Abstract:
In a switching power supply circuit, completion of discharging of a capacitance provided in a soft-start circuit requires a period longer than a cycle with which an activating/deactivating portion switches switching control operation of a driving portion (which performs switching control to turn on/off a switching device provided in a stepping-up DC-DC converter) between an activated state and a deactivated state. Moreover, the soft-start circuit is prevented from performing soft-start operation until discharging of the capacitance is completed. Furthermore, the activating/deactivating portion prevents the switching control operation of the driving portion from being switched from the deactivated state to the activated state from a time point that constant voltage feeding operation of a constant voltage portion (which feeds, to a comparing portion that sends an output signal to the driving portion, a constant voltage as the drive voltage) is switched from the deactivated state to the activated state until a predetermined period elapses.

Description:
This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2004-299572 filed in Japan on Oct. 14, 2004, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a switching power supply circuit that steps up the input voltage from a direct-current source and then feeds it to a load. More particularly, the present invention relates to a switching power supply circuit that activates/deactivates stepping-up operation according to an external signal. 
   2. Description of Related Art 
   In recent years, white light-emitting diodes that are excellent in durability, luminous efficiency, and space-saving, etc., have come to be used as one of the illumination light sources (backlights or frontlights) of liquid crystal display devices (LCDs) incorporated in electronic apparatuses such as cellular phones, PDAs (personal digital assistants), and digital cameras. This white light-emitting diode requires a relatively high forward direction voltage to emit light, and, in general, as an illumination light source, a plurality of white light-emitting diodes are used and connected in series to make their brightness equal. Thus, driving the white light-emitting diode as an illumination light source requires a direct voltage higher than the direct voltage fed from the battery incorporated in the electronic apparatus. 
   For this reason, a stepping-up switching power supply circuit is used as a circuit for driving the white light-emitting diode.  FIG. 11A  shows an example of the configuration of a conventional stepping-up switching power supply circuit. The switching power supply circuit shown in  FIG. 11A  is composed of an input capacitor  2 , a coil  3 , a diode  4  that is a rectifying device, an output capacitor  5 , an output current detection resistance R 1 , and a stepping-up chopper regulator  10  that takes the form of a single IC package and performs stepping-up operation by controlling the charge and discharge of energy to the coil  3 . The switching power supply circuit shown in  FIG. 11A  steps up the direct voltage fed from a direct-current source  1  such as a lithium-ion battery, then feeds the stepped-up voltage to the loads, namely, six white light-emitting diodes LED 1  to LED 6 , and thus drives them. Note that a power source switch (not shown) is provided between the direct-current source  1  and the switching power supply circuit. When the power source switch is on, the direct voltage Vin is fed from the direct-current source  1  to the switching power supply circuit; when the power source switch is off, the direct voltage Vin is not fed from the direct-current source  1  to the switching power supply circuit. 
   The direct-current source  1  has the negative terminal connected to the ground, and has the positive terminal that is connected to the ground via the input capacitor  2  and is also connected to the one end of the coil  3 . The other end of the coil  3  is connected to the anode of the diode  4 , and the cathode of the diode  4  is connected to the ground via the output capacitor  5 . The white light-emitting diodes LED 1  to LED 6  and the output current detection resistance R 1  are connected in series to form a series circuit, and this series circuit is connected in parallel with the output capacitor  5 . 
   The stepping-up chopper regulator  10  is provided with, as the terminals for external connection, a power supply terminal T VIN , a ground power supply terminal T GND , an output voltage monitor terminal T VO , a feedback terminal T FB , a switch terminal T VSW , and a control terminal T CTRL . The power supply terminal T VIN  is connected to the positive terminal of the direct-current source  1 , and the ground power supply terminal T GND  is connected to the ground. With these terminals, the stepping-up chopper regulator  10  can operate from the direct-current source  1 . The switch terminal T VSW  is connected to the node at which the coil  3  and the diode  4  are connected together, the output voltage monitor terminal T VO  is connected to the cathode of the diode  4 , and the feedback terminal T FB  is connected to the node at which the white light-emitting diode LED 6  and the output current detection resistance R 1  are connected together. The control terminal T CTRL  receives a brightness adjusting signal, which will be described below. 
   Next, the internal configuration of the stepping-up chopper regulator  10  will be described. The stepping-up chopper regulator  10  is provided with N-channel MOSFETs  11  and  12  (hereinafter referred to as Nch transistors), a drive circuit  13 , a current detection comparator  14 , an oscillation circuit  15 , an amplifier  16 , a PWM comparator  17 , an error amplifier  18 , a reference power source  19 , resistances R 2  to R 4 , a soft-start circuit  20 , an on/off circuit  21 , an overheating detection circuit  22 , an overvoltage detection circuit  23 , a constant voltage circuit  24 , and a switch  25 . 
   When the switch  25  is on, the constant voltage circuit  24  converts the direct voltage Vin from the power supply terminal T VIN  into a voltage having a predetermined value, and then feeds the converted voltage to the PWM comparator  17  and the error amplifier  18  as the drive voltage. When the switch  25  is on, the direct voltage Vin is fed, as the drive voltage, to the other circuits constituting the stepping-up chopper regulator  10 . 
   The switch  25  is provided between the power supply terminal T VIN  and the signal receiving side of the constant voltage circuit  24 . When a high level signal is fed to the control terminal, the switch  25  is turned on; when a low level signal is fed to the control terminal, the switch  25  is turned off. The brightness adjusting signal received from the outside by the control terminal T CTRL  is fed to the control terminal of the switch  25 . Therefore, when the brightness adjusting signal takes a low level, electric power is not fed to the circuits of the stepping-up chopper regulator  10 . This reduces the electric power consumption to near zero. This advantageously contributes to low electric power consumption. 
   The drains of the Nch transistor  11  and the Nch transistor  12  are both connected to the switch terminal T VSW , and the gates of the Nch transistor  11  and the Nch transistor  12  are both connected to the drive circuit  13 . The source of the Nch transistor  12  is directly connected to the ground, and the source of the Nch transistor  11  is connected to the ground via the resistance R 2 . This makes the ratio of the drain current of the Nch transistor  11  to that of the Nch transistor  12  equal to the ratio of the gate width/gate length of the Nch transistor  11  to that of the Nch transistor  12 . 
