JP2009055752A - Dc/dc converter and electronic apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a step-up DC/DC converter capable of improving both efficiency characteristics and ripple characteristics without depending on the variation in an input voltage. <P>SOLUTION: This DC/DC converter for stepping up the voltage of an input voltage VBAT to generate a desired output voltage VOUT by switching-driving a power transistor N1 has: a control section CTRL for starting switching drive of the power transistor N1 when the output voltage VOUT decreases to a lower limit voltage VL and stopping the switching drive of the power transistor N1 when the output voltage VOUT increases to an upper limit voltage VH; and an ON-period setting section OPS for variably controlling an ON-period Ton of the power switching transistor N1 in the switching drive in response to the input voltage VBAT. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a DC / DC converter that boosts an input voltage to generate a desired output voltage, and an electronic device using the DC / DC converter.

  In recent years, various portable electronic devices that use batteries as a power source, such as mobile phone terminals and digital still cameras, have been put into practical use. The step-up DC / DC converters installed in these devices have efficiency characteristics and ripple characteristics. Improvement is required.

In addition, patent documents 1-5 can be mentioned as an example of the prior art relevant to the above.
JP-A-6-217538 JP-A-7-227082 JP-A-10-225104 JP 2000-228869 A JP 2004-166391 A

  By the way, as a control method of the DC / DC converter, as shown in FIGS. 3A and 3B, when the output voltage VOUT falls to a predetermined lower limit voltage VL, switching driving of the power transistor is started, and the output voltage There is a control method (so-called hysteresis control method) that stops switching driving of the power transistor when VOUT rises to a predetermined upper limit voltage VH. In the conventional DC / DC converter, an on period Ton ( In the examples of FIGS. 3A and 3B, the high level period of the pulse signal PULSE is set to a predetermined fixed value.

  Certainly, if the input voltage VBAT does not fluctuate at all, even in the conventional DC / DC converter described above, desired efficiency characteristics and ripple characteristics can be obtained by appropriately setting the ON period Ton at the time of switching. Is possible.

  However, for example, in a portable electronic device that uses a battery as a power source, the input voltage VBAT fluctuates greatly as the battery is consumed, and the DC / DC converter boost ratio (one on period) depends on this. The rise of the output voltage VOUT obtained by Ton also varies greatly.

  Therefore, when the input voltage VBAT is low, as shown in FIG. 3A, the increase width of the output voltage VOUT obtained in one ON period Ton becomes small, and the number of times the power transistor is switched (and thus switching loss). As a result, the efficiency characteristics deteriorated.

  On the other hand, when the input voltage VBAT is high, as shown in FIG. 3B, the increasing range of the output voltage VOUT obtained in one ON period Ton becomes large, and the output voltage VOUT exceeds the predetermined upper limit voltage VH. The ripple characteristics were worse.

  In view of the above problems, the present invention provides a step-up DC / DC converter capable of improving both efficiency characteristics and ripple characteristics without depending on fluctuations in input voltage, and an electronic device using the same ( For example, an object is to provide a portable electronic device using a battery as a power source.

  In order to achieve the above object, a DC / DC converter according to the present invention is a DC / DC converter that generates a desired output voltage by boosting an input voltage by switching driving a power transistor. A control unit that starts switching driving of the power transistor when the voltage falls to a predetermined lower limit voltage, and stops switching driving of the power transistor when the output voltage rises to a predetermined upper limit voltage; according to the input voltage; And an on-period setting unit that variably controls the on-period during switching driving of the power transistor (first configuration).

  In the DC / DC converter having the first configuration, the on-period setting unit sets a longer on-period during switching driving of the power transistor as the input voltage is lower. The higher the voltage, the shorter the ON period during switching driving of the power transistor (second configuration) may be used.

  In the DC / DC converter having the second configuration, the on-period setting unit includes a current source that generates a variable current according to the input voltage, a capacitor that is charged or discharged by the variable current, It is preferable that the time until the terminal voltage of the capacitor reaches a predetermined value is set as an ON period during the switching drive of the power transistor (third configuration).

