Abstract:
A voltage drop DC-DC converter includes a switch circuit including a plurality of switches for alternately connecting a first and second capacitors in series and in parallel in response to an output signal of an oscillator circuit oscillating at a constant frequency. An efficient voltage dropping conversion is possible by deriving a voltage divided by the first and second capacitors when the first and second capacitors are connected in series and deriving terminal voltages of the first and second capacitors when the first and second capacitors are connected in parallel. When the derived voltage is increased beyond an aimed reference output voltage, the oscillation of the oscillator circuit is stopped.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage drop DC-DC converter and, in particular, to a voltage drop DC-DC converter for use as a power source circuit of a portable telephone set of such as PHS (Personal Handyphone System) or other portable telephone system or a portable electronic device such as electronic book or PDA (Personal Digital Assistance), which DC-DC converter is high efficiency with low noise and occupies a small area. 
     2. Description of the Prior Art 
     In a conventional battery-powered electronic device such as portable telephone set of a portable telephone system or a portable electronic device, an operating voltage thereof is lowered in order to reduce power consumption thereof and, recently, such battery-powered electronic device is operated with a 1.8V power source since a LSI mounted thereon is operated with a 3V power source. On the other hand, the development of LSI operating with a 1.5 V power source has been started most recently. 
     Incidentally, an output voltage of a power source such as a lithium ion battery to be used in an electronic device of such kind is in a range from 3.0V to 4.2V and is usually about 3.6 V. 
     The operating voltage of such LSI is usually obtained by dropping a battery voltage. In the conventional series regulator including a transistor for dropping a battery voltage, voltage-dropping efficiency is as low as about 50%. 
     On the other hand, the voltage-dropping efficiency of a voltage-dropping circuit constructed with a charge-pump circuit and a voltage-dropping transistor is 50% or higher. Therefore, such voltage-dropping circuit has been used as a power source circuit of a battery-powered electronic device. 
     A conventional voltage drop DC-DC converter of switched capacitor type, which is an example of such voltage-dropping circuit, is disclosed in JP H8-205524A. 
     FIG. 3 shows a construction of the disclosed voltage drop DC-DC converter. In FIG. 3, a voltage converting circuit  9  of the voltage drop DC-DC converter includes a capacitor C 1 , a capacitor C 2 , switches SWa, SWb and SWc and a switch control circuit  8  for switching a connection of the capacitors C 1  and C 2  between a series connection and a parallel connection. 
     The voltage converting circuit  9  operates to charge the capacitors C 1  and C 2  having substantially the same capacitance with power of an input voltage Vin when the capacitors are connected in series by on-off switching of the switches SWa and SWb under control of the switch control circuit  8 . After the capacitors C 1  and C 2  are charged, the series connection of the capacitors C 1  and C 2  is changed to a parallel connection by controlling the switches SWa, SWb and SWc to obtain a voltage across the capacitor C 1  as well as the capacitor C 2 , which is substantially a half of the charged voltage Vin across the series circuit of the capacitors C 1  and C 2 . The thus obtained voltage Vin/2 is transferred to a power capacitor C 3  to charge the latter and a power of the charge capacitor C 3 , that is, charges of the capacitor C 3 , is outputted externally from an output terminal  9 A of the voltage converting circuit  9 . Incidentally, a reference numeral  9 B in FIG. 3 depicts an input terminal to which the input voltage Vin is applied. 
     The output voltage at the output terminal  9 A of the voltage converting circuit  9  is dropped by a MOSFET Q, which is applied to an output terminal  11 A of the DC-DC converter as an output voltage Vout. A gate of the MOSFET Q is connected to an output terminal of a differential amplifier  7  and the output voltage Vout is controlled by an output voltage of the MOSFET Q. The differential amplifier  7  has a plus input terminal supplied with a reference voltage Vref from a reference voltage generator circuit  6  and a minus input terminal supplied with the output voltage from the output terminal  11 A of the DC-DC converter and performs a control for making the output voltage Vout at the output terminal  11 A becomes equal to the reference voltage Vref. 
     Although the output voltage Vout at the output terminal  11 A of the DC-DC converter is a detection voltage in this case, the voltage converting circuit  9  may output a constant voltage obtained by dividing the voltage Vout by a voltage dividing circuit as the detection voltage. 
     In such voltage drop DC-DC converter of switched capacitor type, however, there is problems of necessity of a power transistor as the voltage dropping transistor circuit, difficulty of obtaining conversion efficiency of 90% or higher and heat generation and increased area of a power source circuit. 
