JP5407548B2 - Switching power supply - Google Patents

Description

  The present invention relates to an output abnormality detection technique in a DC power supply apparatus, and more particularly to a technique that is effective for detecting output overvoltage in a step-up switching power supply apparatus.

  In general, a DC-DC converter including a step-up switching regulator is used in a DC power supply device that generates a driving power source for an LED such as a WLED (white light emitting diode). In such an LED drive power supply device, as shown in FIG. 8, the drive current flowing through the LED is converted into a voltage by a sense resistor Rs, fed back to the control circuit CNT, and compared with a reference voltage by an error amplifier in accordance with the potential difference. Feedback control is performed in which a drive pulse having a pulse width is generated and a switching element SW that intermittently passes a current through an inductor (coil) L is turned on and off to keep the drive current constant.

  In such a power supply device, when the LED is disconnected from the output terminal, the feedback voltage FB falls to the ground potential, so the control circuit CNT increases the ON time of the switching element SW in an attempt to increase the output voltage. Then, as shown in FIG. 9, the output voltage Vout rises, and in some cases, the withstand voltage of peripheral elements and ICs may be exceeded, and the elements and components may be destroyed. Therefore, a technique is known in which an overvoltage detection circuit OVP for monitoring the output voltage is provided, and when the overvoltage detection circuit OVP detects an overvoltage, the control circuit CNT controls the switching element SW so that the output voltage does not increase any further. It has been.

  In addition, as invention regarding the overvoltage detection circuit in a power supply device, there exist some which are described in patent document 1 and patent document 2, for example.

JP-A-61-222174 Japanese Patent Laid-Open No. 2003-143838

  Since the overvoltage detection circuit in the conventional power supply apparatus has a fixed overvoltage detection level, the output voltage cannot be made higher than the overvoltage detection level. Therefore, in the power supply device for LED drivers, the number of connectable LEDs is limited, and the withstand voltage of the rectifying Schottky barrier diode SBD and the smoothing capacitor CO, which are external components, is also limited. There are restrictions such as the inability to select parts. As a result, it has been clarified that there is a problem that a high-cost element having a withstand voltage corresponding to the detection level of the overvoltage detection circuit has to be used.

  The present invention has been made paying attention to the above-described problems, and the object of the present invention is to allow the user to freely set the overvoltage detection level in the DC power supply device and to freely select external components. The present invention is to provide an overvoltage detection technique in which the number of LEDs that can be connected is not limited as compared with the related art in the LED drive power supply device.

  Another object of the present invention is to provide a technique capable of providing an overcurrent detection function and a low voltage detection function in addition to an overvoltage detection function in a DC power supply device.

In order to achieve the above object, the present invention
An inductor connected between a voltage input terminal to which a DC voltage is input and an output terminal to which a load is connected; a switching element for passing a current intermittently through the inductor; and the switching element in accordance with an output feedback voltage And a control circuit that generates and outputs a signal for controlling on and off of the switching power supply that rectifies the current flowing through the inductor and outputs a voltage having a potential different from the input voltage.
An abnormal voltage detection circuit that detects that the output voltage is equal to or higher than a predetermined potential or lower than a predetermined potential by comparing with a predetermined voltage;
A variable voltage source for generating the predetermined voltage;
A control terminal for setting a voltage to be applied to the abnormal voltage detection circuit from the outside by the variable voltage source, and
The control circuit is configured to change ON / OFF control of the switching element when the abnormal voltage detection circuit detects an abnormal voltage.

  According to the above configuration, when the output voltage becomes equal to or higher than the predetermined potential or lower than the predetermined potential, the control circuit controls the output voltage so as not to be higher than the predetermined potential or lower than the predetermined potential. Thus, the user can freely set the abnormal voltage detection level through the control terminal. As a result, the withstand voltage of the external parts can be freely selected to some extent, so that the degree of freedom in selecting the parts is increased and the number of LEDs that can be connected in the LED drive power supply device is not so limited as compared with the conventional one.