  An electronic apparatus according to the present invention is generated by the DC / DC converter having any one of the first to third configurations, a battery for supplying the input voltage to the DC / DC converter, and the DC / DC converter. And a load using the output voltage as a drive voltage (fourth configuration).

  According to the present invention, although it is a simple system, it is possible to improve both efficiency characteristics and ripple characteristics, which are important characteristics for a step-up DC / DC converter, without depending on fluctuations in input voltage.

  Hereinafter, a detailed description will be given by taking as an example a case where the present invention is applied to a DC / DC converter mounted on a portable electronic device (such as a mobile phone terminal or a digital still camera) that uses a battery as a power source.

  FIG. 1 is a block diagram showing an embodiment of a DC / DC converter according to the present invention.

  As shown in the figure, the DC / DC converter of the present embodiment includes a control unit CTRL, an on period setting unit OPS, an N-channel field effect transistor N1, a coil L1, a diode D1, and resistors R1 and R2. And a capacitor C1.

  The control unit CTRL is driven by receiving the output voltage VOUT (or the input voltage VBAT), and a feedback voltage VFB corresponding to the output voltage VOUT (a part of the output voltage VOUT drawn from the connection node between the resistor R1 and the resistor R2). When the output voltage VOUT drops to a predetermined lower limit voltage VL, the switching drive of the transistor N1 is started, and when the output voltage VOUT rises to the predetermined upper limit voltage VH, the switching drive of the transistor N1 is stopped. It is means to do. The upper limit voltage VH and the lower limit voltage VL are constant voltages generated using a band gap circuit that does not depend on fluctuations in the input voltage VBAT or fluctuations in ambient temperature.

  The on-period setting unit OPS is means for variably controlling the on-period Ton when the transistor N1 is switched according to the input voltage VBAT. The on-period setting unit OPS includes P-channel field effect transistors P1 and P2 and N-channel field effect transistor N2. And a resistor R3, a capacitor C2, a comparator CMP1, and a DC voltage source E1.

  The sources of the transistors P1 and P2 are both connected to the application terminal for the input voltage VBAT. The gates of the transistors P1 and P2 are connected to each other, and the connection node is connected to the drain of the transistor P1.

  One end of the resistor R3 is connected to the drain of the transistor P1. The other end of the resistor R3 is connected to the ground terminal. One end of the capacitor C2 is connected to the drain of the transistor P2. The other end of the capacitor C2 is connected to the ground terminal. The drain of the transistor N2 is connected to the drain of the transistor P2. The source of the transistor N2 is connected to the ground terminal. The gate of the transistor N2 is connected to the charge / discharge control terminal of the control unit CTRL.

  The non-inverting input terminal (+) of the comparator CMP1 is connected to the positive terminal of the DC voltage source E1 (a band gap circuit that generates a constant voltage that does not depend on fluctuations in the input voltage VBAT or fluctuations in ambient temperature). The negative terminal of the DC voltage source E1 is connected to the ground terminal. The inverting input terminal (−) of the comparator CMP1 is connected to one end of the capacitor C2 (the terminal terminal for extracting the terminal voltage Vc). The power supply terminal of the comparator CMP1 is connected to the application terminal of the output voltage VOUT (or the application terminal of the input voltage VBAT).

  That is, the ON period setting unit OPS is charged by the current source (P1, P2, R3) that generates the variable current Im (= α × (VBAT−Vgs) / R) corresponding to the input voltage VBAT, and the variable current Im. And the capacitor C2 is configured to set the time until the terminal voltage Vc of the capacitor C2 reaches a predetermined value as the ON period Ton when the transistor N1 is switched. Note that the parameter α indicates the mirror ratio of the current mirror circuit including the transistors P1 and P2, and the parameter Vgs indicates the gate-source voltage (on threshold voltage) of the transistors P1 and P2. R represents the resistance value of the resistor R3.

