JP2012015399A - Drive circuit of light-emitting diode, and light-emitting device and electronic equipment using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress heat generation from a bipolar transistor.SOLUTION: An output transistor Q1 is a PNP-type bipolar transistor, and its emitter is connected to a cathode of an LED string 6. A heat dissipation resistor R7 and a current control resistor R4 are sequentially provided in series between the emitter of the output transistor Q1 and a fixed voltage terminal. In an operational amplifier OA, its output terminal is connected to the base of the output transistor Q1, and its non-inverting input terminal is connected to the point of connection between the heat dissipation resistor R7 and the current control resistor R4, and also reference voltage Vref is applied to its inverting input terminal. A switching power supply 4 generates a drive voltage Vout so that the base potential of the output transistor Q1 agrees with the reference voltage Vref and supplies the drive voltage to the anode of the LED string 6.

Description

  The present invention relates to a drive circuit for a light emitting diode.

  In recent years, light emitting devices using LEDs (light emitting diodes) have been used as backlights for liquid crystal panels and lighting equipment. FIG. 1 is a circuit diagram showing a configuration of a light emitting device studied by the present inventors. The light emitting device 1003 includes an LED string 1006, a switching power supply 1004, and a current source CS.

  The LED string 1006 includes a plurality of LEDs connected in series. The switching power supply 1004 boosts the input voltage Vin and supplies the drive voltage Vout to one end of the LED string 1006.

The current source CS is provided on the path of the LED string 1006. The current source CS supplies a driving current I _LED corresponding to the target brightness to the corresponding LED string 1006. The current source CS includes an output transistor Q1, a current control resistor R4, and an operational amplifier OA. The output transistor Q1 is a PNP-type bipolar transistor, and is provided on the path of the drive current I _LED . The current control resistor R4 is provided between the collector of the output transistor Q1 and the ground terminal. The output terminal of the operational amplifier OA is connected to the base of the output transistor Q1, its inverting input terminal is connected to the collector of the output transistor Q1, and its non-inverting input terminal receives the reference voltage Vref.

Feedback is applied by the current source CS so that the collector potential of the output transistor Q1, that is, the voltage drop of the current control resistor R4 matches the reference voltage Vref, and the drive current I _LED is set to a value corresponding to the reference voltage Vref. .
I _LED = Vref / R4

  The switching power supply 1004 includes an output circuit 1102 and a control IC 1100. The output circuit 1102 includes an inductor L1, a switching transistor M1, a rectifier diode D1, and an output capacitor C1. The control IC 1100 adjusts the drive voltage Vout by controlling the on / off duty ratio of the switching transistor M1. The error amplifier EA amplifies an error between the base voltage Vb of the output transistor Q1 and the reference voltage Vref. The control IC 1100 receives the output signal of the error amplifier EA and adjusts the output voltage Vout so that the base voltage Vb and the reference voltage Vref match.

JP 2010-015967 A JP 2009-188135 A

  In the light emitting device 1003 of FIG. 1, the base voltage Vb and the collector voltage Vc of the output transistor Q1 are both adjusted to be equal to the reference voltage Vref. Therefore, the base-collector voltage of the output transistor Q1 is equal to the base-emitter voltage Vf.

  Since a transistor having a small base-emitter voltage Vf is expensive, a transistor having a large Vf may be used to reduce the cost. When a transistor having a large Vf is used, the collector-emitter voltage of the output transistor Q1 increases, so that the power consumption of the output transistor Q1 increases and heat generation also increases.

  The present invention has been made in view of these problems, and one of exemplary purposes of an embodiment thereof is to provide a current driving circuit capable of suppressing heat generation of a bipolar transistor.

  One embodiment of the present invention relates to a driving circuit for driving a light emitting diode string. This drive circuit includes an output transistor, which is a PNP bipolar transistor, the emitter of which is connected to the cathode of a light-emitting diode string, and a heat distribution resistor and An operational amplifier in which the current control resistor and its output terminal are connected to the base of the output transistor, its non-inverting input terminal is connected to the connection point of the heat distribution resistor and the current control resistor, and a reference voltage is applied to its inverting input terminal And a switching power supply that generates a drive voltage so that the base potential of the output transistor matches the reference voltage and supplies the drive voltage to the anode of the light-emitting diode string.