Preferably, the abnormal voltage detection circuit detects that the output voltage has become equal to or higher than a predetermined potential by comparing with the first predetermined voltage;
A low voltage detection circuit that detects that the output voltage has become equal to or lower than a predetermined potential by comparing with a second predetermined voltage;
A first variable voltage source for generating the first predetermined voltage;
A second variable voltage source for generating the second predetermined voltage;
A first control terminal for setting a voltage to be supplied from the outside to the overvoltage detection circuit by the first variable voltage source;
And a second control terminal for setting a voltage to be supplied from the outside to the low voltage detection circuit by the second variable voltage source.

  As a result, when the output voltage becomes higher than a predetermined potential, the control circuit prevents the output voltage from becoming higher than that, and when the output voltage becomes lower than the predetermined potential, the control circuit outputs it. Since the voltage is controlled so as not to be lower than that, an abnormal state of the output voltage can be avoided, and the first and second control terminals allow the user to set the overvoltage detection level and the low voltage detection level of the output voltage respectively. It can be set freely.

  Preferably, an overcurrent detection circuit that detects that the output current has exceeded a predetermined current with a third predetermined voltage as a reference, a third variable voltage source that generates the third predetermined voltage, and an external A third control terminal for setting a voltage to be applied to the overcurrent detection circuit by the third variable voltage source, and the control circuit, when the overcurrent detection circuit detects an overcurrent, The on / off control of the switching element is changed.

  Thus, when the output current exceeds a predetermined current, the control circuit is controlled so that the output current does not increase any more, so that an abnormal increase in the output current can be avoided and the third control terminal The user can freely set the overcurrent detection level.

  Further preferably, any one of the control terminals also serves as a terminal for inputting a control signal for instructing operation / non-operation of the control circuit from the outside, and the control signal has a potential higher than a predetermined level. Instructs the operation of the control circuit, a potential lower than a predetermined level is defined as a signal that instructs the non-operation of the control circuit, and a voltage higher than the predetermined level is applied to the control terminal to which the control signal is input In such a state, the magnitude of the applied voltage designates the magnitude of a predetermined voltage to be applied to the detection circuit.

  Accordingly, it is possible to suppress an increase in the number of terminals due to the provision of a control terminal for setting a voltage to be applied to the detection circuit from the outside by the variable voltage source, and a compact switching power supply device and a semiconductor integrated circuit for power control Can be realized.

  In addition, a switching circuit that allows current to flow intermittently through the inductor is provided with a control circuit that generates a signal for on / off control according to the output feedback voltage, and the current flowing through the inductor is rectified to have a potential different from the input voltage. An abnormal voltage detection circuit for detecting that the output voltage is equal to or higher than a predetermined potential or lower than a predetermined potential by comparing with a predetermined voltage in a power supply control semiconductor integrated circuit constituting a switching power supply device that outputs a voltage; A variable voltage source that generates the predetermined voltage; and a control terminal for setting a voltage that the variable voltage source should supply to the abnormal voltage detection circuit from the outside, the control circuit detecting the abnormal voltage When the circuit detects an abnormal voltage, the on / off control of the switching element may be changed.

  Thus, in the switching power supply device using the power control semiconductor integrated circuit, an abnormal state of the output voltage can be avoided, and the user can freely set the detection level of the abnormal voltage by the control terminal.

  According to the present invention, in a DC power supply device, the user can freely set the detection level of overvoltage, low voltage, and overcurrent, and the degree of freedom in selecting external components can be increased, and LED driving can be performed. There is an effect that the number of connectable LEDs is not so limited in the conventional power supply device as compared with the conventional one.

It is a block block diagram which shows one Embodiment at the time of applying this invention to a step-up type DC-DC converter. It is a graph which shows the relationship between the control voltage from the outside in the variable voltage source of the DC-DC converter of embodiment, and an overvoltage detection level. It is a time chart which shows the operation | movement at the time of the overvoltage detection in the DC-DC converter of embodiment. It is a block block diagram which shows the 1st modification of the DC-DC converter to which this invention is applied. It is a block block diagram which shows the 2nd modification of the DC-DC converter to which this invention is applied. It is a block block diagram which shows the 3rd modification of the DC-DC converter to which this invention is applied. It is a block block diagram which shows the 4th modification of the DC-DC converter to which this invention is applied. It is a block diagram which shows the structural example of the conventional DC-DC converter. It is a time chart which shows the operation | movement at the time of the overvoltage detection in the conventional DC-DC converter.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.

  FIG. 1 shows an embodiment in which the present invention is applied to a step-up DC-DC converter.

