CN103680414B - A kind of LCD backlight constant current drive circuit - Google Patents
A kind of LCD backlight constant current drive circuit Download PDFInfo
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- CN103680414B CN103680414B CN201210339228.4A CN201210339228A CN103680414B CN 103680414 B CN103680414 B CN 103680414B CN 201210339228 A CN201210339228 A CN 201210339228A CN 103680414 B CN103680414 B CN 103680414B
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Abstract
The invention discloses a kind of LCD backlight constant current drive circuit, comprise power supply, pwm control circuit, LED drive circuit, bleeder circuit and differential feedback circuit, pwm control circuit controls the time that backlight LED opens and closes, and then controls the brightness of backlight LED; Bleeder circuit, in order to set feedback voltage threshold value, sets the output constant current value of LED drive circuit by the sampling resistor value of mating LED drive circuit; LED drive circuit, according to the adjustment input of Microprocessor S3C44B0X instruction and differential feedback circuit, drives backlight LED with constant electric current, to realize when mains fluctuations, keeping the constant of LCD display brightness.This circuit, compared with traditional voltage-type LCD backlight drive circuit, has that backlight illumination is stable, LED color shift is little, control accuracy advantages of higher.And compared with the constant-current drive circuit of integrated chip, its application is more flexible, and cost advantage is obvious.
Description
Technical field
The invention belongs to constant current driver circuit for LED, especially relate to a kind of LCD backlight constant current drive circuit.
Background technology
At present, the LCD backlight drive circuit of mainstream automotive instrument roughly has two classes: a class is voltage type backlight drive circuit, and another kind of is constant current backlight drive circuit.Wherein, voltage type backlight drive circuit structure is simple, directly drive a string or many strings backlight LED by triode or Darlington transistor, backlight electric power is directly connected with automobile storage battery power supply, the base stage of triode or Darlington transistor is controlled by pwm signal, and then the switch control rule realized the driving circuit of LED, then pass through frequency and the dutycycle of reasonable set PWM, just can realize the effect that human eye is difficult to " the constant backlight brightness " of differentiating.But the effect of this " constant backlight brightness " is hypothetical, and that is exactly that generator output voltage is constant.If generator output voltage has fluctuation, the electric current flowing through LED will be caused to change thereupon, finally cause backlight illumination to fluctuate or flicker.And this phenomenon there will not be in the instrument of current constant control LCD backlight.Except generator starts and has impact to battery voltage, the problem of the start and stop of some high power loads and generator self rectification circuit can cause battery voltage to fluctuate equally, thus causes similar problem.Therefore, middle-to-high end car instrument is mostly applied constant current and drives backlight circuit, therefore do not deposit the problems referred to above.
Also there is a potential defect in voltage type backlight drive circuit, that is exactly: affect comparatively significantly LED for color and predominant wavelength by forward current, as blue, very green and White LED, this kind of type of drive is cannot ensure backlight color conforming when voltage fluctuation.Therefore, for instrument that is blue, very green and white backlight, for ensureing backlight effect, it is necessary that constant current drives, therefore usually more expensive than the instrument of other backlight colors.
For constant current backlight drive circuit, scheme more common in the market utilizes integrated chip, realizes current constant control at some peripheral circuits of coupling.The great advantage of this scheme is applied simply exactly, current stabilization, accurately, because its Current adjustment and feedback circuit mostly realize at chip internal.But its shortcoming is also very outstanding; High expensive is generally tens times of voltage type backlight circuit.For the low and middle-end instrument that Cost Competition is more and more fierce, the current constant control scheme of application integration chip is difficult to obtain living space.This is also why constant current backlight drive circuit is only just comparatively common in high-end instrument.
Summary of the invention
Technical matters to be solved by this invention is to provide a kind of LCD backlight constant current drive circuit, and based on triode difference negative-feedback circuit, to meet when car load battery voltage fluctuates, LCD still can realize good, stable display effect.
In order to solve the problems of the technologies described above, the present invention adopts following technical scheme:
A kind of LCD backlight constant current drive circuit, comprises power supply, pwm control circuit and LED drive circuit, also comprises bleeder circuit and differential feedback circuit, wherein, pwm control circuit is connected with bleeder circuit, and controls the time of backlight LED opening and closing, and then controls the brightness of backlight LED; Bleeder circuit is connected with differential feedback circuit, in order to set feedback voltage threshold value, is set the output constant current value of LED drive circuit by the sampling resistor value of mating LED drive circuit; Differential feedback circuit is connected with LED drive circuit, by dynamic adjustments to realize the pressure drop constant on sampling resistor, and then controls LED drive circuit output constant current; LED drive circuit is connected with power supply, according to the adjustment input of Microprocessor S3C44B0X instruction and differential feedback circuit, drives backlight LED with constant electric current, to realize when mains fluctuations, keeping the constant of LCD display brightness.
