CN102111932B

CN102111932B - A Light Emitting Diode Current Balance Circuit

Info

Publication number: CN102111932B
Application number: CN 200910261687
Authority: CN
Inventors: 林立韦; 李振强; 李吉欣; 黄元伯; 林文明
Original assignee: Top Victory Investments Ltd
Current assignee: Top Victory Investments Ltd
Priority date: 2009-12-24
Filing date: 2009-12-24
Publication date: 2013-10-30
Anticipated expiration: 2029-12-24
Also published as: CN102111932A

Abstract

The invention discloses a current balancing circuit of a light emitting diode, and belongs to the technical field of light emitting diode driving. The light emitting diode current balancing circuit comprises a reference current generator, a current mirror and a voltage compensation circuit. The reference current generator provides a reference current with robust power voltage disturbance resistance for the current mirror to generate a current to be drawn to each string, and the structure adopted by the current mirror is not influenced by the gain ratio of the transistor, so that the current mirror can stably clamp the current flowing through each string. The voltage compensation circuit detects the voltage of each lamp string end point so as to compensate the forward conduction voltage drop of each lamp string, thereby ensuring that each lamp string with different forward conduction voltage drops can be lightened, and ensuring that the current flowing through each lamp string is more balanced. In addition, the invention does not need to adopt a special LED controller, and has quite low cost and competitive power.

Description

A kind of current balance circuit for LED

Technical field

The present invention relates to the light-emitting diode Driving technique, particularly a kind of current balance circuit for LED.

Background technology

LED (Light-Emitting Diode, light-emitting diode) ideal value that forward conduction voltage drop indicates at specifications is 3.3V, will inevitably there be error in actual forward conduction voltage drop with this ideal value of 3.3V after but LED connected circuit, if there is 5% error, then the forward conduction voltage drop of LED reality may be between 3.135V～3.465V.Therefore, even by the formed lamp string of the LED coupled in series of equal number, same type (light bar), on each lamp string, still can produce different forward conduction voltage drops, if applying identical lamp string at each lamp string presses, each lamp string will make the electric current that flows through each lamp string not identical because of the not identical cause of the forward conduction voltage drop of reality, and the brightness that provides of each lamp string is just not identical yet certainly.For the not identical current unevenness problem that flows through that causes of forward conduction voltage drop that solves lamp string reality, current balance circuit just becomes the important driving element of LED lamp string.

Fig. 1 is a kind of circuit diagram of existing single led lamp string mobile equilibrium circuit.See also Fig. 1, lamp string 11 comprises the light-emitting diode D1～Dn (n is positive integer) of a plurality of coupled in series, and the forward conduction voltage drop Vf1 of lamp string 11 is the summation of the forward conduction voltage drop of each light-emitting diode D1～Dn.The first end of lamp string 11 presses VBUS to obtain the required bias voltage of conducting by the receiving light crosstalk, and the second end of lamp string 11 is coupled to current balance circuit.Current balance circuit comprises transistor Q, detect resistance R and operational amplifier OP, wherein, detect resistance R and detect the current value that flows through lamp string 11, the inverting input of operational amplifier OP receives the detection resistance R and detects the current value (its corresponding current actual value) that flows through lamp string 11, non-inverting input receives control command Vset (its corresponding current set point), send signal to the control end of transistor Q according to both differences from output again, change pressure drop on the transistor Q with the working point of adjusting transistor Q, and then the forward conduction voltage drop Vf1 of change lamp string 11, make the current actual value convergence current setting value that flows through lamp string 11.

Fig. 2 is a kind of circuit diagram of existing a plurality of LED lamp string mobile equilibrium circuit.See also Fig. 2, when using a plurality of LED lamp string 11～1m (m is positive integer), the second end of each lamp string 1i (i is arbitrary positive integer among 1～m) all needs to be coupled to a current balance circuit as shown in Figure 1, make current setting value corresponding to current value approach control order Vset that flows through each lamp string 1i, and then the current value that flows through each lamp string 1i is equated or in the certain error scope, (namely reach current balance type).When the quantity larger (or m is larger) of lamp string 11～1m, the number of needed current balance circuit is certainly also more and more, except increasing cost and circuit area, each current balance circuit also can cause because of the error that included separately transistor Q, detection resistance R and operational amplifier OP itself exist the electric current current-sharing deleterious between each lamp string 1i.