  The DC-DC converter of this embodiment includes a coil (inductor) L1 and a rectifying diode D1, which are connected in series between a voltage input terminal IN to which a DC voltage Vin from the battery 20 is input and an output terminal OUT. A driving switching transistor SW1 composed of an N-channel MOSFET (insulated gate field effect transistor) connected between a connection node between the coil L1 and the diode D1 and the ground point GND, and the transistor SW1 is turned on / off. The switching control circuit 10 and the smoothing capacitor CO connected between the output terminal OUT and the ground point constitute a step-up type switching regulator.

  Further, between the output terminal OUT and the grounding point, a plurality of LEDs in series as a load and a sense resistor R1 for current-voltage conversion are connected in parallel with the smoothing capacitor CO, so that a current flowing through the LED is supplied. The voltage is changed by the resistor R1 and input to the switching control circuit 10 as the feedback voltage FB.

  The switching control circuit 10 includes an error amplifier 11 that outputs a voltage corresponding to a potential difference between the feedback voltage and the reference voltage Vref1 by applying the reference voltage Vref1 to the non-inverting input terminal and the feedback voltage FB to the inverting input terminal. A PWM (pulse width modulation) comparator 12 in which the output of the error amplifier 11 is input to the inverting input terminal, and an overvoltage detection comparator 13 in which the output voltage Vout is applied to the non-inverting input terminal.

  A waveform signal from a waveform generation circuit (not shown) that generates a waveform signal (RAMP signal) such as a triangular wave or sawtooth wave having a predetermined frequency is input to the non-inverting input terminal of the PWM comparator 12 and fed back. Depending on the voltage FB, control is performed such that when the voltage is high, that is, when the LED drive current is large, the pulse width of the output drive pulse is narrowed, and when the feedback voltage is low, that is, when the LED drive current is small, the pulse width is widened. Do.

  The PWM pulse signal generated by the PWM comparator 12 and the output signal of the overvoltage detection comparator 13 are supplied to the RS flip-flop 15 via the OR gate 14, and a drive circuit (driver) is output by the output of the flip-flop 15. ) 16 is configured to generate a gate control signal for driving the switching transistor SW1 on and off.

  The drive circuit 16 normally turns on the switching transistor SW1 with a PWM pulse, passes current through the coil L1, accumulates energy, and turns off SW1, releases energy from the coil, rectifies with the diode D1, and outputs to the output terminal. An electric charge is supplied to the capacitor C1 connected to, and an output voltage Vout obtained by boosting the input voltage from the battery is generated. Then, by feedback control based on the voltage from the sense resistor R1, the PWM comparator 12 controls the pulse width of the PWM pulse so that the LED drive current becomes constant.

  In this embodiment, a clock CK having a predetermined frequency is input to the set terminal of the flip-flop 15, the output of the flip-flop 15 rises by the clock CK, and the flip-flop 15 is reset by the output from the PWM comparator 12. When the output falls, a PWM pulse is generated and supplied to the drive circuit 16 to generate a drive pulse for SW1. Then, SW1 is turned on only during a period when the drive pulse is at a high level, and a current flows through the coil L1. By raising a pulse for turning on the switching transistor SW1 by the clock CK, a pulse can be generated even when, for example, the output of the PWM comparator 12 is stuck at a high level (or low level). A circuit (oscillator) that generates the clock CK is provided in the switching control circuit (IC) 10, but may be configured to be input from outside the IC chip.

  In the present embodiment, the output voltage Vout is input to the non-inverting input terminal of the overvoltage detection comparator 13, and the voltage Vref2 from the variable voltage source 17 is input to the inverting input terminal as a comparison voltage. The voltage generated is variable by the control voltage Vc applied to the external terminal P1 from the outside. Further, in the present embodiment, the external terminal to which the control voltage Vc is applied is configured to also serve as a terminal for inputting an enable signal SHDN that commands the operation / non-operation of the switching control circuit (IC) 10. ing. The comparator 18 is a circuit for discriminating the signal SHDN applied from the outside to the external terminal P1, its non-inverting input terminal is connected to the external terminal P1, and the reference voltage Vref3 is applied to the inverting input terminal. .

