JP2007195345A - Dc-dc converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a DC-DC converter of a switched capacitor type that eliminates a resistor-voltage dividing circuit at an output stage and is variable in output. <P>SOLUTION: The DC-DC converter comprises: a DC power supply; a plurality of unit boosting circuits that perform charging operations for charging capacitors by connecting the capacitors to the DC power supply in parallel therewith, and discharging operations that discharge the capacitors by connecting the capacitors to the DC power supply in series thereto, and are connected in series between the DC power supply and an output end; and a voltage control circuit that makes all the unit boosting circuits perform the charging and discharging operations after making the unit boosting circuits of the number corresponding to command signals perform the charging operations. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a voltage control circuit of a power supply circuit such as a display device using a DC power supply voltage such as an electrophoretic display device (EPD).

  A display device such as EPD, which is being developed as a display device for a portable device, requires a drive voltage of 15 to 20 [V] for pixel display. However, since a portable device uses a battery as a power source, the power supply voltage is often about 3 [V]. For this reason, it is necessary to boost the power supply voltage 5 to 6 times. A DC-DC converter is used for this boosting. Examples using a DC-DC converter for the display are described in Patent Documents 1 and 2, for example.

By the way, in a display device such as an EPD, display quality (image contrast, holding time, etc.) varies depending on the magnitude of the drive voltage applied to the pixel electrode. For example, when the drive voltage is increased, the contrast and holding time of the image are improved. On the other hand, since the power consumption increases when the drive voltage is increased, the drive voltage can be set to be variable so that it can be set to an appropriate drive voltage according to circumstances in a display or a portable information device that uses a battery as a power source. Is desirable. For this reason, the drive voltage can be adjusted by combining the resistance voltage dividing circuit and the output selection circuit (switch element).
JP2003-15507 JP 2005-315919 A

  However, when the voltage boosted by the DC-DC converter is variably adjusted by the resistance voltage dividing circuit and the voltage dividing output selection circuit, useless current flows to the resistance voltage dividing circuit. Further, when the current supply capability of the DC-DC converter is not sufficient, the boosted voltage is lowered. Further, in order to operate the switch element (transistor) of the output selection circuit, a gate voltage larger than the boosted high voltage is required, and a circuit (level shifter) for this purpose is required.

  Accordingly, an object of the present invention is to provide a switched-capacitor variable output DC-DC converter that eliminates the need for a resistive voltage divider circuit.

  In order to achieve the above object, a DC-DC converter according to the present invention comprises a DC power supply, a charging operation in which a capacitor is connected in parallel to the DC power supply, and a capacitor connected in series to the DC power supply. A plurality of unit booster circuits connected in series between the DC power supply and the output terminal, and the number of unit booster circuits corresponding to the command signal perform the charging operation or the command A voltage control circuit that causes all the unit booster circuits to perform the discharge operation after the number of unit booster circuits corresponding to the signal has stopped the charging operation.

  With such a configuration, the output voltage can be made variable in the switched capacitor type DC-DC converter without using a resistance voltage dividing circuit. Therefore, it is possible to avoid the power loss due to the resistance voltage dividing circuit, the switch element (transistor) group for selecting the resistance voltage dividing output, the high-level drive signal group necessary for operating the switch element group, and the generation circuit of those signals. It becomes possible to do. Further, a resistor voltage dividing circuit having a large current capacity, a drive circuit for a selection switch element, and the like are not necessary, which is convenient for an integrated circuit.

  Preferably, the unit booster circuit includes a first switch element connected between a circuit input terminal and a circuit output terminal, and second and third switches connected in series between the circuit input terminal and a reference potential. And a capacitor connected between the circuit output terminal and the connection point between the second and third switch elements, and the voltage control circuit corresponds to the charging operation. And the third switch element is turned on, the second switch element is turned off, the first and third switch elements are turned off and the second switch element is turned on in response to the discharge operation. .

  With such a configuration, a switched capacitor unit DC-DC converter is formed.

  Preferably, the command signal is a signal indicating an output voltage. The output voltage is variably set by determining the number of unit-unit DC-DC converters that perform the charging operation (corresponding unit DC-DC converters) corresponding to the output voltage indicated by the command signal by the voltage control circuit. The

  The display device of the present invention includes the above-described DC-DC converter. Accordingly, a display device with less power consumption can be obtained, which is advantageous for a battery-powered display device.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. 1 to 4 are explanatory diagrams for explaining the basic operation of a switched capacitor DC-DC converter that boosts an input DC voltage. In the drawings, corresponding parts are denoted by the same reference numerals, and description thereof will be omitted.