  The gate of the transistor N1 is connected to the pulse signal output terminal of the control unit CTRL. The drain of the transistor N1 is connected to one end of the coil L1 and the anode of the diode D1. The other end of the coil L1 is connected to the application end of the input voltage VBAT. The cathode of the diode D1 is connected to the load. The source of the transistor N1 is connected to the ground terminal. One end of the capacitor C1 is connected to the cathode of the diode D1. The other end of the capacitor C1 is connected to the ground terminal. The resistors R1 and R2 are connected in series between one end of the capacitor C1 and the ground end, and the connection nodes of the resistors R1 and R2 are respectively connected to the feedback input ends of the control unit CTRL.

  Although not explicitly shown in FIG. 1, an electronic apparatus (for example, a digital still camera) according to the present invention includes a battery (for example, a lithium ion battery) that supplies an input voltage VBAT to a DC / DC converter, and a DC / DC Loads using the output voltage VOUT generated by the converter as a drive voltage (for example, driving a lens driver that is a heavy load, and a standby state and an idle state of a sub-microcomputer that is a light load) are included.

  The basic operation (DC / DC conversion operation) of the DC / DC converter having the above configuration will be described in detail.

  The transistor N1 is an output power transistor that is switched and driven in accordance with a pulse signal PULSE (switching drive signal) from the control unit CTRL.

  When the transistor N1 is turned on, a switch current flows to the coil L1 toward the ground terminal via the transistor N1, and the electrical energy is stored. Note that if the charge is already accumulated in the capacitor C1 during the ON period of the transistor N1, the current from the capacitor C1 flows through the load. At this time, since the anode potential of the diode D1 is lowered to almost the ground potential via the transistor N1, the diode D1 is in a reverse bias state, and no current flows from the capacitor C1 toward the transistor N1.

  On the other hand, when the transistor N1 is turned off, the electric energy stored therein is released by the counter electromotive voltage generated in the coil L1. At this time, since the diode D1 is in a forward bias state, the current flowing through the diode D1 flows into the load and also flows into the ground terminal through the capacitor C1, thereby charging the capacitor C1. By repeating the above operation, the load is supplied with the DC output boosted and smoothed by the capacitor C1.

  As described above, the DC / DC converter of the present embodiment drives the coil L1, which is an energy storage element, by on / off control of the transistor N1, thereby boosting the input voltage VBAT and generating a desired output voltage VOUT. Functions as a chopper type booster circuit.

  Next, the hysteresis control of the DC / DC converter configured as described above and the variable control of the ON period according to the input voltage VBAT will be described in detail with reference to FIG. 2 together with FIG.

  FIG. 2 is a timing chart for explaining the hysteresis control of the DC / DC converter according to the present invention and the variable control of the ON period according to the input voltage VBAT. FIG. 2A shows a case where the input voltage VBAT is low. (B) shows a case where the input voltage VBAT is high. 2A and 2B show the behaviors of the output voltage VOUT and the pulse signal PULSE. The solid line represents the behavior of the present invention, and the broken line represents the conventional behavior (FIGS. 3A and 3B). The same behavior as in b) is shown.

  When the input voltage VBAT is low, the variable current Im is small in the ON period setting unit OPS, and therefore the capacitor C2 is slowly charged. Therefore, the time until the terminal voltage Vc of the capacitor C2 reaches a predetermined value is lengthened, and the ON period Ton is set to be long when the transistor N1 is switched.

  Therefore, even when the input voltage VBAT is low, the number of switching times (and hence switching loss) of the transistor N1 does not increase unnecessarily, and it is possible to maintain good efficiency characteristics. The solid line in FIG. 2A shows a state in which the ON period Ton is ideally variably controlled so that the output voltage VOUT can be increased to the upper limit voltage VH without excess or deficiency in one switching drive. Yes.