Described differential feedback circuit comprises a pair triode Q2, Q3 and resistance R3, triode Q2, Q3 connect in differential amplifier circuit mode, the base stage of triode Q2 is connected with bleeder circuit, the base stage of triode Q3 is connected with LED drive circuit, and the emitter of triode Q2, Q3 is connected and is connected with resistance R3.
Described differential feedback circuit also comprises triode Q1 and resistance R9, and the collector of triode Q1 is connected with resistance R9 and LED drive circuit respectively, and resistance R9 other end connecting to neutral, the base stage of triode Q1 is connected to the collector of triode Q2.
Described bleeder circuit comprises resistance R1, R2 and electric capacity C1, and the output terminal of pwm control circuit is in parallel with resistance R2, electric capacity C1 after connecting with resistance R1 respectively.
The output terminal of described power supply is also connected with low-pass filter circuit.
Described each several part circuit is built by discrete element device.
Adopt LCD backlight constant current drive circuit of the present invention, tool has the following advantages:
When 1, can guarantee mains fluctuations, backlight illumination is constant.
2, can ensure that blueness, very green and White LED predominant wavelength and color do not offset, be controlled by PWM, realize brightness adjustable.
3, compared with traditional integrated chip type constant-current drive circuit, there is flexible configuration, regulate simply, the advantages such as low price.
Accompanying drawing explanation
Below in conjunction with the drawings and specific embodiments, the present invention is described in detail:
Fig. 1 is theory diagram of the present invention.
Fig. 2 is circuit diagram of the present invention.
Embodiment
LCD backlight constant current drive circuit of the present invention as shown in Figure 1, comprise power supply, pwm control circuit (pulse-width modulator) and LED drive circuit, also comprise bleeder circuit and differential feedback circuit, wherein, pwm control circuit is connected with bleeder circuit, and control the time of backlight LED opening and closing, and then control the brightness of backlight LED; Bleeder circuit is connected with differential feedback circuit, in order to set feedback voltage threshold value, set the output constant current value of LED drive circuit by the sampling resistor value of mating LED drive circuit, therefore need to adopt precision higher, the precision resistance of good stability does dividing potential drop and feedback resistance; Differential feedback circuit is connected with LED drive circuit, by dynamic adjustments to realize the pressure drop constant on sampling resistor, and then reaches the object flowing through its current constant, namely controls LED drive circuit output constant current; LED drive circuit is connected with power supply, according to the adjustment input of Microprocessor S3C44B0X instruction and differential feedback circuit, drives backlight LED with constant electric current, to realize when mains fluctuations, keeping the constant of LCD display brightness.
The physical circuit figure of LCD backlight constant current drive circuit of the present invention as shown in Figure 2, described differential feedback circuit comprises a pair triode Q2, Q3 and resistance R3, triode Q2, Q3 connect in differential amplifier circuit mode, the base stage of triode Q2 is connected with bleeder circuit, the base stage of triode Q3 is connected with LED drive circuit, and the emitter of triode Q2, Q3 is connected and is connected with resistance R3.Described differential feedback circuit also comprises triode Q1 and resistance R9, and the collector of triode Q1 is connected with resistance R9 and connecting to neutral, and the base stage of triode Q1 is connected to the collector of triode Q2.Described bleeder circuit comprises resistance R1, R2 and electric capacity C1, and the output terminal of pwm control circuit is in parallel with resistance R2, electric capacity C1 after connecting with resistance R1 respectively.The output terminal of described power supply is also connected with low-pass filter circuit, forms primarily of electric capacity C2, C3 and resistance R12.Described LED drive circuit is primarily of the formation such as diode D1-D7, triode Q4 ~ Q7, resistance R4 ~ R6, R6b, R10.
Above-mentioned triode Q2, Q3 form differential input amplifying circuit to pipe, and be load with R3, the object of this stage circuit is the pressure drop at steady resistance R4 two ends, namely Vbase, makes it be approximately equal to Vref.And the size of Vbase is directly proportional to the electric current flowing through LED, therefore reasonable set Vref select sampling resistor R4, just can determine the current value flowing through LED.The value of advising Vref is here sufficiently large, to such an extent as to the Parameters variation caused due to mains fluctuations and temperature variation can be left in the basket.
Vref is determined by the output of the pwm control circuit of microcontroller, and the ON time of the direct control LED of pwm signal, therefore the brightness of LED is directly associated with the dutycycle of pwm signal.