  1 and 2 show the charging operation of the DC-DC converter. As shown in FIG. 1, in this embodiment (description example), five stages of unit DC-DC converters are connected in series (in series) between an input terminal IN and an output terminal OUT. A low voltage LVDD (for example, 3 [V]) of a battery (not shown) is applied to the input terminal IN, and a boosted DC high voltage HVDD (for example, 18 [V]) is output to the output terminal OUT. Each unit DC-DC converter includes three switch elements and one capacitor. For example, as indicated by a dotted line in the drawing, the first unit DC-DC converter is configured by switch elements SW1a, SW2a, SW2a 'and a capacitor Ca.

  In the circuit configuration of the first unit DC-DC converter, a switch SW1a is connected between an input end and an output end. Switch elements SW2a and SW2a 'are connected in series between the input terminal of the unit DC-DC converter and a reference potential (for example, ground potential). A capacitor Ca is connected between the connection point between the switch elements SW2a and SW2a 'and the output terminal of the unit DC-DC converter. The switch elements SW2a and SW2a 'are switches that operate complementarily, and the switch elements SW1a and SW2a are the same kind of switch elements.

  The second to fifth unit DC-DC converters are similarly configured by switch elements SW1b to SW1e, switch elements SW2b to SW2e, switch elements SW2b 'to SW2e', and capacitors Cb to Ce. A switch element SW1f for preventing backflow is provided between the output terminal of the fifth unit DC-DC converter and the circuit output terminal OUT.

  In the charging operation of the DC-DC converter configured as described above, the switch elements SW1a to SW1e are turned on, the switch element SW1f is turned off, the switch elements SW2a to SW2e are turned off, and the switch elements SW2a ′ to SW2e ′ are turned on. Is done by.

  As a result, as shown in FIG. 2, the capacitors Ca to Ce are connected in parallel between the DC power supply LVDD and the reference potential, and are charged respectively.

  FIG. 3 shows the discharge operation of the DC-DC converter. The discharging operation is performed by turning off the switch elements SW1a to SW1e, turning on the switch element SW1f, turning on the switch elements SW2a to SW2e, and turning off the switch elements SW2a 'to SW2e'.

  As a result, as shown in FIG. 4, the capacitors Ca to Ce are connected in series between the DC power supply LVDD and the output terminal OUT, and each discharges. If the voltage due to the charges charged in the capacitors Ca to Ce is Va to Ve, the voltage at the output terminal OUT is LVDD + Va + Vb + Vc + Vd + Ve [V].

  Here, if the capacitors Ca to Ce are configured in the same manner, the voltage of the output terminal OUT is LVDD + 5 × Va [V], and if LVDD is 3 volts, a DC output voltage of 18 volts can be obtained as HVDD. If the capacitors Ca to Ce are not charged, the voltage at the output terminal OUT is LVDD + 0 [V], which is 3 [V]. Therefore, the setting of the output voltage of the DC-DC converter can be variably set from LVDD (for example, 3 [V]) to LVDD + 5 × Va (for example, 18 [V]) depending on whether or not the capacitor of each stage is charged. . According to such a method, it is not necessary to use a resistance voltage dividing circuit.

  FIG. 5 shows a case where the above-described DC-DC converter device is constituted by an integrated circuit.

  As shown in FIG. 1, the DC-DC converter device 1 includes a switch control unit 10 and a DC-DC converter unit 20.

  In the switch control unit 10, for example, an output voltage to be set in the DC-DC converter device 1 is supplied to terminals DD0 and DD1 as a command signal from a brightness control circuit of an EPD not shown. In addition, power LVDD is supplied. The switch control unit 10 outputs selection signals XHVSEL4 to 6 corresponding to the command signals DD0 and 1 to the DC-DC converter unit 20. In the embodiment, as described later, the charging operation of the third to fifth unit DC-DC converters among the five unit DC-DC converters can be selected.

