JP2011138666A - Led backlight drive circuit and on-board equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress mechanical noises generated from a smoothing capacitor equipped with an LED backlight drive circuit. <P>SOLUTION: The LED backlight drive circuit is constituted of: a switching-type DC/DC boosting circuit 11 which supplies a drive current to the LED backlight 3 where a single or a plurality of column light source 9 configured by connecting a plurality of LEDs 7 in series are connected in parallel, and in which a smoothing ceramic capacitor C1 is equipped at the output end; an LED driver IC13 which carries out on/off of the drive current which flows from the DC/DC boosting circuit 11 into the LED backlight 3 based on PWM control signal Sp; and a load circuit 15 which is connected to the output end N1 of the DC/DC boosting circuit 11, and in which the current equivalent to the drive current in a turned-off period Ta when the drive current of the LED backlight 3 is turned off. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an LED backlight driving circuit for driving an LED backlight used for a backlight of a liquid crystal panel display, and an in-vehicle device.

A light source called a backlight is arranged on the back surface of the liquid crystal display panel. As a light source of this backlight, an LED is widely attracting attention because it can cover a wide color space and is suitable for reduction in size and weight.
A backlight using an LED as a light source (hereinafter referred to as an “LED backlight”) is generally configured by connecting one or a plurality of column light sources in which a plurality of LEDs are arranged in a row, and the LED backlight drive circuit. A drive current is supplied to each of the column light sources and driven. When adjusting the LED backlight brightness, the white balance cannot be kept constant if the current value for driving the LED is changed. Therefore, the brightness of the LED is usually adjusted by changing the duty ratio of the on / off period of the LED. Adjustable PWM (Pulse Width Modulation) control is used.
Specifically, the LED backlight driving circuit converts a voltage supplied from a power source such as a battery into a driving voltage for the LED backlight and supplies the LED backlight with a switching DC / DC booster circuit (DC-DC). Converter) and an LED driver IC that controls the on / off period of the LED backlight by switching the DC / DC booster circuit based on the PWM control signal. Also, a smoothing capacitor is provided at the output end of this type of DC / DC booster circuit (see, for example, Patent Document 1).

JP 2001-166278 A

  As the smoothing capacitor, a multilayer ceramic capacitor is preferably used. However, since the ceramic capacitor has a property of generating a piezoelectric phenomenon when an AC signal is applied, the ceramic capacitor is mechanically synchronized with the PWM control frequency. There is a problem that noise is generated. To solve this problem, a multi-channel drive type LED backlight drive circuit that can be lit and driven independently for each column light source (channel) of the LED backlight is configured. A countermeasure can be taken by suppressing the voltage fluctuation of the ceramic capacitor. However, since it is necessary to drive and turn on a plurality of channels, the circuit configuration and control are complicated, and there is a problem that a countermeasure cannot be taken in the case of a single channel.

  The present invention has been made in view of the above-described circumstances, and an object thereof is to provide an LED backlight driving circuit and an in-vehicle device that can suppress mechanical noise generated from a smoothing capacitor.

  In order to achieve the above object, the present invention provides a switching-type power supply that supplies a driving current to an LED backlight in which a plurality of column light sources in which a plurality of LEDs are connected in series is connected in parallel, and a smoothing capacitor is provided at the output end. A DC / DC converter; an LED driver IC for turning on / off a drive current flowing from the DC / DC converter to the LED backlight based on a PWM control signal; and an output terminal of the DC / DC converter; There is provided an LED backlight drive circuit comprising a load circuit in which a current equivalent to the drive current flows from the DC / DC converter during a period in which the backlight drive current is off.

  According to the present invention, in the LED backlight driving circuit, the load circuit causes the current to flow when the driving current to the LED backlight is turned off and / or during the period when the driving current is off. It is characterized by comprising a circuit for thinning out the time of flowing.

  According to the present invention, in the LED backlight drive circuit, the period for thinning out the number of times the current flows is such that the frequency obtained from the period is outside the audible band.

  The present invention also provides a display comprising a liquid crystal display panel, an LED backlight in which one or more column light sources each having a plurality of LEDs connected in series are connected in parallel, and an LED backlight driving circuit that drives the LED backlight. A vehicle-mounted device having a panel, wherein the LED backlight driving circuit supplies a driving current to the LED backlight and includes a smoothing capacitor at an output end, and a PWM control signal. Based on the LED driver IC for turning on / off the driving current flowing from the DC / DC converter to the LED backlight and the output terminal of the DC / DC converter, and during the period when the driving current of the LED backlight is off Negative current flowing from the DC / DC converter is equivalent to the drive current. And a circuit, the number of the current flowing through the load circuit, or through the time, when the motor vehicle is running, or wherein the reducing when little remaining capacity of the battery of the vehicle.