There is in recent years the integrated circuit of the led controller 22 of many moneys special use to be devised.Led controller 22; for example with a plurality of as shown in Figure 1 the current balance circuit miniaturization and be integrated on the wafer; each current balance circuit is coupled to the second end of corresponding lamp string 1i by tunnel ends CHi; usually also can adjust lamp string by feedback end FB control DC/DC (Direct Current/Direct Current, dc-dc) transducer 21 and press the VBUS value.Although integrated circuit can reach more accurate control and less circuit area, but the reliability of integrated circuit is not as traditional circuit, and electric current and the power loss limited (being generally less than 60mA) that can bear because of integrated circuit, in the LED of high-voltage great-current lamp string is used, also need on the contrary to add transistor and other element and come balanced balanced current, for the designer, not only can not save cost, circuit more complicated and be not easy control.

Summary of the invention

The electric current and the circuit that flow through each lamp string for more stable strangulation are controlled simply easily, the invention provides a kind of current balance circuit for LED.Described technical scheme is as follows:

A kind of current balance circuit for LED is used for driving a plurality of lamp strings, and each lamp string includes the light-emitting diode of a plurality of coupled in series, and each lamp string first end all is coupled to a lamp string and presses.This current balance circuit for LED comprises:

One current mirror, a reference current generator and a voltage compensating circuit.Current mirror is used for producing a plurality of current drawn according to a reference current when opening, and each current drawn provides to the second end of a corresponding lamp string, make to flow through each lamp string stream and reach balance, and to make the current value that flows through each lamp string when closing is zero.Reference current generator comprises one first bipolar transistor, one adjustable shunt regulator, one first resistance and one second resistance, adjustable shunt regulator has cathode terminal, anode tap and reference edge, the first bipolar transistor collector terminal is coupled to the first end of a supply voltage and the first resistance, the base terminal of the first bipolar transistor is coupled to the second end of the first resistance and the cathode terminal of adjustable shunt regulator, the emitter terminal of the first bipolar transistor is coupled to the reference edge of adjustable shunt regulator and the first end of the second resistance, the anode tap of adjustable shunt regulator is coupled to a ground connection, the second end output reference current of the second resistance.Voltage compensating circuit comprises a plurality of the first diodes, one second bipolar transistor, one the 3rd resistance, one the 4th resistance, one the 5th resistance, one the 6th resistance, one the 7th resistance 1 the 8th resistance and one first electric capacity, the cathode terminal of each the first diode is coupled to the second end of a corresponding lamp string, the anode tap of each the first diode all is coupled to the first end of the 3rd resistance and the first end of the 4th resistance, the second end of the 3rd resistance is coupled to the reference edge of adjustable shunt regulator and the first end of the 5th resistance, the second end of the 4th resistance and the second end of the 5th resistance are coupled to respectively base terminal and the collector terminal of the second bipolar transistor, the first end of the 6th resistance and the second end are coupled to respectively emitter terminal and the ground connection of the second bipolar transistor, the first end of the 7th resistance is coupled to the collector terminal of the second bipolar transistor and the first end of the first electric capacity, the second end of the 7th resistance is coupled to the first end of the 8th resistance, the second end of the 8th resistance is coupled to the second end and the ground connection of the first electric capacity, the first end of the 8th resistance is exported a compensating signal, and compensating signal is used for adjusting lamp string pressure value.

Further, current mirror comprises the first transistor and a transistor seconds of a plurality of couplings, each the first transistor and transistor seconds all have first end, the second end and control end, the first end of each the first transistor is coupled to the second end of a corresponding lamp string so that a corresponding current drawn to be provided, the second end that the first end of transistor seconds is coupled to control end and is coupled to the second resistance is to receive reference current, the second end of each the first transistor and transistor seconds all is coupled to ground connection, and the control end of each the first transistor and transistor seconds is coupled to each other.

Further, each the first transistor and transistor seconds are bipolar transistor or are field-effect transistor.Further, the second end of each the first transistor and transistor seconds all is coupled to ground connection by a corresponding resistance.

Further, current mirror is not opened when the control end of transistor seconds is coupled to ground connection, and closes when the control end of transistor seconds is coupled to ground connection.