  The DC-DC converter unit 20 is further supplied with a clock signal DCK and a power supply voltage LVDD corresponding to the charging operation mode and the discharging operation mode. Although the DC-DC converter unit 20 is configured by an integrated circuit, capacitors Ca to Ce having large capacities are externally attached to external terminals C2 to C6 of the DC-DC converter unit 20. A capacitor CO is connected to the output terminal VSC of the DC-DC converter unit 20 that outputs the boosted DC voltage, and holds the output high voltage HVDD.

  FIG. 6 shows a specific configuration example of the DC-DC converter unit 20.

  In the figure, parts corresponding to those in FIG. As described above, the DC-DC converter is composed of a 5-stage unit DC-DC converter. Among these, the charging operation of the third to fifth unit DC-DC converters can be selectively set.

  In other words, the one-conductivity type (PMOS) transistors M3, M6, M9, M12, M15, and M16 correspond to the switch elements SW1a to SW1f, respectively. M16 is diode-connected to prevent a backflow of current from the output capacitor CO during the charging operation. One conductivity type (PMOS) transistors M1, M4, M7, M10, and M13 correspond to the switch elements SW2a to SW2e, respectively. Other conductivity type (NMOS) transistors M2, M5, M8, M11, and M14 correspond to the switch elements SW2a 'to SW2e', respectively. In addition, capacitors Ca to Ce in the figure correspond to Ca to Ce shown in FIG.

  A clock signal XDCK6 is supplied to each gate of the transistors M3, M6, M9, M12, and M15 of the first to fifth unit DC-DC converters. The clock signal XDCK6 is a clock signal that is level-shifted by inverting the clock signal DCK by an inverter composed of complementary transistors M20 and M21 to which a boosted output voltage is applied at both ends.

  A clock signal XDCK obtained by inverting the clock signal DCK via an inverter is supplied to the gates of the complementary transistors M1 and M2, M4, and M5 of the first and second unit DC-DC converters.

  Output signals of NOR gates X1 to X3 are supplied to the gates of complementary transistors M7 and M8, M10 and M11, M13 and M14 of the third to fifth unit DC-DC converters, respectively.

  A clock signal DCK is supplied to each one input of the NOR gates X1 to X3. Selection signals XHVSEL4 to XHVSEL6 are supplied from the switch control unit 10 to the other inputs of the NOR gates X1 to X3, respectively. When the low level of the selection signals XHVSEL4 to XHVSEL6 is supplied to the NOR gates X1 to X3, the supplied unit DC-DC converter repeats the charging operation and the discharging operation according to the clock signal DCK, but the selection signals XHVSEL4 to XHVSEL6 When a high level is supplied to the NOR gates X1 to X3, the outputs of the NOR gates X1 to X3 are in a low level state, and the third to fifth unit DC-DC converters are not charged and are in a discharging operation state. Therefore, the potential difference between both ends of Cc to Ce becomes 0 V regardless of the DCK level.

  In the above-described configuration, the first to fifth unit DC-DC converters perform the charging operation to each capacitor when the clock signal DCK is at a low level and the XDCK is at a high level. In addition, when the clock signal DCK is at a high level and the XDCK is at a low level, the first to fifth unit DC-DC converters perform discharge operations by connecting the capacitors in series. As described above, since the charging operation of each capacitor of the third to fifth unit DC-DC converters is controlled by the selection signals XHVSEL4 to XHVSEL6, in the embodiment, for example, when the power supply voltage LVDD is 3 [V] The output voltage HVDD can be changed in steps of 3 [V] from 9 to 18 [V].

  In addition, each operation of the unit DC-DC converter causes the number of unit DC-DC converters corresponding to the command signal to perform the charging operation, or causes the number of unit DC-DC converters corresponding to the command signal to stop the charging operation. Thereafter, the same result can be obtained even if all the unit DC-DC converters are discharged.

  FIG. 7 shows a specific configuration example of the switch control unit 10. As shown in the figure, the switch control unit 10 includes a logic circuit 11, an AND gate, and an inverter.

  FIG. 8 shows signals input to the logic circuit 11 and output signals output from the corresponding circuit. As shown in the input / output table of the figure, for example, when the DC-DC converter performs the triple boosting operation of the power supply voltage LVDD, the command signal DD0 to the A input terminal and the B input terminal of the logic circuit 11 is used. And DD1 are set to levels of “1” and “1”, respectively. Accordingly, levels of “1”, “1”, “1”, and “0” are output to the output terminals Y0 to Y3 by logic (not shown) inside the logic circuit 11, respectively. The output signal at the output terminal Y0 is the above-described selection signal XHVSEL6. An AND output of the output terminals Y0 and Y1 becomes the selection signal XHVSEL5. A signal obtained by inverting the output signal Y3 of the output terminal Y3 through the inverter becomes the selection signal XHVSEL4. Similarly, four types of output voltages VDD3 (3 times boost) to VDD6 (6 times boost) are set by the command signals DD0 and DD1.