  According to this aspect, the collector-emitter voltage of the output transistor is reduced by the voltage drop of the heat dispersion resistor, and the power consumption of the output transistor, in other words, the amount of heat generation can be reduced.

The resistance value of the heat distribution resistor may be determined such that the voltage drop of the heat distribution resistor is 20% or more of the base-emitter voltage of the output transistor.
In this case, power consumption can be suitably distributed between the output transistor and the heat distribution resistor.

  Another embodiment of the present invention is a light-emitting device. This device includes a light-emitting diode string and a drive circuit according to any one of the above-described modes for driving the light-emitting diode string.

  Yet another embodiment of the present invention is an electronic device. This electronic device includes a liquid crystal panel and the above-described light emitting device provided as a backlight of the liquid crystal panel.

  Note that any combination of the above-described constituent elements and the constituent elements and expressions of the present invention replaced with each other among methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  According to an aspect of the present invention, phase shift burst dimming can be easily realized.

It is a circuit diagram which shows the structure of the light-emitting device which this inventor examined. It is a circuit diagram which shows the structure of an electronic device provided with the switching power supply which concerns on embodiment. 3 is a flowchart for determining an operation mode of the current drive circuit of FIG. 2.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are electrically connected in addition to the case where the member A and the member B are physically directly connected. It includes the case of being indirectly connected through another member that does not affect the connection state.
Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as an electrical condition. It includes the case of being indirectly connected through another member that does not affect the connection state.

  FIG. 2 is a circuit diagram illustrating a configuration of an electronic device including the switching power supply according to the embodiment.

  The electronic device 2 is a battery-driven device such as a notebook PC, a digital camera, a digital video camera, a mobile phone terminal, or a PDA (Personal Digital Assistant), and includes a light emitting device 3 and an LCD (Liquid Crystal Display) panel 5. The light emitting device 3 is provided as a backlight of the LCD panel 5.

  The light emitting device 3 includes LED strings 6_1 to 6_n that are light emitting elements, a current driving circuit 8, and a switching power supply 4. The number of channels n is 8 at the maximum, and the designer of the electronic device 2 is determined according to the size of the LCD panel 5, the type of the electronic device 2, and the like. That is, the channel number n can be any number from 1 to 8.

  Each LED string 6 includes a plurality of LEDs connected in series. The switching power supply 4 is a step-up DC / DC converter that boosts an input voltage (for example, battery voltage) Vin input to the input terminal P1 and outputs an output voltage (drive voltage) Vout from the output terminal P2. One end (anode) of each of the plurality of LED strings 6_1 to 6_n is commonly connected to the output terminal P2.

  The switching power supply 4 includes a control IC 100 and an output circuit 102. The output circuit 102 includes an inductor L1, a rectifier diode D1, a switching transistor M1, and an output capacitor C1. Since the topology of the output circuit 102 is general, description thereof is omitted.

  The switching terminal P4 of the control IC 100 is connected to the gate of the switching transistor M1. The control IC 100 adjusts the ON / OFF duty ratio of the switching transistor M1 by feedback so that the output voltage Vout necessary for lighting the LED string 6 is obtained. The switching transistor M1 may be built in the control IC 100.

  The resistors R1 and R2 divide the output voltage Vout to generate a feedback voltage Vout ′ corresponding thereto. The feedback voltage Vout 'is input to the feedback terminal P3 (OVP terminal). An overvoltage protection circuit (not shown) (not shown) performs overvoltage protection when the feedback voltage Vout 'exceeds a threshold value.

The current drive circuit 8 is provided on the other end (cathode) side of the plurality of LED strings 6_1 to 6_n. Current drive circuit 8, each LED string 6_1～6_N, supplies intermittent drive current _I LED1 _{~I LEDn} corresponding to the target luminance.

The current driving circuit 8 includes a plurality of current sources CS _{1 to} CS _n provided for each channel, a burst controller 9, a control input terminal P 5, a standby terminal (STB terminal) P 6, burst dimming terminals BS 1 to BS 8 for each channel, Current control terminals CL1 to CL8 for each channel, comparators COMP1 to COMP8, and comparator COMP9 for each channel are provided.