  According to the present invention, the DC / DC converter includes the load circuit that is connected to the output end of the LED backlight and flows a current equivalent to the drive current from the DC / DC converter during the period when the drive current of the LED backlight is off. The voltage fluctuation at the output terminal of the DC converter is suppressed. Thereby, an AC signal is not applied to the smoothing capacitor provided at the output end, and the generation of mechanical noise of the capacitor is suppressed.

It is a circuit diagram which shows the structure of the LED backlight apparatus which concerns on embodiment of this invention. It is a timing chart which shows operation | movement of a switching control circuit. It is a timing chart which shows operation | movement of a LED backlight apparatus. It is a figure which shows typically the structure of the vehicle equipment which concerns on this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram showing a configuration of an LED backlight device 1 according to the present embodiment.
The LED backlight device 1 is a device suitably used for a backlight of a liquid crystal display panel used for a display panel of information equipment or the like, and includes an LED backlight 3 as a light source and an LED backlight driving circuit 5. I have. The LED backlight 3 is configured by connecting a plurality of column light sources 9 in which a plurality of LEDs 7 are arranged in a line in series in a plane. A white LED chip is preferably used for the LED 7.

The LED backlight drive circuit 5 is a circuit that drives the LED backlight 3 and also allows luminance adjustment based on an external PWM control signal. 15.
The DC / DC booster circuit 11 is connected to an external power supply P, converts the power supply voltage of the power supply P into a drive voltage Vd (FIG. 3) suitable for driving the LED backlight 3, and outputs the converted drive voltage. The DC / DC booster circuit 11 is configured by a switching (chopper-type) DC / DC converter. The DC / DC booster circuit 11 is connected in series to the coil L1 connected to the power source P and the coil L1. And a switching element Tr1 such as a MOSFET provided between the coil L1, the diode D1, and the ground, and a smoothing ceramic capacitor C1 is provided at the output end of the diode D1. ing.

  In such a configuration, current energy from the power source P is stored in the coil L1 while the switching element Tr1 is on, and the coil L acts to maintain the previous current value when the switching element Tr1 is off. At this time, since the potential at the input terminal of the coil L is fixed to the power supply voltage of the power supply P, the coil L outputs power so as to add the voltage corresponding to the current energy accumulated in the power supply voltage. As a result, the power supply voltage is boosted. The diode D1 provided at the subsequent stage of the coil L1 functions as an element that circulates the current of the coil L1 when the switching element Tr1 is off. The diode D1 can be replaced by a switching element such as a MOSFET that operates in a phase opposite to that of the switching element Tr1.

Further, since the smoothing ceramic capacitor C1 is connected to the output terminal N1 of the DC / DC booster circuit 11, the coil current is output from the coil L1 to the ceramic capacitor C1, thereby driving voltage to the output terminal N1. A voltage corresponding to Vd is generated. The drive voltage Vd is applied in parallel to each column light source 9 of the LED backlight 3, so that a drive current flows through each column light source 9, and each LED 7 of each column light source 9 has the same operating point (drive). (Current value). For the ceramic capacitor C1, a multilayer ceramic capacitor using a high dielectric constant ceramic as a dielectric between electrodes is used because of excellent frequency characteristics and the like.
Note that the DC / DC booster circuit 11 may use a DC / DC converter having another configuration (type) depending on use conditions.

The LED driver IC 13 turns on / off the switching element Tr1 at a duty ratio corresponding to the target value of the drive voltage Vd, and turns on / off the drive current to the LED backlight 3 based on the PWM control signal Sp from the outside. Then, lighting / extinguishing of the LED backlight 3 is controlled.
That is, the LED driver IC 13 turns on / off the drive current that flows to each column light source 9 of the LED backlight 3 in synchronization with the on / off of the PWM control signal Sp, and sets the current value of the drive current that flows when the LED driver IC 13 is on. A constant current drive circuit 17 is provided to keep constant. Further, the LED driver IC 13 detects the current value of the drive current flowing through the constant current drive circuit 17 and feeds back to the switching of the switching element Tr1 of the DC / DC booster circuit 11 so as to stabilize the current value. Is also provided.

  The PWM control signal Sp is a pulse signal having a duty ratio corresponding to the target value of the luminance of the LED backlight 3, and is generated and input by a higher-order circuit of the LED backlight device 1. Then, the constant current drive circuit 17 of the LED driver IC 13 sends a drive current to the LED backlight 3 in synchronization with the PWM control signal Sp, and controls the duty ratio when the LED backlight 3 is turned on / off. The brightness of the LED backlight 3 is adjusted.