   The ends of the resistance R 2  are respectively connected to the two input terminals of the current detection comparator  14 . The output of the current detection comparator  14  and one output of the oscillation circuit  15  are added together by the amplifier  16 , and their sum is fed to the inverting input terminal of the PWM comparator  17 . The output of the PWM comparator  17  and the other output of the oscillation circuit  15  are fed to the drive circuit  13 . 
   The output of the error amplifier  18  is fed to the non-inverting input terminal of the PWM comparator  17 , and the non-inverting input terminal of the error amplifier  18  is connected to the feedback terminal T FB . The inverting input terminal of the error amplifier  18  is connected to one end of the resistance R 3  and to one end of the resistance R 4 . The other end of the resistance R 4  is connected to the ground, and the other end of the resistance R 3  is connected to the positive terminal of the reference power source  19 . The negative terminal of the reference power source  19  is connected to the ground. 
   The output of the soft-start circuit  20  is fed to the node at which the non-inverting input terminal of the PWM comparator  17  and the output terminal of the error amplifier  18  are connected together, and the outputs of the on/off circuit  21 , the overheating detection circuit  22 , and the overvoltage detection circuit  23  are fed to the drive circuit  13 . The brightness adjusting signal received from the outside by the control terminal T CTRL  is fed to the soft-start circuit  20  and the on/off circuit  21 . The output voltage Vout is fed through the output voltage monitor terminal T VO  to the overvoltage detection circuit  23 . 
   Next, the operation of the switching power supply circuit having the configuration shown in  FIG. 11A  will be described. The switching power supply circuit shown in  FIG. 11A  causes, by making the drive circuit  13  turn on/off the Nch transistor  12 , the output voltage Vout obtained by stepping up the input voltage Vin from the direct-current source  1  to appear across the output capacitor  5  and thus drives the white light-emitting diodes LED 1  to LED 6 . 
   Specifically, when the drive circuit  13  applies a predetermined gate voltage to the gate of the Nch transistor  12  and the Nch transistor  12  is on, a current flows from the direct-current source  1  to the coil  3 . As a result, energy is accumulated in the coil  3 . On the other hand, when the drive circuit  13  does not apply a predetermined gate voltage to the gate of the Nch transistor  12  and the Nch transistor  12  is off, the accumulated energy is released, resulting in a back electromotive force generated in the coil  3 . The back electromotive force generated in the coil  3  is added to the input voltage Vin of the direct-current source  1 , and then charges through the diode  4  the output capacitor  5 . By repeating the operations described above, stepping-up operation is performed, resulting in appearance of the output voltage Vout across the output capacitor  5 . This output voltage Vout makes the output current lout flow through the white light-emitting diodes LED 1  to LED 6 , and thereby makes the white light-emitting diodes LED 1  to LED 6  emit light. 
   Then, a feedback voltage Vfb obtained by multiplying a current value of the output current lout by a resistance value of the output current detection resistance R 1  is fed through the feedback terminal T FB  to the non-inverting input terminal of the error amplifier  18 , and then compared with a reference voltage Vref to be fed to the inverting input terminal of the error amplifier  18 . With this comparison, a voltage according to the difference between the feedback voltage Vfb and the reference voltage Vref appears in the output of the error amplifier  18 . This voltage is fed to the non-inverting input terminal of the PWM comparator  17 . Note that the reference voltage Vref is a voltage obtained by dividing the output voltage of the reference power source  19  by the resistances R 3  and R 4 . 
   A signal proportional to the current that flows through, when the Nch transistor  11  is turned on, the resistance R 2  and a sawtooth waveform signal outputted from the oscillation circuit  15  are added together and then amplified by the amplifier  16 . The resultant signal is inputted to the inverting input terminal of the PWM comparator  17 , and then compared with the output voltage level of the error amplifier  18  by the PWM comparator  17 . As a result, when the output voltage level of the error amplifier  18  is higher than the output signal level of the amplifier  16 , the PWM output of the PWM comparator  17  takes a high level. On the other hand, when the output voltage level of the error amplifier  18  is lower than the output signal level of the amplifier  16 , the PWM output of the PWM comparator  17  turns to a low level. 
   Upon receiving the PWM output of the PWM comparator  17 , the drive circuit  13  feeds a pulse signal having a duty according to the received PWM output to the gates of the Nch transistors  11  and  12  so as to turn them on/off. Specifically, when the PWM output of the PWM comparator  17  takes a high level, the drive circuit  13  starts, at the beginning of each cycle of the sawtooth waveform signal outputted from the oscillation circuit  15 , feeding of the predetermined gate voltage to the Nch transistors  11  and  12  so as to turn them on. When the PWM output of the PWM comparator  17  becomes a low level, the drive circuit  13  stops feeding of the predetermined gate voltage to the Nch transistors  11  and  12  so as to turn them off. 
   When the above-described on/off control of the Nch transistors  11  and  12 , namely, switching control operation of the drive circuit  13  is in the activated state, the stepping-up operation is activated so as to make the feedback voltage Vfb and the reference voltage Vref equal. As a result, the output current lout is stabilized to a current value obtained by dividing the reference voltage Vref (=feedback voltage Vfb) by the resistance value of the output current detection resistance R 1 . 
   The signals inputted to the inverting input terminal of the PWM comparator  17  include a signal according to the current flowing through the resistance R 2 , i.e., a signal according to the current flowing through the coil  3  when the Nch transistors  11  and  12  are turned on. This makes it possible to control the peak current of the coil  3 . Moreover, upon detecting the output voltage Vout exceeding a predetermined voltage, the overvoltage detection circuit  23  makes the drive circuit  13  deactivate the switching control operation. This makes it possible to prevent the overvoltage exceeding the predetermined voltage from being applied to the white light-emitting diodes LED 1  to LED 6 , which are loads, and the output capacitor  5 . Furthermore, upon detecting overheating, especially around the Nch transistor  12 , resulting from the switching control operation of the drive circuit  13 , the overheating detection circuit  22  makes the drive circuit  13  deactivate the switching control operation. This makes it possible to prevent a breakdown or the like of the stepping-up chopper regulator  10  due to overheating. 