The i-th current source CS _i supplies the drive current I _LEDi to the corresponding LED string 6 — _i . The current source CS _i includes an output circuit CSb _i and a control unit CSa _i . The output circuit CSb _i includes an output transistor Q1, a current control resistor R4, a pull-up resistor R5, and a heat distribution resistor R7. The output transistor Q1 is a PNP-type bipolar transistor, and its emitter is connected to the cathode of the LED string 6_i. The heat distribution resistor R7 and the current control resistor R4 are provided in series between the emitter of the output transistor Q1 and the fixed voltage terminal (ground terminal). Voltage V _R4 at the connection point of the heat dissipation resistor R7 and the current control resistor _R4, that is the voltage drop of the current control resistor R4 is input to the current control terminal CLi. The pull-up resistor R5 is provided between the base and emitter of the output transistor Q1. Other channels are similarly configured.

The resistor R4, the voltage drop _{V R4} proportional to the drive current _{I LEDi} occurs.
V _R4 = I _LEDi × R4
Controller CSa _i, as the corresponding voltage drop _{V R4} coincides with the reference voltage Vref, modulate the base voltage of the output transistor Q1. In other words, during the lighting period
I _LEDi = Vref / R4
Holds.

Controller CSa _i includes an operational amplifier OA1, the transistors M4. The transistor M4 is provided between the burst dimming terminal BSi and the ground terminal. The non-inverting input terminal of the operational amplifier OA1 (+) the reference voltage Vref is input to its inverting input terminal - the voltage of the current control terminal CL, that is, the voltage drop V _R4 of the current control resistor R4 is input to the () . The output voltage of the operational amplifier OA1 is connected to the gate of the transistor M4. Feedback is applied by the current source CS _{i so} that V _R4 = Vref is established, and the drive current I _LEDi corresponding to the reference voltage Vref can be generated in each channel. The transistor M4 may be regarded as a part of the output stage of the operational amplifier OA4, or the transistor M4 may be omitted.

The control input terminal P5 receives a pulse width modulated dimming pulse signal PWM that is used when performing burst dimming. The first level (e.g., high level) of the dimming pulse signal PWM is a lighting period _{T ON} of the LED string 6, the second level (e.g., low level) instructs the turn-off period _{T OFF.} The duty ratio of the PWM dimming pulse signal PWM, that is, the lighting period T _ON and the extinguishing period T _OFF is used in common for all channels.

  A standby signal STB for instructing a standby state and an operating state of the current driving circuit 8 is input to the standby terminal P6. Specifically, when the standby signal STB is at a low level (for example, 0 to 0.8 V), the current driving circuit 8 is in a standby state. When the standby signal STB is at a high level (> 0.8 V), the current driving circuit 8 is in an operating state and supplies a driving current to the LED string 6.

The burst controller 9 can switch between the following modes based on the voltage level V _STB of the standby signal STB and the voltage levels V _{BS1 to} V _BS8 of the burst dimming terminals _{BS1 to} BS8 of the eight channels.

a. Common mode for all channels φ _COM
In this mode, the burst controller 9 does not perform phase shift, and drives the LED strings of all the channels to be driven with the phases of their drive currents I _LED being aligned, regardless of the number of connected LED strings 6. To do. Also referred to as phi ₀ since the phase difference between the drive current of each channel is zero in this mode.

b. Phase shift mode φ _SHIFT
In this mode, the burst controller 9 drives the light emitting diode strings of the respective channels so that the phases of the respective driving currents are shifted. The phase shift mode b includes the following three modes.

b1. 90 degree phase shift mode φ ₉₀
In this mode, the first to fourth channels are targeted for driving, the driving current _I LED1 _{~I LED 4} of the phase for the LED string 6_1～6_4 is 1/4 period shift dimming pulse signal PWM to each other.

b2. 60 degree phase shift mode φ ₆₀
In this mode, the first to sixth channel of the phase of the drive current _I LED1 _{~I LED 6} to the LED string 6_1～6_6 is 1/6 cycle shift in the dimming pulse signal PWM to each other.

b3. 45 degree phase shift mode φ ₄₅
In this mode, the phase of the drive current _I LED1 _{~I LED 8} from the first to the LED string 6_1～6_8 eighth channel is 1/8 cycle shift in the dimming pulse signal PWM to each other.

The burst controller 9 generates burst control signals PWM _{1 to} PWM ₈ corresponding to each mode and supplies them to the current sources CS _{1 to} CS ₈ . When the burst control signal PWM _i is high, the current source CS _i generates a driving current _{I LEDi} become an operating state, it is the lighting period _{T ON.} On the other hand, when the burst control signal PWM _i is at a low level, the current source CS _i is in a stopped state, which is the extinguishing period T _OFF .