The load circuit 15 is a circuit that suppresses voltage fluctuation at the output terminal N1 of the DC / DC booster circuit 11 and prevents an AC signal from being applied to the smoothing ceramic capacitor C1. Since the LED backlight drive circuit 5 is provided with the load circuit 15, the occurrence of piezoelectric phenomenon in the ceramic capacitor C <b> 1 can be suppressed, so that no mechanical noise is generated from the ceramic capacitor C <b> 1.
The load circuit 15 has a load resistor R1 connected in parallel with the input end of the LED backlight 3 with respect to the output end N1 of the DC / DC booster circuit 11, that is, the connection end of the ceramic capacitor C1, and the load resistor R1. A switching element Tr2 such as a MOSFET for turning on / off the current flow and a switching control circuit 21 for controlling on / off of the switching element Tr2 in synchronization with the PWM control signal Sp are configured.

  The load resistor R1 is set to a resistance value that makes the current value flowing through the load resistor R1 equivalent to the drive current flowing through the LED backlight 3 when the switching element Tr2 is on. As a result, even when the LED backlight 3 is off (extinguished), the voltage at the output terminal N1 of the DC / DC booster circuit 11 is maintained constant by flowing a current to the load resistor R1 side.

  The switching control circuit 21 includes an inverter circuit 23 that generates a pulse signal having a phase opposite to that of the PWM control signal Sp so as to turn on the switching element Tr2 during the extinguishing period of the LED backlight 3. Furthermore, since the switching control circuit 21 acts so that energization of the load circuit 15 leads to the power consumption of the power source P and increases the power consumption, the switching element Tr2 is turned on to suppress the power consumption. A thinning circuit 25 is provided for thinning out the number of times and the ON time, that is, the number of times the current flows through the load resistor R1 and the time during which the current flows.

  Specifically, the thinning circuit 25 includes a frequency divider 27 that divides the output of the inverter circuit 23, a clock circuit 29 that outputs a clock signal having a predetermined frequency, and a multiplier 31 that multiplies the frequency of the clock signal. A logical product circuit 33 that outputs a logical product of the outputs of the frequency divider 27 and the multiplier 31 and inputs the logical product to the switching element Tr2.

  As the frequency divider 27, a variable frequency divider that can dynamically set the frequency division ratio to 1/1 to 1 / m (m is an integer of 2 or more) is used. In the present embodiment, as shown in the timing chart of FIG. 2, the frequency divider is configured so that the output pulse of the inverter circuit 23 and the divided pulse are alternately output from the frequency divider 27. 27, the 1/1 frequency division ratio and the 1 / m frequency division ratio are alternately set for each period of the PWM control signal Sp. Thereby, as shown in FIG. 2, the frequency divider 27 outputs a pulse signal that becomes H level in synchronization with the extinguishing period Ta when the LED backlight 3 is extinguished (off) and becomes L level in synchronization with the lighting period Tb. In this case, the H-level pulse signal is frequency-divided by 1 / m every cycle, and an output signal S1 that is a frequency-divided pulse Pa is output. By turning on the switching element Tr2 in synchronization with the H level of the output signal S1, the H level period in the extinguishing period Ta is reduced by the frequency division in the divided pulse Pa. The time during which the current flows through the load resistor R1 is thinned out to reduce power consumption.

  Further, the power consumption can be further suppressed by eliminating the H level pulse signal itself from the output signal S1 of the frequency divider 27 and thinning out the number of times the current flows through the load resistor R1 during the extinguishing period Ta. Therefore, the multiplier 31 outputs a mask signal Sm that defines a mask period Ma in which the H level pulse signal of the output signal S1 of the frequency divider 27 is masked. As shown in FIG. 2, the mask signal Sm is a signal that is at an L level over the mask period Ma, and the pulse signal is set so that the mask period Ma masks one pulse of the output signal S1. It is. By inputting the mask signal Sm and the output signal S1 of the frequency divider 27 to the logical product circuit 33, the logical product circuit 33 outputs the output signal of the switching control circuit 21 as shown in FIG. Ma becomes an L level, and an output signal Sout that is a pulse signal synchronized with the output signal S1 of the frequency divider 27 is output only during the non-mask period Mb. Then, the switching element Tr2 is turned on in synchronization with the H level of the output signal Sout. The period Tc of the mask period Ma is set so that the frequency f (f = 1 / Tc) obtained from the period Tc is outside the human audible band, which will be described later.