Further, current balance circuit for LED also comprises an over-voltage detection circuit, over-voltage detection circuit is coupled to the second end of each lamp string, be used for exporting when the second terminal voltage that detects arbitrary lamp string surpasses an overvoltage threshold rub-out signal, it is zero that rub-out signal is used for making lamp string pressure value.

Further, over-voltage detection circuit comprises a plurality of the second diodes, one Zener diode, one the 9th resistance, 1 the tenth resistance and one second electric capacity, the anode tap of each the second diode is coupled to the second end of a corresponding lamp string, the cathode terminal of each the second diode all is coupled to the cathode terminal of Zener diode, the anode tap of Zener diode is coupled to the first end of the 9th resistance, the second end of the 9th resistance is coupled to the first end of the tenth resistance and the first end of the second electric capacity, the second end of the tenth resistance is coupled to the second end and the ground connection of the second electric capacity, the first end output error signal of the tenth resistance.

Further, at supply voltage value during less than a constant current threshold value, the cut-off of the first bipolar transistor, the reference current value of reference current generator output determined by supply voltage value, by variable supply voltage as dim signal to realize simulating light modulation.

Further, at supply voltage value during greater than a constant current threshold value, the first bipolar transistor conducting and the normal operation of adjustable shunt regulator, the reference current of reference current generator output constant current, current balance circuit for LED also comprises a light adjusting circuit, light adjusting circuit is coupled to reference current generator and/or current mirror, be used for receiving a pulse-width signal and according to pulse-width signal alternately opening and closing reference current generator and/or current mirror from a light modulation termination, by with the pulse-width signal of pulse width variability as dim signal to realize digital dimming.Here, " reference current generator and/or current mirror " expression " reference current generator ", " current mirror ", " reference current generator and current mirror " three select one.

Further, supply voltage is coupled to the light modulation end instead, in the duty ratio of pulse-width signal during greater than a duty cycle threshold, pulse-width signal level during the high level activation is constant voltage and greater than the constant current threshold value, and in the duty ratio of pulse-width signal during less than duty cycle threshold, pulse-width signal level during the high level activation is variable and less than the constant current threshold value.

The present invention can provide the reference current with strong anti-supply voltage disturbance characteristic to produce current drawn to each lamp string for current mirror because of the reference current generator that adopts, and the framework that current mirror adopts is not affected by the transistor gain ratio, therefore current mirror can be stablized the electric current that each lamp string is flow through in strangulation, and add voltage compensating circuit and detect the voltage of each lamp string end points in order to the forward conduction voltage drop of each lamp string is compensated, different forward each lamp string of conduction voltage drop all can be lighted except guaranteeing to have, also can make the electric current balance more that flows through each lamp string, and because not adopting special-purpose led controller and cost quite cheap and have more competitiveness.

Description of drawings

Fig. 1 is the circuit diagram of a kind of single led lamp string mobile equilibrium circuit that provides of prior art;

Fig. 2 is the circuit diagram of a kind of a plurality of LED lamp string mobile equilibrium circuit of providing of prior art;

Fig. 3 is the block diagram of a kind of LED current balance circuit of providing of the embodiment of the invention 1;

Fig. 4 is the circuit diagram of a kind of LED current balance circuit of providing of the embodiment of the invention 1;

Fig. 5 is component symbol and the functional schematic of the adjustable shunt regulator shown in Figure 4 that provides of the embodiment of the invention 1;

Fig. 6 A is the signal mode graphoid of the first LED current balance circuit shown in Figure 4 of providing of the embodiment of the invention 1;

Fig. 6 B is the signal mode graphoid of the second LED current balance circuit shown in Figure 4 of providing of the embodiment of the invention 1;

Fig. 7 is the actual signal survey map of the LED current balance circuit shown in Figure 4 that provides of the embodiment of the invention 1.