During a certain time after the standby signal STB is asserted from the low level to the high level, the determination period T _JDG is entered. The determination period T _JDG is, for example, several cycles of the dimming pulse signal PWM, specifically about three cycles. In this determination period T _JDG , the burst controller 9 determines a mode based on the voltage level V _STB of the standby signal STB and the voltage levels V _{BS1 to} V _BS8 of the burst dimming terminals _{BS1 to} BS8 of the eight channels. FIG. 3 is a flowchart for determining the operation mode of the current drive circuit 8 of FIG.

First, the burst controller 9 determines an operation mode according to the voltage level V _STB of the standby signal STB. When the voltage level V _STB of the standby signal STB is included in a predetermined first range, it is set to all channels common mode phi _0. The comparator COMP9 compares the voltage V _STB with the threshold voltage Vth1, and outputs a determination signal S9 indicating the comparison result. If the determination signal S9 indicates the _V STB> Vth1 (S100 of Y), the burst controller 9 is set to all channels common mode φ ₀ (S102).

When the voltage level _{V STB} of the standby signal STB is included in a predetermined second voltage range, the burst controller 9 is set to the phase shift mode phi _SHIFT. Since the second voltage range is V _STB <Vth1, when the determination signal S9 indicates V _STB <Vth1 (N in S100), the burst controller 9 enters the phase shift mode _φSHIFT .

Burst controller 9 is set to the phase shift mode phi _SHIFT is subsequently based on the voltage level _V BS1 _{~V BS8} burst dimming terminal BS of each channel, 90 °, 60 °, to one of the 45 degree phase shift mode Is set.

Comparator COMP1~COMP8 is provided for each channel, the voltage _V BS1 _{~V BS8} the corresponding channel, is compared with a predetermined threshold voltage Vth2. The threshold voltage Vth2 is preferably about 0.1 V, for example. The comparator COMPi of the i-th channel outputs a detection signal Si that is at a high level (H) when V _BSi <Vth2, and is at a low level (L) when V _BSi > Vth2.

When the LED string 6_i is connected to the i-th burst dimming terminal BSi, if the driving current I _LEDi is zero, the voltage level V _BSi rises to the vicinity of the output voltage Vout. On the other hand, when the LED string 6 — i is not connected to the burst dimming terminal BSi, the voltage level V _BSi decreases to near the ground voltage. That is, the output signal Si of the comparator COMPi indicates whether or not the LED string 6_i is connected.

When the potentials V _{BS5 to} V _BS8 of all the burst dimming terminals _{BS5 to} BS8 of the fifth to eighth channels are lower than the predetermined second threshold voltage Vth2 in the determination period T _JDG , in other words, the burst controller 9 S5 == H && S6 = H && S7 = H && S8 = H
When the condition is satisfied (Y in S104), the 90-degree phase shift mode φ ₉₀ is set (S106). This indicates a state in which the LED strings 6_5 to 6_8 are not connected to the fifth to eighth channels. (A = B) is an operator indicating true (1) when A and B are equal, and false (0) otherwise, and “&&” is an operator indicating logical product.

When the conditional expression is not satisfied (N in S104), the process proceeds to S108. 7, burst dimming terminal of the 8 channel BS 7, BS8 potential _V BS _{7, V BS8} is is lower than the second threshold voltage Vth2, in other words, condition S7 = H && S8 = H
When satisfying (S108 of Y), first to sixth channel becomes driven, it is set to 60 degree phase shift mode φ ₆₀ (S110).

Otherwise (S108 of N), all the channels becomes driven, is set to 45 degree phase shift mode φ ₄₅ (S112).

In this way, it is determined whether or not the LED string 6 is connected for each channel. The error amplifier EA1 is in the driving period, among the channels each voltage _{V BS} which LED string 6 is connected, and amplifies the lowest one, the error of the reference voltage Vref (for example 0.3V), corresponding to the error An error voltage Verr is generated. The error voltage Verr is output from the FB terminal via the transistor Q2 and the resistor R6, and input to the feedback terminal of the control IC 100. Control IC100 is in the driving period, to adjust the lowest one of the channels each voltage _{V BS} which LED string 6 is connected to the reference voltage Vref coincide, the output voltage Vout.

