrtc is thus adjustable. When voltage of the battery power Vbat is lower than the predetermined value, the enable signal en switches to a logic state “low” and the control bit latch 440 latches the control signal Sc. As a result, the state of the control bit is retained at the data output Q, and the RTC generator continues to function normally even when the system power is lost. In this embodiment, the second regulated voltage Vrtc is selected among different voltage levels based on the control bit.
While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
This application claims the benefit of U.S. Provisional Application No. 60/746,175, filed on May 2, 2006.
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Number | Date | Country | |
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Number | Date | Country | |
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60746175 | May 2006 | US |