In the accompanying drawing, the list of parts of each label representative is as follows:

11～1m: the lamp string, the 21:DC/DC transducer, the 22:LED controller,

CH1～CHm: tunnel ends, FB: feedback end, the 31:DC/DC transducer,

32: reference current generator, 33: current mirror, 34: voltage compensating circuit,

35: over-voltage detection circuit, 36: light adjusting circuit, DIM: the light modulation end, C1: the first electric capacity,

C2: the second electric capacity, D1～Dn: light-emitting diode, D11～D1m: the first diode,

D21～D2m: the second diode, OP: operational amplifier, Q: transistor,

Q11～Q1m: the first transistor, Q22: transistor seconds, Q1: the first bipolar transistor,

Q2: the second bipolar transistor, Q3～Q6: transistor switch, R: detect resistance,

R1～R10: the first resistance～the tenth resistance, R11～R1m, R22: degeneration resistance,

R31～R34: current-limiting resistance, TL1: adjustable shunt regulator, ZD1: Zener diode,

I1～Im: current drawn, Iref: reference current, VIN: input voltage, VBUS: lamp string is pressed,

VCC, VEE: supply voltage, VREF: reference voltage, Vcomp: compensating signal,

Vf1～Vfm: lamp string conduction voltage drop, Vfault: rub-out signal, Von-off: power switch signal,

Vpwm: pulse-width modulation (PWM) signal, Vset: control command.

Embodiment

For making the purpose, technical solutions and advantages of the present invention clearer, embodiment of the present invention is described further in detail below in conjunction with accompanying drawing.

Embodiment 1

At first need to prove, those skilled in the art know bipolar transistor (Bipolar JunctionTransistor, abbreviation BJT) has first end (being collector terminal), the second end (being emitter terminal) and control end (being base terminal), field-effect transistor (Field-Effect Transistor, abbreviation FET) has first end (being drain electrode end), the second end (being source terminal) and control end (being gate terminal), resistance (resistor) and electric capacity (capacitor) all have first end and the second end, diode, light-emitting diode and Zener diode (Zener diode) all have anode tap and cathode terminal, below repeat no more.

Fig. 3 and Fig. 4 are respectively block diagram and the circuit diagram of the LED current balance circuit that the embodiment of the invention provides.Please be simultaneously referring to Fig. 3 and Fig. 4, the LED current balance circuit is used for driving a plurality of lamp string 11～1m, and each lamp string 1i includes the light-emitting diode D1～Dn of a plurality of coupled in series, and wherein m, n are positive integer, and i is arbitrary positive integer among 1～m.Each lamp string 1i all has first end and the second end, and the first end of each lamp string 1i all presses VBUS to obtain the required bias voltage of conducting by being coupled to lamp string.Those skilled in the art know in order to allow these lamp strings 11～1m be worked, in each lamp string 1i, the anode tap of light-emitting diode D1 should be coupled to the first end of lamp string 1i, the cathode terminal of light-emitting diode Dk should be coupled to the anode tap of light-emitting diode Dk+1, and the cathode terminal of light-emitting diode Dn should be coupled to the second end of lamp string 1i, wherein, k is arbitrary positive integer in 1～(n-1).The led light source that these lamp strings 11～1m forms can be used the backlight as liquid crystal display, for example straight-down negative or lateral incident type backlight source.

The LED current balance circuit comprises DC/DC transducer 31, reference current generator 32, current mirror 33, voltage compensating circuit 34, over-voltage detection circuit 35 and light adjusting circuit 36.

DC/DC transducer 31 is the buck or boost transducer, is used for providing the DC input voitage VIN of common 5V, 12V or 24V equal-specification to be converted to the direct current lamp string power supply unit (Fig. 3 and Fig. 4 all do not draw) and presses VBUS to drive lamp string 11～1m.DC/DC transducer 31 also receives power switch signal Von-off, rub-out signal Vfault and compensating signal Vcomp.Wherein, power switch signal Von-off for example can make DC/DC transducer 31 press VBUS to supply with lamp string 11～1m and produce supply voltage VCC for generation of lamp string when high level and supply with LED current balance circuit internal circuit, and DC/DC transducer 31 is no longer powered.

Reference current generator 32 comprises the first bipolar transistor Q1, adjustable shunt regulator (programmable shunt regulator) TL1, the first resistance R 1 and the second resistance R 2.Wherein, the TL431 that the integrated circuit of modal adjustable shunt regulator TL1 has Texas Instruments to produce, its component symbol has cathode terminal, anode tap and reference edge shown in the left figure of Fig. 5, and its functional schematic is shown in the right figure of Fig. 5.Can see from the right figure of Fig. 5, adjustable shunt regulator TL1 inside has a constant pressure source provides the inverting input of the reference voltage VREF of fixed value 2.5V to operational amplifier OP, the non-inverting input of operational amplifier OP is coupled to reference edge, only at reference voltage terminal during very near reference voltage VREF, transistor Q just has a stable unsaturation electric current and flows through, and along with the minor variations of reference voltage terminal, the electric current that flows through transistor Q will change at 1A～100mA.