  The reference voltage Vref3 is set to a relatively low voltage such as 1 V, for example. The comparator 18 activates an internal circuit such as the error amplifier 11 or the PWM comparator 12 when the signal SHDN is higher than Vref3 ( When the signal SHDN is lower than Vref3, a signal (low level) for deactivating the internal circuit is output.

  The variable voltage source 17 is configured such that a voltage (Vc) applied from the outside to the external terminal P1 reacts in a predetermined range of 1V or more. Specifically, as shown in FIG. 2, the comparison voltage Vref2 output from the variable voltage source 17 changes in proportion to the voltage Vc in a range where the voltage Vc applied to the external terminal P1 is higher than Vref3. It is configured. As described above, the comparison voltage Vref2 that is the detection level of the overvoltage detection comparator 13 is changed according to the control voltage Vc from the outside, so that the user can arbitrarily set Vref2 according to the withstand voltage of the component to be used, for example. can do.

  Therefore, as shown in FIG. 3, in the system of FIG. 1, even if the output voltage Vout suddenly rises due to the LED being disconnected from the output terminal, the output of the comparator 13 is output when the output voltage Vout reaches the comparison voltage Vref2. Is inverted and is supplied to the flip-flop 15 through the OR gate 14, and the drive circuit 16 turns off the transistor SW1 based on the output of the flip-flop 15, and the output voltage Vout further increases. Do not. Even when the reset terminal is fixed at a high level, the flip-flop 15 can operate as described above by configuring the output to be at a high level for a very short time each time a clock is input to the set terminal. . Further, the frequency of the clock CK may be lowered when the output of the comparator 13 changes to a high level.

  With the above configuration, it is possible to prevent the peripheral components to be applied from being destroyed due to the output voltage Vout becoming abnormally high. In addition, since the user can freely set the level of the comparison voltage Vref2 in accordance with the breakdown voltage of the component to be used, the degree of freedom in selecting the component can be increased. In addition, by setting the comparison voltage Vref2 according to the forward voltage of the LED to be used, it is possible to avoid a reduction in the number of connectable LEDs.

  In the above embodiment, the output voltage Vout is directly input to the overvoltage detection comparator 13, but a voltage dividing circuit including a plurality of resistors connected in series between the output terminal and the ground point is provided. The voltage divided by the voltage dividing circuit may be input to the overvoltage detection comparator 13. Thereby, there is an advantage that it is not necessary to apply a voltage higher than the input voltage Vin to the circuit (IC) from the outside.

  Further, although not particularly limited, the switching control circuit 10 is formed as a semiconductor integrated circuit (IC) on one semiconductor chip, and the coil L1, the diode D1, and the smoothing capacitor C1 are constituted by discrete components and are described above. It can be configured to be connected to the IC as an external element. The switching transistor SW1 may also be configured as an external element of the IC.

  FIG. 4 shows a first modification of the DC-DC converter. In this modification, as the variable voltage source 17 for generating the comparison voltage Vref2 input to the overvoltage detection comparator 13, a plurality of constant voltage sources having different potentials and any one of the voltages are selected to detect the overvoltage as the comparison voltage Vref2. A circuit having a plurality of switches to be input to the comparator 13, and external control terminals P1 and P2 for inputting control codes C1 and C2 for determining ON / OFF states of the switches from the outside, and a control code C1 , C2 and a decoder 19 for generating a switch control signal.

  In the embodiment (FIG. 1), the comparison voltage Vref2 is continuously changed by the control voltage Vc from the outside, whereas this modification (FIG. 4) is configured to change the comparison voltage Vref2 stepwise. However, substantially the same effect as that of the DC-DC converter of the embodiment can be obtained.

  In this modification, a plurality of constant voltage sources and switches and a decoder are provided, but instead of the decoder, a variable current source (or a plurality of fixed current sources and switches) capable of switching a current value is also variable. A resistor for current-voltage conversion is provided in place of the voltage source 17, and the current generated by the variable current source is passed through the resistor for current-voltage conversion to generate the comparison voltage Vref2 having a variable voltage value. You may do it. Further, a current mirror circuit for copying the current generated by the variable current source is provided, and the current copied by the current mirror circuit is passed through a current-voltage conversion resistor to generate the comparison voltage Vref2. Also good.