The above is the configuration of the light-emitting device 3. Next, the operation will be described.
Among the plurality of channels, the base voltage V _BS of the output transistor Q1 is focused on the lowest channel.
Base voltage _{V BS} of the output transistor Q1 is feedback-controlled so as to coincide with the reference voltage Vref by switching power supply 4. The voltage drop _{V R4} of the current control resistor R4, is feedback controlled to coincide with the reference voltage Vref by the current source CS. When writing the base-emitter voltage of the output transistor Q1 as Vf, the collector-emitter voltage as V _CE , and the voltage drop of the heat dispersion resistor R7 as _VR7 , the following equation holds for the emitter voltage of the output transistor Q1.
Vref + Vf = Vref + V _R7 + V _CE (1)

When formula (1) is transformed, formula (2) is obtained.
V _CE = Vf−V _R7 (2)

As can be seen from equation (2), the collector-emitter voltage V _CE of the output transistor Q1, as compared with the case without the heat distribution resistors R7, can be reduced by the voltage drop V _R7 minutes Heat dissipation resistor R7. This means that the power consumption of the output transistor Q1, that is, the heat generation amount can be reduced.

  By reducing the amount of heat generated by the output transistor Q1, the electronic device 2 in which the light-emitting device 3 is mounted can be easily protected from heat, and the cost required for it can be reduced.

In order to obtain a sufficient effect of heat dissipation, the voltage drop V _R7 Heat dissipation resistor R7, about 20% of the base-emitter voltage Vf of the output transistor Q1, or such that more, the heat dissipation resistor R7 It is desirable to design the resistance value.
0.2 × Vf <V _R7 <Vf (3)
The voltage drop V _R7 of the heat dispersion resistor R7 is given by equation (4).
V _R7 = I _LED × R7 (4)
Therefore, the amount of heat generated by the output transistor Q1 can be suitably reduced by designing the resistance value of the heat dispersion resistor R7 within the following range.
0.2 × Vf / I _LED <R7 <Vf / I _LED

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and various modifications may exist in each of those constituent elements, each processing process, and a combination thereof. Hereinafter, such modifications will be described.

  In the embodiment, a non-insulated switching power supply using an inductor has been described, but the present invention can also be applied to an insulating switching power supply using a transformer.

  In the embodiment, the electronic apparatus has been described as an application of the light emitting device 3, but the application is not particularly limited and can be used for lighting or the like.

  Although the present invention has been described using specific terms based on the embodiments, the embodiments only illustrate the principles and applications of the present invention, and the embodiments are defined in the claims. Many variations and modifications of the arrangement are permitted without departing from the spirit of the present invention.

Q1 ... output transistor, L1 ... inductor, C1 ... output capacitor, D1 ... rectifier diode, M1 ... switching transistor, 2 ... electronic device, 3 ... light emitting device, 4 ... switching power supply, R4 ... current control resistor, R7 ... heat dispersion resistor 5 ... LCD panel, BS ... Burst dimming terminal, CL ... Current control terminal, R5 ... Pull-up resistor, 6 ... LED string, 8 ... Current drive circuit, 9 ... Burst controller, 100 ... Control IC, 102 ... Output circuit , CS ... current source.

Claims (4)

A driving circuit for driving a light emitting diode string,
An output transistor that is a PNP-type bipolar transistor, the emitter of which is connected to the cathode of the light-emitting diode string;
A heat distribution resistor and a current control resistor provided in series between the emitter of the output transistor and a fixed voltage terminal in order;
An operational amplifier having an output terminal connected to a base of the output transistor, a non-inverting input terminal connected to a connection point of the heat distribution resistor and the current control resistor, and a reference voltage applied to the inverting input terminal;
A switching power supply that generates a drive voltage so that the base potential of the output transistor matches a reference voltage, and supplies the drive voltage to the anode of the LED string;
A drive circuit comprising:   2. The drive circuit according to claim 1, wherein the resistance value of the heat distribution resistor is determined such that a voltage drop of the heat distribution resistor is 20% or more of a base-emitter voltage of the output transistor. A light emitting diode string;
The drive circuit according to claim 1 or 2, which drives the light emitting diode string;
A light emitting device comprising: LCD panel,
The light emitting device according to claim 3 provided as a backlight of the liquid crystal panel;
An electronic device comprising:

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