   The on/off circuit  21  makes the drive circuit  13  activates/deactivates the switching control operation according to the brightness adjusting signal inputted to the control terminal T CTRL . When the drive circuit  13  activates the switching control operation, the switching power supply circuit activates the stepping-up operation; when the drive circuit  13  deactivates the switching control operation, the switching power supply circuit deactivates the stepping-up operation. Used as the brightness adjusting signal is, for example, a PWM (pulse width modulation) signal. When the brightness adjusting signal inputted to the control terminal T CTRL  takes a high level, the on/off circuit  21  makes the drive circuit  13  activate the switching control operation, and thus makes the output current lout flow through the white light-emitting diodes LED 1  to LED 6 . On the other hand, when the brightness adjusting signal takes a low level, the on/off circuit  21  makes the drive circuit  13  deactivate the switching control operation, and thus lowers the output voltage Vout. This makes the average current flowing through the white light-emitting diodes LED 1  to LED 6  vary according to the duty of the brightness adjusting signal. The brightness of the white light-emitting diodes LED 1  to LED 6  is proportional to the average current flowing therethrough. This makes it possible to adjust the brightness of the white light-emitting diodes LED 1  to LED 6  by changing the duty of the brightness adjusting signal. 
   When the drive circuit  13  starts the switching control operation, the soft-start circuit  20  gradually changes the output duty of the drive circuit  13 , and thereby makes the output voltage Vout rise gently. In other words, the soft-start circuit is a circuit that performs so-called soft-start operation. If the output voltage Vout does not rise gently, when the capacitor  5  is uncharged, an excessive charge current flows from the direct-current source  1  to charge it. The problem here is that when the direct-current source  1  is a battery such as a lithium-ion battery, such an excessive charge current puts an extra strain on the battery. Furthermore, this excessive charge current lowers the battery voltage, and thus makes it impossible to use the battery to its end voltage. 
   As shown in  FIG. 11B , the soft-start circuit  20  consists of terminals T 1  to T 3 , switches SW 1  and SW 2 , a constant current source I 1 , a capacitance C 1 , and a P-channel MOSFET Q 1  (hereinafter referred to as a Pch transistor). The terminal T 1  is connected to the control terminals of the switches SW 1  and SW 2 , and the terminal T 2  is connected, via the switch SW 1  and the constant current source I 1 , to the gate of the Pch transistor Q 1 , to one end of the capacitance C 1 , and to one end of the switch SW 2 . The other end of the capacitance C 1 , the other end of the switch SW 2 , and the drain of the Pch transistor Q 1  are connected to the ground, and the source of the Pch transistor Q 1  is connected to the terminal T 3 . Note that the terminal T 1  is connected to the control terminal T CTRL , the terminal T 2  is connected to the power supply terminal T VIN  via the switch  25 , and the terminal T 3  is connected to the node at which the non-inverting input terminal of the PWM comparator  17  and the output terminal of the error amplifier  18  are connected together. The switch SW 1  is turned on when a high level signal is fed to its control terminal, and is turned off when a low level signal is fed thereto. The switch SW 2  is turned off when a high level signal is fed to its control terminal, and is turned on when a low level signal is fed thereto. 
   In the switching power supply circuit shown in  FIG. 11A , every time a brightness adjusting signal V CTRL  inputted to the control terminal T CTRL  turns from a low level to a high level, the soft-start circuit  20  performs soft-start operation so as to cause the output voltage Vout to rise gently. This prevents the output voltage Vout from rising quickly to a predetermined voltage value V 1 . The problem here is that when the brightness adjusting signal V CTRL  has a short cycle and thus remains at a high level only for a short period of time, it is impossible to apply to the white light-emitting diodes LED 1  to LED 6  a voltage required by them to emit light. This makes it impossible to perform desired brightness adjustment according to the duty of the brightness adjusting signal. 
   In view of the conventionally experienced inconveniences and disadvantages described above, the inventors of the present invention have devised a switching power supply circuit that activates/deactivates stepping-up operation according to an external signal and that can make the output voltage reach a target value even when a cycle with which the stepping-up operation is switched between an activated state and a deactivated state is short. Such a switching power supply circuit has been proposed, in the Japanese Patent Application filed as No. 2004-65427, by the applicant of the present invention. 
   The switching power supply circuit proposed in the above Patent Application differs from the switching power supply circuit shown in  FIG. 11A  only in that the soft-start circuit  20  is replaced with a soft-start circuit shown in  FIG. 12  or a soft-start circuit shown in  FIG. 13 . Note that, in  FIGS. 12 and 13 , such members as are found also in  FIG. 11B  will be identified with common reference characters. 
   The soft-start circuit shown in  FIG. 12  consists of terminals T 1  to T 3 , a switch SW 1 , a constant current source I 1 , a capacitance C 1 , a Pch transistor Q 1 , a switch SW 2 , and a constant current source I 2 . The terminal T 1  is connected to the control terminals of the switches SW 1  and SW 2 , and the terminal T 2  is connected, via the switch SW 1  and the constant current source I 1 , to the gate of the Pch transistor Q 1 , to one end of the capacitance C 1 , and to one end of the switch SW 2 . The other end of the capacitance C 1  and the drain of the Pch transistor Q 1  are connected to the ground, the source of the Pch transistor Q 1  is connected to the terminal T 3 , and the other end of the switch SW 2  is connected to the ground via the constant current source I 2 . Note that the terminal T 1  is connected to the control terminal T CTRL , the terminal T 2  is connected to the power supply terminal T VIN , and the terminal T 3  is connected to the node at which the non-inverting input terminal of the PWM comparator  17  and the output terminal of the error amplifier  18  are connected together. The switch SW 1  is turned on when a high level signal is fed to its control terminal, and is turned off when a low level signal is fed thereto. The switch SW 2  is turned off when a high level signal is fed to its control terminal, and is turned on when a low level signal is fed thereto. 