FIG. 3 is a timing chart showing the operation of the LED backlight 3.
In this figure, the PWM control signal Sp, the output signal Sout of the switching control circuit 21, the voltage change of the output terminal N2 of the LED backlight 3, the voltage change of the output terminal N3 of the load resistor R1 of the load circuit 15, and the DC / The voltage change at the output terminal N1 of the DC booster circuit 11 is shown.
As shown in this figure, the voltage at the output terminal N2 of the LED backlight 3 is at the H level when the LED backlight 3 is turned off and at the L level when the LED backlight 3 is turned on, and thus changes in a phase opposite to the PWM control signal Sp.
On the other hand, the voltage at the output terminal N3 of the load resistor R1 of the load circuit 15 is L level while the switching element Tr2 is turned on and a current flows through the load resistor R1, and is H level when the switch element is off and no current flows. Therefore, the output signal Sout of the switching control circuit 21 that drives the switching element Tr2 is inverted.

Since the voltage (drive voltage Vd) at the output terminal N1 of the DC / DC booster circuit 11 rises when both the LED backlight 3 and the load circuit 15 are in the non-drive state, as shown in FIG. The voltage rises when both the voltage at the output terminal N2 of the light 3 and the voltage at the output terminal N3 of the load resistor R1 become H level.
At this time, in the load circuit 15, when there is no thinning operation by the thinning circuit 25, the load circuit 15 is driven over the turn-off period Ta of the LED backlight 3, so that the voltage at the output terminal N <b> 1 does not change. It is maintained substantially constant.

On the other hand, when the thinning operation is performed, as shown in FIG. 3, when the load circuit 15 is driven by the divided pulse Pa, the switching element Tr2 is turned off in the divided pulse Pa. The voltage at the output terminal N1 rises, and during the mask period Ma, the voltage at the output terminal N1 rises during the turn-off period Ta of the LED backlight 3.
At this time, the period during which the load circuit 15 is not driven in the frequency-divided pulse Pa is shorter than the mask period Ma. Therefore, the voltage increase at the output terminal N1 during the period is smaller than the mask period Ma. For this reason, since the deformation due to the piezoelectric effect of the ceramic capacitor C1 due to the voltage fluctuation of the output terminal N1 is limited, mechanical noise is also reduced.

  Further, since the voltage increase at the output terminal N1 during the mask period Ma is relatively large, mechanical noise generated from the ceramic capacitor C1 also increases. Therefore, in the present embodiment, as described above, the frequency T obtained from the period Tc of the mask period Ma, that is, the period Tc in which mechanical noise is generated by the mask period Ma, is outside the human audible band. This makes it difficult to hear mechanical noise.

  Thus, according to the present embodiment, a current equivalent to the drive current is connected to the output terminal N1 of the DC / DC booster circuit 11 during the extinction period Ta when the drive current of the LED backlight 3 is off. The load circuit 15 flowing from the / DC booster circuit 11 is provided. With this configuration, voltage fluctuation at the output terminal N1 of the DC / DC booster circuit 11 is suppressed, and an AC signal is not applied to the smoothing ceramic capacitor C1 provided at the output terminal N1, and the machine of the ceramic capacitor C1 Generation of noise is suppressed.

  In addition, according to the present embodiment, the load circuit 15 includes the thinning circuit 25 that thins out the number of times the current flows when the extinguishing period Ta is triggered and the time during which the current flows during the extinguishing period Ta. Power consumption in the load circuit 15 can be suppressed while suppressing mechanical noise.

  In particular, according to the present embodiment, the period Tc for thinning out the number of times the current flows is set such that the frequency f obtained from the period Tc is outside the audible band, so that power consumption is effectively suppressed while making it difficult to hear mechanical noise. be able to.

The embodiment described above shows one aspect of the present invention, and can be arbitrarily modified and applied without departing from the spirit of the present invention.
For example, in the above-described embodiment, the multilayer ceramic capacitor C1 is exemplified as the smoothing capacitor. However, the present invention is not limited to this, and the type of capacitor (capacitance) in which mechanical noise is generated due to voltage fluctuation of the output terminal N1 is ceramic. The present invention can also be applied when used in place of the capacitor C1.

Moreover, the LED backlight device 1 described above is suitable for use as a backlight of a display panel of an in-vehicle device.
FIG. 4 is a diagram schematically showing the configuration of the in-vehicle device 50 according to the present invention.
As shown in this figure, the in-vehicle device 50 is mounted on a vehicle and operates by receiving power supply from a battery 52 provided in the vehicle. The in-vehicle device 50 includes a control circuit 54 and a display panel 56, and the display panel 56 includes the liquid crystal display panel 58 and the above-described LED backlight device 1 provided behind the liquid crystal display panel 58. It is prepared for.
The in-vehicle device 50 detects the remaining amount of the battery 52 and outputs it to the control circuit 54, and detects the operating state of the motor 62 of the vehicle and outputs the operating state of the prime mover to the control circuit 54. Circuit 64. The prime mover 62 is an engine or / and an electric motor.