     According to this voltage drop DC-DC converter of switched capacitor type, it may be possible to regulate the output voltage by directly controlling the switches for changing the connection of the capacitors by the voltage converting circuit  9 , without using the voltage dropping transistor. In such case, however, it is necessary in order to stabilize the output voltage Vout of the DC-DC converter to control the switching frequency or switching period-of the switch control circuit  8 . Since, in such control, the switching is performed before the charging of the capacitors is completed, there is a problem that noise may be generated on the output voltage Vout. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a voltage drop type DC-DC converter, which is high efficiency with reduced noise generation and occupies a small area. 
     In order to achieve the above object, the voltage drop DC-DC converter according to the present invention is featured by comprising a first and second capacitors, an oscillator circuit for generating a signal having a specific frequency, a switch circuit including a plurality of switches for alternately connecting the first and second capacitors in series and in parallel in response to the output signal of the oscillator circuit, a DC power source for charging the first and second capacitors with a DC voltage when the first and second capacitors are connected in series, an output terminal for deriving a junction voltage at a junction point of the first and second capacitors when the first and second capacitors are connected in series and deriving a terminal voltage of the first and second capacitors when the first and second capacitors are connected in parallel and a comparator for comparing the voltage at said output terminal with a predetermined reference voltage and generating a signal for stopping the oscillation of said oscillator circuit when the voltage at said output terminal is larger than the reference voltage, wherein the first and second capacitors are connected in parallel when the oscillation of the oscillator circuit is stopped. 
     In the present invention, it is possible to effectively drop a power source voltage by providing the switch circuit including the plurality of switches for alternately connecting the first and second capacitors in series and in parallel in response to the output signal of the oscillator circuit oscillating at the specific frequency and by deriving the junction voltage at the junction point of the first and second capacitors when the first and second capacitors are connected in series and deriving the terminal voltage of the first and second capacitors when the first and second capacitors are connected in parallel. 
     Further, since the control of the oscillator circuit, when the aimed voltage is increased beyond the reference output voltage, is to merely stop the oscillation, there is no need of providing a voltage drop transistor, generation of noise and heat of the power source circuit is restricted and an area occupied by the voltage drop DC-DC converter can be small. 
     As a result, the voltage drop DC-DC converter of the present invention can provide effects of high efficiency, low noise, small heat generation and small occupation area. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a voltage drop DC-DC converter of switched capacitor type, according to an embodiment of the present invention; 
     FIG. 2 is a block diagram of a voltage drop DC-DC converter of switched capacitor type, according to another embodiment of the present invention; and 
     FIG. 3 is a schematic block diagram of an example of a conventional voltage drop DC-DC converter of switched capacitor type. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 in which same constructive components as those shown in FIG. 3 are depicted by same reference numerals, respectively, a voltage drop DC-DC converter  10  of switched capacitor type, which is driven by a lithium ion battery  1 , includes an oscillator circuit (OSC)  2 , an inverter  3 , driver circuits  4 A and  4 B, a voltage drop charge pump circuit  5 , a reference voltage generator circuit  6 , a comparator  7 A and an output terminal  10 A from which a regulated output voltage Vout is derived. A reference numeral  10 B indicates an input terminal inputted with an input voltage Vin from the lithium ion battery  1 . The circuit components from the input terminal  10 B to the output terminal  10 A are formed as an IC. 
     As shown in FIG. 1, the voltage drop transistor Q shown in FIG. 3 is removed and, instead thereof, an output SP of the comparator  7 A of the DC-DC converter  10  is supplied to the oscillator circuit  2 . The comparator  7 A has a minus input terminal supplied with a reference voltage Vref from the reference voltage generator circuit  6  and a plus input terminal, which is connected to the output terminal  10 A of the DC-DC converter  10  and provides the output voltage Vout. 
     With this circuit construction, the comparator  7 A generates a high level signal (H) when the output voltage Vout is higher than the reference voltage Vref, upon which the oscillation of the oscillator circuit  2  is stopped. 
     In the DC-DC converter  10  of the present invention, the output voltage Vout at the output terminal  10 A is used as a detection voltage. Alternatively, it is possible to divide the output voltage Vout by a voltage divider circuit and use a resultant voltage divided at a constant ratio as the detection voltage. In the latter case, the reference voltage Vref generated by the reference voltage generator circuit  6  becomes a low voltage corresponding to the constant dividing ratio. 
     The voltage drop charge pump circuit  5  corresponds to the voltage converter  9  shown in FIG.  3  and functions to switch the connection of capacitors C 1  and C 2  between a series connection and a parallel connection by on-off switching of switches SW 1 , SW 2 , SW 3  and SW 4 . 
     One terminal of the capacitor C 1  of the voltage drop charge pump circuit  5  is connected to a positive side (Vin) of the battery  1  through the switch SW 1  and to the output terminal  10 A through the switch SW 2 . The other a terminal of the capacitor C 1  is connected to a negative side of the battery  1  through a ground line and to the output terminal  10 A through the switch SW 4 . The switches SW 1 , SW 2 , SW 3  and SW 4  are constructed with MOSFET&#39;s, which are formed as an IC. 