   When the input voltage Vin is fed to the terminal T 2  and the brightness adjusting signal fed to the terminal T 1  takes a high level, the switch SW 1  is turned on and the switch SW 2  is turned off. This makes the constant current source I 1  output a constant current, and thereby charging of the capacitance C 1  is started. After charging of the capacitance C 1  is started, the gate potential of the Pch transistor Q 1  gradually changes from a low potential to a high potential until the Pch transistor Q 1  is switched from on to off. The gate potential ΔV 1  of the Pch transistor Q 1  is expressed by formula (1) below. It is to be noted that C 1  represents the capacitance value of the capacitance C 1 , I 1  represents the output current value of the constant current source I 1 , and Δt 1  represents the charging time.
 
ΔV1= I   1   ·Δt 1/ C   1   (1)
 
   As the gate potential of the Pch transistor Q 1  changes from a low potential to a high potential, the output potential of the error amplifier  18  gradually increases. This realizes soft-start operation. The rising speed of the output voltage Vout with soft-start operation can be easily set by adjusting the capacitance value C 1  of the capacitance C 1  and the output current value I 1  of the constant current source I 1 . 
   On the other hand, when the brightness adjusting signal fed to the terminal T 1  turns to a low level, the switch SW 1  is turned off and the switch SW 2  is turned on. This makes the constant current source I 2  extract the charges accumulated in the capacitance C 1 . The charge extraction described above causes a potential drop ΔV 2  across the gate of the Pch transistor Q 1 , which is expressed by formula (2) below. It is to be noted that C 1  represents the capacitance value of the capacitance C 1 , I 2  represents the output current value of the constant current source I 2 , and Δt 2  represents the charge extraction time.
 
ΔV2= I   2   ·Δt 2/ C   1   (2)
 
   By reducing the potential drop ΔV 2 , it is possible to suppress extraction of the charges accumulated in the capacitance C 1 . Thus, it is necessary to make the ratio I 1 /I 2  greater to suppress extraction of the charges accumulated in the capacitance C 1 . This makes it possible to suppress reduction in the output potential of the error amplifier  18 . 
   When reduction in the output potential of the error amplifier  18  is suppressed, the soft-start circuit shown in  FIG. 12  cannot perform soft-start operation. This allows the output voltage Vout to reach the target value (=V 1 ) even when the brightness adjusting signal V CTRL  remains at a high level only for a short period of time, as seen in  FIG. 14  showing the waveforms of the brightness adjusting signal V CTRL  fed from the outside of the stepping-up switching power supply device provided with the soft-start circuit shown  FIG. 12  or  FIG. 13 , of the output voltage Vout, and of the input current Iin fed from the direct-current source  1 . This makes it possible to make the LED 1  to LED 6  emit light. This makes it possible, even when the brightness adjusting signal has a short cycle and thus remains at a high level only for a short period of time, to perform a desired brightness adjustment according to the duty of the brightness adjusting signal. Note that the ratio I 1 /I 2  described above is so set that the period required to complete discharging of the capacitance is made longer than a cycle with which the on/off circuit  21  switches the switching control operation of the drive circuit  13  between the activated state and the deactivated state. 
   In  FIG. 14 , assuming that the switching power supply circuit is started up at the time point when the brightness adjusting signal V CTRL  turns from a low level to a high level for the first time, and, when the switching power supply circuit is started up, the output voltage Vout is 0 (V), and the output capacitor  5  is not charged at all. 
   When a brightness adjusting signal that turns to a low level when the power source switch provided between the direct-current source  1  and the switching power supply circuit is off, i.e. when the input voltage Vin is not fed to the switching power supply circuit is inputted to the stepping-up switching power supply circuit provided with the soft-start circuit shown in  FIG. 12 , the charges accumulated in the capacitance C 1  are extracted by the constant current source I 2  when the power source switch is off, i.e. when the input voltage Vin is not fed to the switching power supply circuit. In that case, the time when the power source switch is off, i.e. the charge extraction time Δt 2  is long enough to allow the constant current source I 2  to extract a sufficient amount of charges from the capacitance C 1 . This makes the gate potential of the Pch transistor Q 1  equal to or nearly equal to the ground potential. This permits the soft-start circuit shown in  FIG. 12  to perform soft-start operation when the power source switch is switched from off to on. 
   The soft-start circuit shown in  FIG. 13  consists of terminals T 1  to T 3 , a switch SW 1 , a constant current source I 3 , a capacitance C 1 , a Pch transistor Q 1 , and a constant current source I 4 . The terminal T 1  is connected to the control terminal of the switch SW 1 , and the terminal T 2  is connected, via the switch SW 1  and the constant current source I 3 , to the gate of the Pch transistor Q 1 , to one end of the capacitance C 1 , and to one end of the constant current source I 4 . The other end of the capacitance C 1 , the drain of the Pch transistor Q 1 , and the other end of the constant current source I 4  are connected to the ground, and the source of the Pch transistor Q 1  is connected to the terminal T 3 . Note that the terminal T 1  is connected to the control terminal T CTRL , the terminal T 2  is connected to the power supply terminal T VIN , and the terminal T 3  is connected to the node at which the non-inverting input terminal of the PWM comparator  17  and the output terminal of the error amplifier  18  are connected together. The switch SW 1  is turned on when a high level signal is fed to its control terminal, and is turned off when a low level signal is fed thereto. 
   When the input voltage Vin is fed to the terminal T 2  and the brightness adjusting signal fed to the terminal T 1  takes a high level, the switch SW 1  is turned on. This makes the constant current source I 3  output a constant current. Of this constant current, a part is extracted by the constant current source I 4 , and the rest serves as a charge current for the capacitance C 1 . As the capacitance C 1  is charged, the gate potential of the Pch transistor Q 1  gradually increases from a low potential to a high potential until the Pch transistor Q 1  is switched from on to off. The gate potential ΔV 1  of the Pch transistor Q 1  is expressed by formula (3) below. It is to be noted that C 1  represents the capacitance value of the capacitance C 1 , I 3  represents the output current value of the constant current source I 3 , I 4  represents the output current value of the constant current source I 4 , and Δt 1  represents the charging time.