  FIG. 5 shows a second modification of the DC-DC converter. In this modification, in addition to the overvoltage detection comparator 13a and its variable voltage source 17a, a low voltage detection comparator 13b that detects a low voltage state of the output voltage Vout and a comparison voltage Vref4 that is input to the comparator are generated. The variable voltage source 17b is provided, and even when the output voltage Vout becomes lower than a predetermined voltage, the switching transistor SW1 is controlled so that the output voltage does not decrease any more. For example, when the comparator 13b detects a low voltage state, by stopping the clock CK supplied to the flip-flop 15 at its output, the transistor SW1 is continuously turned off, and the output voltage Vout becomes the input voltage Vin. Can be maintained at a slightly lower value. Further, the frequency of the clock CK may be changed according to the output of the comparator 13b.

  By providing a low-voltage detection comparator, for example, when the voltage of the battery 20 decreases and a sufficient output voltage Vout cannot be obtained even if the voltage is boosted by the converter, this is detected and the converter operates. Can be stopped. In this modification as well, as in the above-described embodiment (FIG. 1), either one of the external terminals P1 and P2 can also be used as a terminal for inputting the enable signal SHDN. Further, the low voltage detection comparator 13b may be configured such that a voltage obtained by dividing the output voltage Vout by a resistance voltage dividing circuit is input and compared with the comparison voltage Vref4.

  FIG. 6 shows a third modification of the DC-DC converter. In this modification, in addition to the overvoltage detection comparator 13a and its variable voltage source 17a, an overcurrent detection comparator 13c that detects an overcurrent state, and a variable voltage source 17c that generates a comparison voltage Vref5 input to the comparator. And a resistor R2 connected in series with the switching transistor SW1, and the converted voltage is input to the comparator 13c and compared with the comparison voltage Vref5, so that when the overcurrent is detected, the switching transistor SW1 Is controlled so that the time for turning off is longer, so that no larger current flows. Thereby, deterioration of an element can be prevented.

  In this modified example, as in the above-described embodiment, any one of the external terminals can also be used as a terminal for inputting the enable signal SHDN. In addition, a comparator is provided to detect that a larger amount of current has flowed, so that, for example, when the load is short-circuited and a very large output current flows, the converter operation is stopped. Also good.

  FIG. 7 shows a fourth modification of the DC-DC converter. In this modification, in addition to the overvoltage detection comparator 13a, a low voltage detection comparator 13b that detects a low voltage state of the output voltage Vout, and an overcurrent detection comparator 13c that detects an overcurrent state are provided, and the output voltage When Vout becomes higher than the predetermined voltage, when the output voltage Vout becomes lower than the predetermined voltage, or when the output current starts to flow excessively, these are detected and the switching transistor SW1 is detected. The on / off control is changed.

  Although the invention made by the present inventor has been specifically described based on the embodiment, the present invention is not limited to the embodiment. For example, in the modification, the overvoltage detection comparator 13a is provided, but there is no overvoltage detection comparator 13a, only the low voltage detection comparator 13b, only the overcurrent detection comparator 13c, or low voltage detection. The present invention can also be applied to a DC-DC converter having a comparator 13b and an overcurrent detection comparator 13c.

  In the above-described embodiment, the PWM comparator 12 is provided after the error amplifier 11. However, instead of the PWM comparator, a PFM (pulse frequency modulation) comparator is provided and the switching transistor is controlled to be turned on and off by the PFM method. The present invention can also be applied to a DC converter.

  Furthermore, in the above embodiment, the OR gate 14 and the flip-flop 15 are provided in the subsequent stage of the PWM comparator 12, but for example, the OR gate 14 is replaced with an AND gate and the flip-flop 15 is omitted to directly output the AND gate. A circuit form to be supplied to the drive circuit 18 is also possible. Further, a latch circuit may be provided instead of the flip-flop 15 and the output of the latch circuit may be supplied to the drive circuit 16.

  In the above embodiment, the diode rectification type converter in which the rectification diode D1 is connected between the coil L1 and the output terminal is shown. However, a transistor is used instead of the diode, and the switching transistor SW1 is almost complementary. The present invention can also be applied to a synchronous rectification type converter that is controlled on and off. Further, the present invention can be applied not only to a step-up DC-DC converter but also to a step-down DC-DC converter.

