In certain electronic circuits, a converted voltage may be used. For example, in a modern PC (personal computer) system, a real time clock (RTC) produces a frequency output that is then used to provide a time base for the system, which thus requires constant power. For this purpose, an RTC Crystal Oscillator (RTCCO) resides on an I/O (input/output) controller hub chip, which is sometimes referred to as the “south-bridge”. An RTC circuit provides an accurate oscillator output (commonly a frequency of 32.768 kHz) that is used as the main clock to maintain system time. The output of the RTC circuit is divided to obtain time in units of seconds, minutes, and hours. The time is stored by the system and used as the time basis for the system, which is maintained when the system power is either on or off.
When PC system is powered down, the RTC circuit derives power from another power source, such as a self-contained source in the PC. A 3.0-volt coin cell lithium battery is generally used because such batteries are widely available and very inexpensive. In certain systems, another power source, such as a charged capacitor, may provide the power for the RTC circuit when the system is powered down. A PC system may be turned off for long periods of time, possibly for years, depending upon usage and the length of time a system may stay in storage. Therefore, an RTC circuit may potentially need to derive power from a coin cell battery or other such power source for a period of years to maintain system time.
As computer processes move towards lower voltages in order to reduce power consumption and to increase speed in digital sections, the voltage of a coin cell may need to be stepped down to a lower voltage, such as a voltage range of less than 2 volts, depending upon the process voltage. The process of converting a DC voltage to a lower voltage consumes some amount of power, thereby reducing the length of time that the system can maintain the system time. Further, a certain minimum voltage is needed to operate the supplied circuit. Because the voltage of a battery or capacitor power source will fall over time as power is consumed, the voltage response of the DC-to-DC converter has an impact on the operation of the supplied circuit.
FIG. 1 illustrates one example of a conventional DC-to-DC converter. A voltage supply 105, such as a coin battery, provides a voltage to the circuit. The voltage supply is connected to the source of circuit is comprised of diode-connected transistors Q2 110 and Q3 115, which provide voltage drops and step down the voltage to the gate of output device 135. A reference load is provided, shown in FIG. 1 as comprising diode-connected transistors Q4 120 and Q5 125. Connected between Q4 120 and Q5 125 is transistor device Q1 130. A current through Q1 130 to adjust the reference load is provided by a clamping control circuit 160, which is controlled by a signal 165. The output voltage 140 from the circuit is supplied to certain devices, shown as an RTC oscillator 145 and RTC logic 150 utilized in maintenance of system time.
FIG. 2 is a graph of voltage output for a conventional DC-to DC converter, such as that shown in FIG. 1. The graph is provided for illustration and is not necessarily drawn to scale. FIG. 2 shows Vin 205 on the X-axis versus Vout 210 on the Y-axis for various voltages. Vin 205 is the voltage supplied by the power source, such as a battery or capacitor. Vout is the voltage output provided by the converter. A value for Vs 215 is shown on the Y-axis, Vs being the supply voltage required for operation of the devices that receive the output voltage. The graph curve 220 for Vin versus Vout falls off relatively quickly as the value of Vin drops. Therefore, the output voltage will drop off as a battery or other power source is depleted, eventually dropping below the needed voltage for operation of supplied devices, such as an RTC circuit.