 
ΔV1=( I   3   −I   4 )· Δt 1/ C   1   (3)
 
   As the gate potential of the Pch transistor Q 1  changes from a low potential to a high potential, the output potential of the error amplifier  18  gradually increases. This realizes soft-start operation. The rising speed of the output voltage Vout with soft-start operation can be easily set by adjusting the capacitance value C 1  of the capacitance C 1 , the output current value I 3  of the constant current source I 3 , and the output current value I 4  of the constant current source I 4 . In the soft-start circuit shown in  FIG. 12 , when the gate potential of the Pch transistor Q 1  is equal to or smaller than a predetermined value, there is no choice but to use a leakage current to extract the charges accumulated in the capacitance C 1  due to the resistance of the switch SW 2 . On the other hand, in the soft-start circuit shown in  FIG. 13 , it is possible to reliably extract the charges accumulated in the capacitance C 1  by using the output current of the constant current source I 4 . 
   On the other hand, when the brightness adjusting signal fed to the terminal T 1  turns to a low level, the switch SW 1  is turned to off. This makes the constant current source I 4  extract the charges accumulated in the capacitance C 1 . The charge extraction described above causes a potential drop ΔV 2  across the gate of the Pch transistor Q 1 , which is expressed by formula (4) below. It is to be noted that C 1  represents the capacitance value of the capacitance C 1 , I 4  represents the output current value of the constant current source I 4 , and Δt 2  represents the charge extraction time.
 
ΔV2= I   4   ·Δt 2/ C   1   (4)
 
   By reducing the potential drop ΔV 2 , it is possible to suppress extraction of the charges accumulated in the capacitance C 1 . Thus, it is necessary to make the ratio I 3 /I 4  greater to suppress extraction of the charges accumulated in the capacitance C 1 . This makes it possible to suppress reduction in the output potential of the error amplifier  18 . Here, as far as charging of the capacitance C 1  is concerned, the current extracted by the constant current source I 4  is wasted. Thus, the ratio I 3 /I 4  is preferably made greater than the ratio I 1 /I 2  by making the output current value I 4  of the constant current source I 4  smaller. 
   When reduction in the output potential of the error amplifier  18  is suppressed, the soft-start circuit shown in  FIG. 13  cannot perform soft-start operation. This allows the output voltage Vout to reach the target value (=V 1 ) even when the brightness adjusting signal V CTRL  remains at a high level only for a short period of time, as seen in  FIG. 14  showing the waveforms of the brightness adjusting signal V CTRL  fed from the outside of the stepping-up switching power supply device provided with the soft-start circuit shown  FIG. 12  or  FIG. 13 , of the output voltage Vout, and of the input current Iin fed from the direct-current source  1 . This makes it possible to make the LED 1  to LED 6  emit light. This makes it possible, even when the brightness adjusting signal has a short cycle and thus remains at a high level only for a short period of time, to perform a desired brightness adjustment according to the duty of the brightness adjusting signal. Note that the ratio I 3 /I 4  described above is so set that the period required to complete discharging of the capacitance is made longer than a cycle with which the on/off circuit  21  switches the switching control operation of the drive circuit  13  between the activated state and the deactivated state. 
   When the power source switch provided between the direct-current source  1  and the switching power supply circuit is off, i.e. when the input voltage Vin is not fed to the switching power supply circuit, the charges accumulated in the capacitance C 1  are extracted by the constant current source I 4 . In that case, the time when the power source switch is off, i.e. the charge extraction time Δt 2  is long enough to allow the constant current source I 4  to extract a sufficient amount of charges from the capacitance C 1 . This makes the gate potential of the Pch transistor Q 1  equal to or nearly equal to the ground potential. This permits the soft-start circuit shown in  FIG. 13  to perform soft-start operation when the power source switch is switched from off to on. 
   In order to produce ICs at lower cost, it is necessary to provide a single terminal with a plurality of capabilities and use a package with a small number of pins. Thus, in the stepping-up chopper regulator provided with the soft-start circuit shown in  FIG. 12  or  FIG. 13 , the control terminal T CTRL  is given capabilities of reducing the power consumption to nearly zero when the switching power supply circuit is off, of performing soft-start operation when the switching power supply circuit is switched from off to on, and of stopping soft-start operation when the brightness of the white LED is adjusted according to the duty of the brightness adjusting signal. However, power supply to all the circuits is stopped when the switching power supply circuit is off, because the power consumption is reduced to nearly zero when the switching power supply circuit is off. Therefore, it takes some time to start all the circuit portions after the switching power supply circuit is turned on. This is especially disadvantage because the feedback system does not operate properly until the circuit portion that serves as a power source for various amplifiers is started up. 
   When the constant voltage circuit  24  that converts the input voltage Vin fed from the direct-current source  1  into a predetermined voltage Vc has a configuration shown in  FIG. 15 , the constant voltage circuit  24  is not started up until a parasitic capacitance PC is fully charged. In a case where the Nch transistor  12  is turned on when the constant voltage circuit  24  is not started up, the Nch transistor  12  is kept turned on until the constant voltage circuit  24  is started up because the amplifiers (the PWM comparator  17 , the error amplifier  18 , and the like) are not controlled. 
   This does not cause any problem for the switching power supply circuit shown in  FIG. 11A , because soft-start operation is performed when the switching power supply circuit is turned on. However, in the switching power supply circuit provided with the soft-start circuit shown in  FIG. 12  or  FIG. 13 , when the Nch transistor  12  is kept turned on until the constant voltage circuit  24  is started up, a flow-through current flows through the Nch transistor  12  (see  FIG. 16 ). In  FIG. 16 , V CTRL  represents the brightness adjusting signal fed from the outside to the switching power supply circuit provided with the soft-start circuit shown in  FIG. 12  or  FIG. 13 , Vout represents the output voltage of the switching power supply circuit provided with the soft-start circuit shown in  FIG. 12  or  FIG. 13 , I SW  represents the drain current of the Nch transistor  12  of the switching power supply circuit provided with the soft-start circuit shown in  FIG. 12  or  FIG. 13 , and PI represents the flow-through current flowing through the Nch transistor  12  of the switching power supply circuit provided with the soft-start circuit shown in  FIG. 12  or  FIG. 13 . 