  Note that the number of unit DC-DC converters connected to the DC-DC converter and the number of unit DC-DC converters to be charged can be arbitrarily set, and are not limited to the above-described embodiments.

  In this way, the boosted output voltage of the DC-DC converter can be changed by individually setting the presence / absence of the charging operation of the plurality of unit DC-DC converters constituting the switched capacitor type DC-DC converter. It becomes.

  FIG. 9 shows a comparative example. In this example, the booster circuit shown in FIG. 1 boosts the voltage to the maximum voltage, and the boosted voltage VSC is changed using a resistor voltage divider circuit composed of resistors R1 to R4 and an output selection circuit composed of transistors T1 to T4. ing.

  As described above, in this configuration, there are power loss in the resistors R1 to R4 of the resistance voltage dividing circuit, and difficulty (heating, large area consumption, etc.) of providing the resistance voltage dividing circuit in the integrated circuit. Further, since the transistors T1 to T4 to which a high voltage is applied cannot be controlled by the low level selection signals SEL1 to SEL4, the level shift circuits LS1 to LS4 are separately provided and the low level selection signals SEL1 to SEL4 are set to the high level, respectively. It is necessary to convert to level selection signals HSEL1 to HSEL4. For this reason, there is a problem that the DC-DC converter circuit becomes complicated by the level shift circuits LS1 to LS4.

  According to the embodiment of the present invention described above, such a problem is solved.

  It should be noted that the DC-DC converter circuit of the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

FIG. 1 is an explanatory diagram for explaining the operation of each switch in the case of a charging operation in a switched capacitor type DC-DC converter circuit. FIG. 2 is an explanatory diagram for explaining a charging operation in a switched capacitor type DC-DC converter circuit. FIG. 3 is an explanatory diagram for explaining the operation of each switch in the case of a discharging operation in a switched capacitor type DC-DC converter circuit. FIG. 4 is an explanatory diagram for explaining the discharge operation in the switched capacitor type DC-DC converter circuit. FIG. 5 is an explanatory diagram illustrating an integrated circuit of a switched capacitor type DC-DC converter circuit. FIG. 6 is an explanatory diagram illustrating a specific circuit configuration example of a switched capacitor type DC-DC converter circuit. FIG. 7 is an explanatory diagram for explaining a control circuit of a switched capacitor type DC-DC converter circuit. FIG. 8 is a table for explaining the input / output operation of the logic circuit 11. FIG. 9 is an explanatory diagram illustrating a reference example of the resistance voltage dividing circuit.

Explanation of symbols

1 DC-DC converter device, 10 switch control unit, 11 logic circuit, 20 DC-DC converter unit

Claims (4)

DC power supply,
A series connection between the DC power supply and the output terminal is performed. The charging operation is performed by charging a capacitor connected in parallel to the DC power supply, and the discharging operation is performed by discharging the cupacitor in series with the DC power supply. A plurality of unit booster circuits,
After causing the number of unit booster circuits corresponding to the command signal to perform the charging operation or stopping the number of unit booster circuits corresponding to the command signal to stop the charging operation, all the unit booster circuits are configured to perform the charging operation. A voltage control circuit for performing a discharge operation;
DC-DC converter provided with. The unit booster circuit includes: a first switch element connected between a circuit input terminal and a circuit output terminal; second and third switch elements connected in series between the circuit input terminal and a reference potential; A capacitor connected between the circuit output terminal and the connection point between the second and third switch elements,
The voltage control circuit makes the first and third switch elements conductive in response to the charging operation, and makes the second switch element non-conductive, and responds to the discharge operation with the first and third switch elements. The DC-DC converter according to claim 1, wherein a switch element is non-conductive and the second switch element is conductive.   The DC-DC converter according to claim 1 or 2, wherein the command signal is a signal indicating an output voltage. A display device comprising the DC-DC converter according to claim 1.

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