     One terminal of the capacitor C 2  of the voltage drop charge pump circuit  5  is connected to the output terminal  10 A and the other terminal thereof is connected to the negative side of the battery  1  through the ground line. 
     The switches SW 1  and SW 4  respond to an output pulse P having duty cycle of about 50% and supplied from the oscillator circuit  2  through the driver circuit  4 A and are normally OFF except a period during which the output pulse P is “H”. The switches SW 2  and SW 3  are normally OFF except a period during which the output pulse P is “L”. Therefore, the switches SW 2  and SW 3  are ON/OFF switched complementarily to the switches SW 1  and SW 4 . 
     Therefore, in the case where the output pulse P of the oscillator circuit  2  is “H”, the capacitors C 1  and C 2  are connected in series when the switches SW 1  and SW 4  are in ON state and, in the case where the output pulse P of the oscillator circuit  2  is “L”, the capacitors C 1  and C 2  are connected in parallel when the switches SW 2  and SW 3  are in ON state. As a result, the output voltage at the output terminal  10 A becomes slightly lower than a half of the output voltage Vin of the battery  1 . In this embodiment, the reference voltage Vref is set to a voltage (=Vin/2−α) slightly lower than Vin/2. Assuming that the output voltage of the battery  1  is, for example, 3.8V, the reference voltage Vref is 1.8V (&lt;1.9V) and the output voltage Vout is stabilized to 1.8V. Incidentally, the battery voltage is not always limited to 3.8V and is usually selected from the range from 3.0V to 4.2V. However, the battery voltage range is not limited to this range. 
     An operation of the whole DC-DC converter of the present invention will be described. 
     The switching of the connection of the capacitors between the series connection and the parallel connection is started by the pulse output of the oscillator circuit  2  and the output voltage Vout at the output terminal  10 A is dropped to a value (≈Vin/2−α) by dropping the battery voltage Vin to substantially a half. Incidentally, α is a regulation voltage, which may be in a range from 0.05V to 0.5V when the battery voltage Vin is in the range from 4.2V to 3.0V. 
     When the output voltage Vout becomes higher than Vin/2−α, in other words, when the output voltage Vout becomes higher than the reference voltage Vref, the comparator  7 A generates an output pulse SP, which is in H level. In response to the output pulse SP, the oscillator circuit  2  stops to operate and the output pulse P of the oscillator circuit  2  becomes “L”. 
     When the oscillator circuit  2  stops to operate, the output pulse P thereof is maintained in the level “L”, so that the switches SW 2  and SW 3  are kept in ON state and the switches SW 1  and SW 4  are kept in OFF state. Therefore, the parallel connection of the capacitors C 1  and C 2  is maintained. Consequently, charge on the parallel capacitors charged by the battery  1  appears on the output terminal  10 A and, then, is discharged. The voltage at the output terminal  10 A is reduced by this discharge. When the output voltage Vout becomes a value slightly lower than Vin/2−α, the output pulse SP from the comparator  7 A is stopped and the oscillation of the oscillation circuit  2  is restarted. Therefore, the capacitors C 1  and C 2  are connected in series and charged. As a result, the control is performed such that the output voltage Vout is maintained at Vin/2−α (=1.8V). That is, the aimed voltage to be stabilized is Vin/2−α corresponding to the reference voltage Vref. 
     In the described embodiment, the oscillation frequency of the oscillator circuit  2  is set to a constant value, with which the switching of the switches is performed after the charging of the capacitors C 1  and C 2  is completed. The voltage drop charge pump circuit  5  always performs a reliable voltage dropping operation with the charging operation of the capacitors C 1  and C 2  being completed. Therefore, switching noise appearing on the output voltage Vout is restricted. 
     As described hereinbefore, the DC-DC converter  10  according to the present invention performs the voltage reduction by half. However, by the DC-DC converter  100  using n capacitors (where n is an integer larger than 2) as shown in FIG. 2, switching n capacitors to form a series connection of them and complementarily switching them to form a parallel connection of them, the DC-DC converter  100  can regulate the output voltage to a value slightly lower than Vin/n. When the n capacitors are connected in series in the latter case, a junction of the series connection of the capacitors, a voltage at which is the lowest one of voltages divided by the capacitors, is connected to the output terminal. 
     Although the capacitors C 1  and C 2  have substantially equal capacitances, it is possible to output an arbitrarily dropped voltage by selecting capacitances of them. 
     Further, although the battery is used as the DC power source in the described embodiment, the DC power source may be realized by a DC power source circuit for generating DC power by rectifying AC power, etc.