   As described above, in the switching power supply circuit provided with the soft-start circuit shown in  FIG. 12  or  FIG. 13 , when the brightness of the white LED is adjusted according to the duty of the brightness adjusting signal, excessive current flows through the Nch transistor  12  every time the switching power supply circuit is turned on. This makes the battery reach its end voltage (about 3.0V) earlier than expected. This disadvantageously restricts the length of the battery time with one charge. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a switching power supply circuit that activates/deactivates stepping-up operation according to an external signal and that can make the output voltage reach a target value even when a cycle with which the stepping-up operation is switched between an activated state and a deactivated state is short, while preventing a flow-through current from flowing through a switching device when the stepping-up operation is switched from a deactivated state to an activated state, and an electronic apparatus provided therewith. 
   To achieve the above object, according to the present invention, a switching power supply circuit is provided with a stepping-up DC-DC converter, a comparing portion, a driving portion, a constant voltage portion, a switching portion, an activating/deactivating portion, and a soft-start circuit. The comparing portion compares a reference voltage with a voltage based on the output current of the stepping-up DC-DC converter. The driving portion performs switching control to turn on/off a switching device provided in the stepping-up DC-DC converter according to the output of the comparing portion. The constant voltage portion feeds a constant voltage as a drive voltage at least to the comparing portion. The switching portion switches constant voltage feeding operation of the constant voltage portion between an activated state and a deactivated state according to an external signal. The activating/deactivating portion switches switching control operation of the driving portion between an activated state and a deactivated state according to the external signal, while preventing the switching control operation of the driving portion from being switched from the deactivated state to the activated state from a time point that the constant voltage feeding operation of the constant voltage portion is switched from the deactivated state to the activated state until a predetermined period elapses. The soft start circuit has a capacitance and a discharging portion, charges the capacitance when the switching control operation of the driving portion is started and thus an output voltage of the stepping-up DC-DC converter rises, controls the driving portion according to a voltage across the capacitance in such a way that, when the output voltage of the stepping-up DC-DC converter rises, the output voltage of the stepping-up DC-DC converter rises gently regardless of the output of the comparing portion, and makes the discharging portion discharge the capacitance when the switching control operation of the driving portion is in the deactivated state, completion of discharging of the capacitance requiring a period longer than a cycle with which the activating/deactivating portion switches the switching control operation of the driving portion between the activated state and the deactivated state. 
   With this configuration, the period required to complete discharging of the capacitance is made longer than a cycle with which the activating/deactivating portion switches the switching control operation of the driving portion between the activated state and the deactivated state, and the soft-start circuit is prevented from performing soft-start operation until discharging of the capacitance is completed. This makes it possible to make the output voltage of the stepping-up DC-DC converter reach a target value even when a cycle with which the activating/deactivating portion switches the switching control operation of the driving portion between the activated state and the deactivated state is short. Thus, it is possible to make the output voltage reach a target value even when a cycle with which stepping-up operation is switched between an activated state and a deactivated state is short. Furthermore, the activating/deactivating portion prevents the switching control operation of the driving portion from being switched from the deactivated state to the activated state from a time point that the constant voltage feeding operation of the constant voltage portion is switched from the deactivated state to the activated state until a predetermined period elapses. Thus, by making the predetermined period longer than the start up time of the constant voltage portion, the switching device is turned and kept off until the constant voltage portion is started up. This prevents a flow-through current from flowing through the switching device. 
   Moreover, an electronic apparatus according to the present invention is provided with a switching power supply circuit configured as described above. The switching power supply circuit feeds electric power, for example, to an illumination light source of a liquid crystal display device incorporated in the electronic apparatus. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing an example of the configuration of a stepping-up switching power supply circuit embodying the invention; 
       FIG. 2  is a time chart showing the voltage and current waveforms appearing at various parts of the stepping-up switching power supply circuit shown in  FIG. 1 ; 
       FIG. 3  is a diagram showing an example of the configuration of a signal correction circuit provided in the stepping-up switching power supply circuit shown in  FIG. 1 ; 
       FIG. 4  is a time chart showing the signal waveforms appearing at various parts of the signal correction circuit shown in  FIG. 3 ; 
       FIG. 5  is a graph showing the characteristics of the signal correction circuit and the constant voltage circuit using a depression transistor; 
       FIG. 6  is a diagram showing another example of the configuration of the signal correction circuit provided in the stepping-up switching power supply circuit shown in  FIG. 1 ; 
       FIG. 7  is a diagram showing an example of the configuration of an oscillation circuit provided in the stepping-up switching power supply circuit shown in  FIG. 1 ; 
       FIG. 8  is a graph showing the static characteristic of the depression transistor; 
       FIG. 9  is a graph showing the oscillation frequency characteristic of the oscillation circuit shown in  FIG. 7 ; 
       FIG. 10  is a graph showing the pre-trimming characteristic of the signal correction circuit shown in  FIG. 6 ; 
       FIGS. 11A and 11B  are diagrams showing an example of the configuration of a conventional stepping-up switching power supply circuit; 
       FIG. 12  is a diagram showing an example of the configuration of a soft-start circuit; 
       FIG. 13  is a diagram showing another example of the configuration of the soft-start circuit; 
       FIG. 14  is a time chart showing the voltage and current waveforms appearing at various parts of the stepping-up switching power supply circuit in which the soft-start circuit of the stepping-up switching power supply circuit shown in  FIG. 11  is replaced with the soft-start circuit shown in  FIG. 12  or  FIG. 13 ; 
       FIG. 15  is a diagram showing an example of the configuration of a constant voltage circuit; and 
       FIG. 16  is a time chart showing the voltage and current waveforms appearing at various parts of the stepping-up switching power supply circuit in which the soft-start circuit of the stepping-up switching power supply circuit shown in  FIG. 11  is replaced with the soft-start circuit shown in  FIG. 12  or  FIG. 13 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   An example of the configuration of a switching power supply circuit embodying the invention is shown in  FIG. 1 . Note that, in  FIG. 1 , such members as are found also in  FIG. 11A  are identified with common reference numerals, and their detailed descriptions will be omitted. 