DESCRIPTION OF SYMBOLS 10 Switching control circuit 11 Error amplifier 12 PWM comparator 13, 13a Overvoltage detection comparator 13b Low voltage detection comparator 13c Overcurrent detection comparator 16 Drive circuit (driver)
17 Variable voltage source 20 Battery L1 Coil (inductor)
CO smoothing capacitor SW1 Coil driving switching transistor

Claims (4)

An inductor connected between a voltage input terminal to which a DC voltage is input and an output terminal to which a load is connected; a switching element for passing a current intermittently through the inductor; and the switching element in accordance with an output feedback voltage A control circuit that generates and outputs a signal for controlling on and off, and rectifies the current flowing through the inductor to output a voltage having a potential different from the input voltage,
An abnormal voltage detection circuit that detects that the output voltage is equal to or higher than a predetermined potential or lower than a predetermined potential by comparing with a predetermined voltage;
A variable voltage source for generating the predetermined voltage;
A control terminal for setting a voltage to be applied to the abnormal voltage detection circuit from the outside by the variable voltage source, and
The control circuit changes the on / off control of the switching element when the abnormal voltage detection circuit detects an abnormal voltage ,
The control terminal also serves as a terminal for inputting a control signal for instructing the operation / non-operation of the control circuit from the outside.
The control signal is defined as a signal in which a potential higher than a predetermined level indicates operation of the control circuit, and a potential lower than the predetermined level indicates non-operation of the control circuit,
In a state where a voltage higher than the predetermined level is applied to the control terminal to which the control signal is input, the magnitude of the applied voltage specifies the magnitude of the predetermined voltage to be applied to the abnormal voltage detection circuit. It is comprised as follows. The switching power supply device characterized by the above-mentioned. The abnormal voltage detection circuit includes:
An overvoltage detection circuit for detecting that the output voltage is equal to or higher than a predetermined potential by comparing with a first predetermined voltage;
A low voltage detection circuit that detects that the output voltage has become equal to or lower than a predetermined potential by comparing with a second predetermined voltage;
A first variable voltage source for generating the first predetermined voltage;
A second variable voltage source for generating the second predetermined voltage;
A first control terminal for setting a voltage to be supplied from the outside to the overvoltage detection circuit by the first variable voltage source;
A second control terminal for setting a voltage to be supplied to the low voltage detection circuit from the outside by the second variable voltage source ,
2. The switching power supply device according to claim 1, wherein any one of the control terminals also serves as a terminal for inputting a control signal for instructing operation / non-operation of the control circuit from the outside . An overcurrent detection circuit for detecting that the output current is equal to or greater than a predetermined current with reference to a third predetermined voltage;
A third variable voltage source for generating the third predetermined voltage;
A third control terminal for setting a voltage to be supplied from the outside to the overcurrent detection circuit by the third variable voltage source;
The control circuit, when the overcurrent detection circuit detects an overcurrent, changes the on / off control of the switching element ,
3. The control terminal according to claim 1, wherein any one of the control terminals is also configured to serve as a terminal for inputting a control signal for instructing operation / non-operation of the control circuit from the outside. Switching power supply. A switching circuit that allows current to flow intermittently through the inductor is provided with a control circuit that generates a signal for on / off control according to the output feedback voltage. A power supply control semiconductor integrated circuit constituting a switching power supply device for outputting,
An abnormal voltage detection circuit that detects that the output voltage is equal to or higher than a predetermined potential or lower than a predetermined potential by comparing with a predetermined voltage;
A variable voltage source for generating the predetermined voltage;
A control terminal for setting a voltage to be applied to the abnormal voltage detection circuit from the outside by the variable voltage source, and
The control circuit changes the on / off control of the switching element when the abnormal voltage detection circuit detects an abnormal voltage ,
The control terminal also serves as a terminal for inputting a control signal for instructing the operation / non-operation of the control circuit from the outside.
The control signal is defined as a signal in which a potential higher than a predetermined level indicates operation of the control circuit, and a potential lower than the predetermined level indicates non-operation of the control circuit,
In a state where a voltage higher than the predetermined level is applied to the control terminal to which the control signal is input, the magnitude of the applied voltage specifies the magnitude of the predetermined voltage to be applied to the abnormal voltage detection circuit. A semiconductor integrated circuit for power control, which is configured as described above.

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