The control circuit 54 has a function of controlling the LED backlight device 1 of the display panel 56. Specifically, the control circuit 54 generates a PWM control signal Sp for brightness adjustment and outputs it, and a load And a load circuit control unit 68 for controlling the operation of the circuit 15.
The load circuit control unit 68 sets the dividing ratio of the frequency divider 27 of the thinning circuit 25 included in the load circuit 15 and the mask period Ma defined by the multiplier 31 to the remaining battery level detection circuit 60 and the motor operating state detection circuit 64. Control based on the detection result.

  More specifically, when the prime mover 62 is operating, mechanical noise generated from the LED backlight device 1 is drowned out by the prime mover sound, road noise during traveling, and the like. Therefore, in this case, the load circuit control unit 68 extends the mask period Ma of the multiplier 31 from a specified value (for example, one cycle of the PWM control signal Sp), so that the output signal Sout of the switching control circuit 21 is at the H level. The number of times of driving the load circuit 15 is reduced or stopped. Thereby, power consumption in the load circuit 15 is suppressed. On the other hand, when the prime mover 62 is stopped, the interior of the vehicle is often quiet, so the load circuit control unit 68 suppresses the generation of mechanical noise from the ceramic capacitor C1 with the mask period Ma as a specified value.

  Further, since the load circuit 15 is driven and current is consumed by the load resistor R1, the load circuit control unit 68 is configured to suppress power consumption of the battery 52 when the remaining amount of the battery 52 is small. The mask period Ma of the multiplier 31 is extended beyond the specified value to reduce or stop the number of times the load circuit 15 is driven.

  When the prime mover 62 is operating or when the remaining amount of the battery 52 is low, the load circuit control unit 68 increases the value of “m” of the frequency division ratio 1 / m of the frequency divider 27. The drive time of the load circuit 15 during the extinguishing period Ta (the time during which current flows) may be reduced.

1 LED backlight device 3 LED backlight 5 LED backlight drive circuit 7 LED
9 row light source 11 DC / DC booster circuit (DC / DC converter)
13 LED driver IC
DESCRIPTION OF SYMBOLS 15 Load circuit 25 Thinning-out circuit 50 In-vehicle apparatus 52 Battery 54 Control circuit 56 Display panel 58 Liquid crystal display panel 60 Battery remaining amount detection circuit 62 Motor | power_engine 64 Motor | power_engine operating state detection circuit 66 PWM control signal generation part 68 Load circuit control part C1 Ceramic capacitor N1 To N3 output terminal Ta extinguishing period Tb lighting period Tc period

Claims (4)

A switching type DC / DC converter that supplies a driving current to an LED backlight in which a plurality of LEDs are connected in series or connected in parallel, and includes a smoothing capacitor at the output end;
An LED driver IC for turning on / off a drive current flowing from the DC / DC converter to the LED backlight based on a PWM control signal;
A load circuit connected to an output terminal of the DC / DC converter, and a current equivalent to the drive current flowing from the DC / DC converter during a period when the drive current of the LED backlight is off. LED backlight drive circuit.   The load circuit includes a circuit that thins out the number of times the current flows when the drive current to the LED backlight is turned off and / or the time during which the drive current is passed during the off-period of the drive current. The LED backlight drive circuit according to claim 1.   3. The LED backlight drive circuit according to claim 2, wherein a cycle for thinning out the number of times of flowing the current is such that a frequency obtained from the cycle is outside the audible band. In-vehicle device having a display panel including a liquid crystal display panel, an LED backlight in which one or more column light sources in which a plurality of LEDs are connected in series are connected, and an LED backlight driving circuit for driving the LED backlight Because
The LED backlight driving circuit is:
A switching type DC / DC converter that supplies a driving current to the LED backlight and includes a smoothing capacitor at an output end;
An LED driver IC for turning on / off a drive current flowing from the DC / DC converter to the LED backlight based on a PWM control signal;
A load circuit connected to the output end of the DC / DC converter, and a current equivalent to the drive current flows from the DC / DC converter during a period when the drive current of the LED backlight is off.
The in-vehicle device, wherein the number of times or the time during which current flows through the load circuit is reduced when the motor of the vehicle is operating or when the remaining battery of the vehicle is low.

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