   The switching power supply circuit shown in  FIG. 1  embodying the invention has a configuration in which the stepping-up chopper regulator  10  of the conventional switching power supply circuit shown in  FIG. 11A  is replaced with a stepping-up chopper regulator  100 . The stepping-up chopper regulator  100  differs from the stepping-up chopper regulator  10  in that the soft-start circuit  20  of the stepping-up chopper regulator  10  is replaced with a soft-start circuit  26 , and a signal correction circuit  27  that receives a brightness adjusting signal from the control terminal T CTRL  and feeds to the on/off circuit  21  a corrected signal obtained by correcting the received brightness adjusting signal is additionally provided. 
   The signal correction circuit  27  feeds to the on/off circuit  21  a corrected signal that is the same as the brightness adjusting signal except that it takes a low level from a time point that the brightness adjusting signal turns from a low level to a high level until a predetermined period t 1  elapses. 
   The soft-start circuit  26  is a soft-start circuit having the configuration shown in  FIG. 12  or  FIG. 13 . With this configuration, even when the brightness adjusting signal has a short cycle and thus remains at a high level only for a short period of time, it is possible to perform a desired brightness adjustment according to the duty of the brightness adjusting signal. 
   When the constant voltage circuit  24  has a configuration shown in  FIG. 15 , the constant voltage circuit  24  is not started up until a parasitic capacitance PC is fully charged. However, the signal correction circuit  27  feeds to the on/off circuit  21  a corrected signal that is the same as the brightness adjusting signal except that it takes a low level from a time point that the brightness adjusting signal turns from a low level to a high level until a predetermined period t 1  elapses. Thus, by making the predetermined period t 1  longer than the start up time of the constant voltage circuit  24 , the Nch transistor  12  is turned and kept off until the constant voltage circuit  24  is started up. This prevents the flow-through current from flowing through the Nch transistor  12  (see  FIG. 2 ). Note that, in  FIG. 2 , V CTRL  represents the brightness adjusting signal fed from the outside to the switching power supply circuit shown in  FIG. 1 , Vout represents the output voltage of the switching power supply circuit shown in  FIG. 1 , and I SW  represents the drain current of the Nch transistor  12  of the switching power supply circuit shown in  FIG. 1 . 
   Here, an example of the configuration of the signal correction circuit  27  is shown in  FIG. 3 . The signal correction circuit shown in  FIG. 3  is provided with Pch transistors Q 2  to Q 9 , Nch transistors Q 10  to Q 17 , an N-channel depression transistor Q 18 , and a capacitor C 2 . 
   The Pch transistors Q 2  to Q 5  and the Nch transistors Q 10  to Q 13  constitute a first signal generation portion. The first signal generation portion outputs a signal S A  (see  FIG. 4 ) that is an inverted signal of the brightness adjusting signal V CTRL  to the node A at which the drain of the Pch transistor Q 5  and the drain of the Nch transistor Q 13  are connected together. 
   The Pch transistors Q 2 , Q 6 , and Q 7 , the Nch transistors Q 10 , Q 14 , and Q 15 , the N-channel depression transistor Q 18 , and the capacitor C 2  constitute a second signal generation portion. The second signal generation portion outputs, to the node B at which the drain of the Pch transistor Q 7  and the drain of the Nch transistor Q 15  are connected together, a signal S B  (see  FIG. 4 ) that takes a high level from a time point that the brightness adjusting signal V CTRL  turns from a low level to a high level until a predetermined period t 1  elapses and that otherwise takes a low level. 
   The Pch transistors Q 8  and Q 9  and the Nch transistors Q 16  and Q 17  constitute a third signal generation portion. The third signal generation portion outputs, to the node C at which the drain of the Pch transistor Q 8 , the drain of the Nch transistor Q 16 , and the drain of the Nch transistor Q 17  are connected together, a signal S C  (see  FIG. 4 ) that takes a low level when one of the signal S A  and the signal S B  takes a low level and the other takes a high level, and that takes a high level when both of the signal S A  and the signal S B  are at a low level. The signal S C  outputted from the third signal generation portion serves as the output signal of the signal correction circuit shown in  FIG. 3 . 
   The signal correction circuit shown in  FIG. 3  has a simple circuit configuration because it determines the predetermined period t 1  by using the time constants of the N-channel depression transistor Q 18  and the capacitor C 2 . This makes it possible to achieve cost reduction. 
   However, a signal correction circuit, like the signal correction circuit shown in  FIG. 3 , that determines the predetermined period t 1  by using the time constants of a depression transistor and a capacitor has the following disadvantage. When there are large variations in the characteristic of the depression transistor of the signal correction circuit, there is a possibility that the predetermined period t 1  is made shorter than the start up time of the constant voltage circuit  24  because of the large characteristic variations of the depression transistor. 
   To prevent the disadvantage described above, the constant voltage circuit  24  may adopt a configuration provided with a constant current source using a depression transistor (e.g., a circuit configuration shown in  FIG. 15 ). When the signal correction circuit  27  adopts the circuit configuration shown in  FIG. 3  and the constant voltage circuit  24  adopts the circuit configuration shown in  FIG. 15 , it is possible to obtain the characteristic curve CL 1  shown in  FIG. 5  of the predetermined period t 1  of the signal correction circuit  27  and the characteristic curve CL 2  shown in  FIG. 5  of the start up time of the constant voltage circuit  24 . Thus, the predetermined period t 1  is made longer than the start up time of the constant voltage circuit  24  unless the threshold voltage Vth of the depression transistor of the signal correction circuit  27  becomes extremely low and the threshold voltage Vth of the depression transistor of the constant voltage circuit  24  becomes extremely high. 
   Furthermore, by making the depression transistors of the signal correction circuit  27  and the constant voltage circuit  24  equal in size and arranging these depression transistors in the same direction, the threshold voltages Vth of the depression transistors of the signal correction circuit  27  and the constant voltage circuit  24  are made approximately equal. This makes it possible to prevent more reliably the predetermined period t 1  from being made shorter than the start up time of the constant voltage circuit  24 . 