  On the other hand, when the input voltage VBAT is high, the variable current Im increases in the ON period setting unit OPS, and thus the capacitor C2 is charged rapidly. Therefore, the time until the terminal voltage Vc of the capacitor C2 reaches a predetermined value is shortened, and as shown in FIG. 2B, the ON period Ton during the switching driving of the transistor N1 is set short.

  Therefore, even when the input voltage VBAT is low, the output voltage VOUT does not exceed the predetermined upper limit voltage VH, and the ripple characteristics can be maintained satisfactorily. The solid line in FIG. 2B shows a state in which the ON period Ton is ideally variably controlled so that the output voltage VOUT is increased to the upper limit voltage VH without excess or deficiency in one switching drive. Yes.

  Thus, in the case of the DC / DC converter of this embodiment, even if the input voltage VBAT varies, the step-up ratio of the DC / DC converter (the increase width of the output voltage VOUT obtained in one ON period Ton) is Since it does not fluctuate greatly, it is possible to improve both efficiency characteristics and ripple characteristics.

  Moreover, if it is the DC / DC converter of this embodiment, as shown in FIG. 1, since the said effect can be show | played using the ON period setting part OPS of a very simple structure, as the whole system There will be no cost increase.

  In the above embodiment, the case where the present invention is applied to a DC / DC converter mounted on a portable electronic device (such as a mobile phone terminal or a digital still camera) using a battery as a power source will be described as an example. However, the present invention is not limited to this application, and can be widely applied to DC / DC converters mounted on other electronic devices.

  The configuration of the present invention can be variously modified within the scope of the present invention in addition to the above embodiment.

  For example, in the above embodiment, the on-period setting unit OPS sets the time until the terminal voltage reaches a predetermined value as the on-period at the time of switching driving of the power transistor, by charging the capacitor with a variable current according to the input voltage. However, the configuration of the present invention is not limited to this, and the capacitor is discharged by a variable current according to the input voltage until the terminal voltage reaches a predetermined value. The time may be set as an on period at the time of switching driving of the power transistor.

  The present invention is a technique suitable for a DC / DC converter mounted on a portable electronic device using a battery as a power source, such as a mobile phone terminal or a digital still camera.

These are block diagrams which show one Embodiment of the step-up type DC / DC converter which concerns on this invention. These are timing charts for explaining hysteresis control of the DC / DC converter according to the present invention and variable control of the ON period according to the input voltage VBAT. These are timing charts which show an example of the step-up operation in the conventional DC / DC converter.

Explanation of symbols

CTRL control unit OPS ON period setting unit N1, N2 N-channel field effect transistor P1, P2 P-channel field effect transistor R1 to R3 Resistor L1 Coil D1 Diode C1, C2 Capacitor CMP1 Comparator E1 DC voltage source

Claims (4)

In a DC / DC converter that boosts an input voltage and generates a desired output voltage by switching driving a power transistor,
A control unit that starts switching driving of the power transistor when the output voltage falls to a predetermined lower limit voltage, and stops switching driving of the power transistor when the output voltage rises to a predetermined upper limit voltage;
An on-period setting unit that variably controls an on-period during switching driving of the power transistor according to the input voltage;
A DC / DC converter characterized by comprising:   The on-period setting unit sets the on-period at the time of switching driving of the power transistor as the input voltage is lower, and conversely, the on-period at the time of switching driving of the power transistor is higher as the input voltage is higher. 2. The DC / DC converter according to claim 1, wherein the DC / DC converter is set short.   The on-period setting unit includes a current source that generates a variable current according to the input voltage, and a capacitor that is charged or discharged by the variable current, and the terminal voltage of the capacitor reaches a predetermined value. 3. The DC / DC converter according to claim 2, wherein the time until is set as an ON period at the time of switching driving of the power transistor.   The DC / DC converter according to any one of claims 1 to 3, a battery for supplying the input voltage to the DC / DC converter, and the output voltage generated by the DC / DC converter as a drive voltage. And an electronic device.

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