The size of the bias current that Differential Input is right is determined by Vref.In fact because the voltage drop at resistance R3 two ends is definite values, so R3 can be considered to the constant current source absorbing steady current.
Therefore, if the emitter current approximately equal of two triodes Q2, Q3, so its base voltage is also with regard to approximately equal, and then the electric current flowing through LED is just directly proportional to input voltage.
What we can be similar to thinks that the Vbe of triode Q1 is invariable, the current value so flowing through resistance R8 is also definite value (only relevant with resistance R8 size), this electric current flows into the collector of triode Q2 as bias current, and in order to make Differential Input to being in equilibrium state, the size of this electric current should be the half flowing through resistance R3 electric current.
Steady state condition: the base current of triode Q1 is sufficiently little, to such an extent as to negligible compared with R8 electric current, and we can think thus: VbeQ2=VbeQ3, IbQ2=IbQ3, IeQ2=IeQ3=Ip/2 (Ip is for flowing through resistance R3 electric current).
Dynamic duty principle of the present invention is as follows: the emitter current of triode Q3 is directly proportional to the forward current of light emitting diode D1-D7.If for a certain reason, cause light emitting diode forward current to fluctuate, so this current fluctuation can cause Vbe, and triode Q3 changes thereupon, and then causes the respective change of collector current of Q3.Because Vref is constant, so the electric current flowing through resistance R3 is also steady state value.Like this, the collector current change being changed the triode Q3 caused by light emitting diode forward current is just converted into the curent change of the collector opposite trend of triode Q2.Again because flow through the current constant of resistance R8, therefore the change of the collector current of triode Q2 is approximately the base current variable quantity of triode Q1, and this curent change directly has influence on again the forward current of light emitting diode D1-D7.Thus, we establish the negative feedback of light emitting diode forward current, make it be tending towards steady state value, i.e. constant current source.Such as: suppose that the electric current flowing through resistance R4 reduces, this will cause Vbase to diminish, and so the collector current of triode Q3 also can correspondingly diminish.And Ip is invariable, this will cause the collector current of triode Q2 to increase, namely the base current of triode Q1 increases (resistance R8 current constant), thus cause the collector current of triode Q1 to increase, the collector current (LED forward current) of triode Q4 increases thereupon, so resistance R4 current value comes back to steady state (SS), the constant current value namely set.Equally, the rising of resistance R4 electric current can cause the change contrary with above-mentioned trend, still causes resistance R4 to get back to steady state (SS), be tending towards constant current eventually through feedback.
It should be noted that at this, this circuit is applicable to the application that maximum drive current is less than 750mA, because only under this prerequisite, the base current of Q1 just can be similar to relative to the constant current source of difference channel ignores.And the drive current of 750mA has reached the ultimate value of triode Q1 power dissipation, so can not by this circuit application in the occasion needing more large-drive-current.
Circuit parameter of the present invention is determined as follows:
Diode D8, the effect of this diode prevents the transient state negative voltage on reverse power connection or power lead from causing damage to circuit.Therefore, the circuit little for current drain generally can choose S1G or BYG60G(VF=1.1VI=500mA, 25 ° of C).
Resistance R12 and electric capacity C2, C3, these devices constitute low-pass filter circuit, in order to the undesired signal of other network launches in suppressing plate, avoid it to impact backlight circuit.
Triode Q2 and Q3, consider from cost and function aspects, conventional BC817 just can meet application demand.But whether want its power dissipation of re-computation to meet the demands, because in normal operation, the pressure drop on Q3 will be maximum.
Resistance R3, in order to set the bias current of difference channel.The selection of this current value will from the viewpoint of two: will try one's best greatly on the one hand, so just can make the injection of Q1 base current, be unlikely to the quiescent point affecting differential pair; On the other hand again can not be too large, because its power dissipation that will directly affect on triode Q2 and Q3.
Resistance R8, in order to set the bias current of triode Q2.
Resistance R1, R2, these two resistor group become bleeder circuit, and in order to set reference voltage Vref, it will directly affect the precision of constant-current circuit.The selection of these two resistance also will consider that the base current of the fan-out capability of microprocessor and triode Q2 is on the impact of partial pressure value.
Resistance R4, from system perspective, the selection of R4 is of paramount importance, because it will have influence on the precision of constant-current circuit, the number of driving LED and flow through the drive current of LED.
Triode Q4, in order to driving LED.Owing to being in magnifying state when Q4 normally works, two ends pressure drop is comparatively large, and therefore dissipated power will select the main factor of this triode.