   Note that the shorter the predetermined period t 1 , the better, for the following reasons. So long as the frequency of the brightness adjusting signal V CTRL  is low, no problem arises. However, when the predetermined period t 1  becomes equal to or longer than about one fourth of the cycle of the brightness adjusting signal V CTRL  due to the high frequency of the brightness adjusting signal V CTRL , the linearity between the duty of the brightness adjusting signal V CTRL  and the brightness of the white light-emitting diodes LED 1  to LED 6  is lost. To solve this problem, it is preferable to use a signal correction circuit, as the signal correction circuit  27 , that determines the predetermined period t 1  by using the time constants of a depression transistor and a capacitor and can trim the depression transistor so that, when the predetermined period t 1  is long, the predetermined period t 1  is made shorter by trimming the depression transistor (by laser trimming or Zener zapping). 
   An example of the configuration of a signal correction circuit that determines the predetermined period t 1  by using the time constants of a depression transistor and a capacitor and that can trim the depression transistor is shown in  FIG. 6 . Note that, in  FIG. 6 , such members as are found also in  FIG. 3  will be identified with common reference characters, and their detailed descriptions will be omitted. In the signal correction circuit shown in  FIG. 6 , depression transistors Q 18 ′ and Q 18 ″ are connected in series to the depression transistor Q 18 , and trimming devices  28  to  30  are provided respectively between the gate and drain of the depression transistors Q 18 , Q 18 ′, and Q 18 ″. 
   When trimming is performed, it is necessary to measure the predetermined period t 1  in a wafer test of the wafer on which the circuits of the stepping-up chopper regulator are formed, and adjust the predetermined period t 1  according to the measurement results. The predetermined period t 1  may be measured, for example, by obtaining the waveforms shown in  FIG. 4  of the brightness adjusting signal V CTRL  and the signal S C . However, this method requires an expensive tester to obtain the signal waveforms. 
   On the other hand, when the oscillation circuit  15  adopts the circuit configuration provided with a constant current source using a depression transistor, it is possible to estimate the predetermined period t 1  only by measuring the value of the oscillation frequency of the oscillation circuit  15 . This eliminates the need to use an expensive tester. Thus, it is preferable that the oscillation circuit  15  adopts the circuit configuration provided with a constant current source using a depression transistor (e.g., a circuit configuration shown in  FIG. 7 ). 
   In the oscillation circuit shown in  FIG. 7 , the constant current source using depression transistors Q 19  and Q 20  outputs a constant current Ic. The depression transistors Q 19  and Q 20  have the static characteristics shown in  FIG. 8 . When the depression transistors Q 19  and Q 20  have the threshold voltage lower than the design value and have the static characteristic indicated by a characteristic curve CL 3 , the value of the constant current Ic is Id 1 . When the depression transistors Q 19  and Q 20  have the threshold voltage equal to the design value and have the static characteristic indicated by a characteristic curve CL 4 , the value of the constant current Ic is Id 2 . When the depression transistors Q 19  and Q 20  have the threshold voltage higher than the design value and have the static characteristic indicated by a characteristic curve CL 5 , the value of the constant current Ic is Id 3 . Specifically, in the oscillation circuit shown in  FIG. 7 , the lower threshold voltage the depression transistors Q 19  and Q 20  serving as the constant current source have, the smaller the constant current Ic becomes; the higher threshold voltage the depression transistors Q 19  and Q 20  serving as the constant current source have, the greater the constant current Ic becomes. As a result, in the oscillation circuit shown in  FIG. 7 , as shown in  FIG. 9 , the lower the threshold voltage Vth the depression transistors Q 19  and Q 20  serving as the constant current source have, the higher the oscillation frequency becomes; the higher the threshold voltage Vth the depression transistors Q 19  and Q 20  have, the lower the oscillation frequency becomes. 
   On the other hand, when the signal correction circuit  27  adopts the circuit configuration shown in  FIG. 6 , as seen in  FIG. 10  showing the characteristic curve of the predetermined period t 1  of the signal correction circuit  27 , the lower the threshold voltage Vth the depression transistors Q 18  to Q 18 ″ have, the shorter the predetermined period t 1  becomes; the higher the threshold voltage Vth the depression transistors Q 18  to Q 18 ″ have, the longer the predetermined period t 1  becomes. Note that the characteristic curve shown in  FIG. 10  of the predetermined period t 1  of the signal correction circuit  27  is not yet trimmed. 
   Based on  FIGS. 9 and 10 , it is possible to estimate the predetermined period t 1  of the signal correction circuit  27  only by measuring the value of the oscillation frequency of the oscillation circuit  15 . By making the depression transistors of the oscillation circuit  15  and the signal correction circuit  27  equal in size and arranging these depression transistors in the same direction, the threshold voltages Vth of the depression transistors of the oscillation circuit  15  and the signal correction circuit  27  are made approximately equal. This enhances the accuracy of the estimation described above. Thus, it is preferable that the depression transistors of the oscillation circuit  15  and the signal correction circuit  27  are made equal in size and are arranged in the same direction. 
   Note that, the above description deals with a case where, when the corrected signal outputted from the signal correction circuit  27  takes a high level, the on/off circuit  21  makes the drive circuit  13  activate the switching control operation, and, when the corrected signal outputted from the signal correction circuit  27  takes a low level, the on/off circuit  21  makes the drive circuit  13  deactivate the switching control operation. It should be understood, however, that the drive circuit  13  may be made to activate the switching control operation when the corrected signal outputted from the signal correction circuit  27  takes a low level, and to deactivate the switching control operation when the corrected signal outputted from the signal correction circuit  27  takes a high level. In that case, the switch SW 1  is turned off when a high level signal is fed to its control terminal and turned on when a low level signal is fed thereto, the switch SW 2  is turned on when a high level signal is fed to its control terminal and turned off when a low level signal is fed thereto, and the switch  25  is turned off when a high level signal is fed to its control terminal and turned on when a low level signal is fed thereto. Furthermore, the above description deals with a case where the present invention is applied to a transformerless stepping-up switching power supply circuit. It should be understood, however, that the present invention can be applied to a stepping-up switching power supply circuit having a switching transformer. 
   Furthermore, an electronic apparatus according to the invention is provided with a load whose driving current has to be adjusted (e.g., an illumination light source of a liquid crystal display device) and a switching power supply circuit according to the invention that drives the load.