Triode Q1, this triode is in order to obtain current fluctuation parameter.In fact, the uneven situation of differential pair has directly been reacted in the change of its base current, and it plays the effect of " comparer " in whole electric current.
Resistance R9, in order to provide the triode Q1 bias current of steady operation.At steady-working state, flow through the electric current of R9 compared with the electric current flowing through R7, should be negligible.
Resistance R11, the bleeder circuit being used for balancing R1 & R2 composition, on the impact of Q2 base voltage, makes the pressure drop of Q3 base stage consistent with the pressure drop of Q2 base stage.Therefore, R3=R1//R2 is got.
In sum, LCD backlight constant current drive circuit of the present invention, compared with traditional voltage type LCD backlight drive circuit, has that backlight illumination is stable, LED color shift is little, control accuracy advantages of higher.And compared with the constant-current drive circuit of integrated chip, its application is more flexible, and cost advantage is obvious.In addition, its each several part circuit all can be built by discrete element device, so while guarantee constant current drives, significantly reduces Material Cost, shows obvious advantage in some applications.
But, those of ordinary skill in the art will be appreciated that, above embodiment is only used to the present invention is described, and be not used as limitation of the invention, as long as in spirit of the present invention, all will drop in Claims scope of the present invention the change of the above embodiment, modification.
Claims (3)
1. a LCD backlight constant current drive circuit, comprise power supply, pwm control circuit and LED drive circuit, it is characterized in that: also comprise bleeder circuit and differential feedback circuit, wherein, pwm control circuit is connected with bleeder circuit, and control the time of backlight LED opening and closing, and then control the brightness of backlight LED; Bleeder circuit is connected with differential feedback circuit, in order to set feedback voltage threshold value, is set the output constant current value of LED drive circuit by the sampling resistor value of mating LED drive circuit; Differential feedback circuit is connected with LED drive circuit, by dynamic adjustments to realize the pressure drop constant on sampling resistor, and then controls LED drive circuit output constant current; LED drive circuit is connected with power supply, according to the adjustment input of Microprocessor S3C44B0X instruction and differential feedback circuit, drives backlight LED with constant electric current, to realize when mains fluctuations, keeping the constant of LCD display brightness; Described differential feedback circuit comprises a pair triode Q2, Q3 and resistance R3, triode Q2, Q3 connect in differential amplifier circuit mode, the base stage of triode Q2 is connected with bleeder circuit, the base stage of triode Q3 is connected with LED drive circuit, and the emitter of triode Q2, Q3 is connected and is connected with resistance R3;
Described differential feedback circuit also comprises triode Q1 and resistance R9, and the collector of triode Q1 is connected with resistance R9 and LED drive circuit respectively, resistance R9 other end connecting to neutral, and the base stage of triode Q1 is connected to the collector of triode Q2;
Described bleeder circuit comprises resistance R1, R2 and electric capacity C1, and the output terminal of pwm control circuit is in parallel with resistance R2, electric capacity C1 after connecting with resistance R1 respectively.
2. LCD backlight constant current drive circuit according to claim 1, is characterized in that: the output terminal of described power supply is also connected with low-pass filter circuit.
3. LCD backlight constant current drive circuit according to claim 1, is characterized in that: described each several part circuit is built by discrete element device.
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| CN201210339228.4A CN103680414B (en) | 2012-09-13 | 2012-09-13 | A kind of LCD backlight constant current drive circuit |
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| CN201210339228.4A CN103680414B (en) | 2012-09-13 | 2012-09-13 | A kind of LCD backlight constant current drive circuit |
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| CN103680414A CN103680414A (en) | 2014-03-26 |
| CN103680414B true CN103680414B (en) | 2016-01-06 |
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| CN106163033B (en) * | 2015-04-08 | 2019-04-19 | 大陆汽车车身电子系统（芜湖）有限公司 | A kind of instrument method for controlling backlight thereof |
| CN106455245B (en) * | 2015-08-10 | 2020-05-12 | 法雷奥电机设备公司 | Protection circuit for driving circuit |
| CN105957475A (en) * | 2015-12-27 | 2016-09-21 | 天津市品通电力科技有限公司 | Display screen control circuit of electric power instrument |
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| CN111883068A (en) * | 2020-06-18 | 2020-11-03 | 惠州市德赛西威汽车电子股份有限公司 | LCD screen constant current drive circuit that is shaded |
| CN113258922B (en) * | 2021-06-16 | 2021-10-08 | 上海芯龙半导体技术股份有限公司南京分公司 | Drive circuit and power supply system |
| CN116321580B (en) * | 2023-05-25 | 2023-08-29 | 杭州海康威视数字技术股份有限公司 | LED driving device, method and equipment |
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