In reference current generator 32, the collector terminal of the first bipolar transistor Q1 is coupled to the first end of DC power supply voltage VCC and the first resistance R 1, the base terminal of the first bipolar transistor Q1 is coupled to the second end of the first resistance R 1 and the cathode terminal of adjustable shunt regulator TL1, the emitter terminal of the first bipolar transistor Q1 is coupled to the reference edge of adjustable shunt regulator TL1 and the first end of the second resistance R 2, the anode tap of adjustable shunt regulator TL1 is coupled to ground connection, the second end output reference current Iref of the second resistance R 2.

In supply voltage VCC value during greater than the constant current threshold value, the first bipolar transistor Q1 conducting and adjustable shunt regulator TL1 normal operation, adjustable shunt regulator TL1 is almost constant voltage and can be used as power supply to the DC power supply voltage VEE of voltage compensating circuit 34 at the voltage of reference edge (or first end of the second resistance R 2), so reference current generator 32 also can arrange by the resistance value that the second resistance R 2 is set reference current Iref value by supply voltage VCC power supply.After the resistance value of the second resistance R 2 was determined, reference current Iref value also was confirmed as constant current, so the voltage (or second terminal voltage of the second resistance R 2) of reference current generator 32 outputs also is confirmed as constant voltage.The constant voltage of reference current generator 32, the output of constant current can make current mirror 33 more stably work.If adjustable shunt regulator TL1 is take integrated circuit TL431 as example, collector terminal is to the about 1V of emitter terminal pressure drop during the first bipolar transistor Q1 conducting, and the voltage of the reference edge of adjustable shunt regulator TL1 is about 2.5V, so the constant current threshold value is about 3.5V.In addition, supply voltage VCC value must have a upper limit, and electric current and power loss that this upper limit can be born during mainly by the first bipolar transistor Q1 conducting are determined.

Current mirror 33 is coupled to the second end of reference current generator 32 and each lamp string 1i, be used for when opening according to reference current Iref produce a plurality of current drawn I1～

, each current drawn Ii provides to the second end of corresponding lamp string 1i, makes the electric current that flows through each lamp string 1i reach balance, and to make the current value that flows through each lamp string 1i when closing be zero.

In the present embodiment, current mirror 33 comprises the first transistor Q11～Q1m and the transistor seconds Q22 of a plurality of couplings, and each the first transistor Q1i and transistor seconds Q22 are the NPN bipolar transistor and have first end (being collector terminal), the second end (being emitter terminal) and control end (being base terminal); But be not limited only to this, for example each the first transistor Q1i and transistor seconds Q22 also can be the N channel fet and have first end (being drain electrode end), the second end (being source terminal) and control end (being gate terminal).The first end of each the first transistor Q1i is coupled to the second end of corresponding lamp string 1i so that corresponding current drawn Ii to be provided, the first end that the first end of transistor seconds Q22 is coupled to control end and connects into diode transistors (diode-connectedtransistor) and transistor seconds Q22 also is coupled to second end of the second resistance R 2 of reference current generator 32 to receive reference current Iref, the second end of each the first transistor Q1i and transistor seconds Q22 all is coupled to ground connection, and the control end of each the first transistor Q1i and transistor seconds Q22 is coupled to each other.The cause that current mirror 33 mates owing to the first transistor Q11～Q1m when opening, the current drawn I1 of inflow the first transistor Q11～Q1m～Im value equates or (namely reach current balance type) in the certain error scope, force the electric current that flows through each lamp string 1i to reach equally balance, make lamp string 11～1m that uniform brightness is provided.

In the present embodiment, the second end of each the first transistor Q1i also is coupled to ground connection by corresponding resistance R 1i, the second end of transistor seconds Q22 also is coupled to ground connection by corresponding resistance R 22, resistance R 11～R1m can make that the first transistor Q11～Q1m produces current drawn I1～Im is not affected than unmatched by transistor gain, so resistance R 11～R1m is called as degeneration resistance (degeneration resistor).In addition, be coupled to ground connection by the control end with transistor seconds Q22, transistor seconds Q22 and each the first transistor Q1i are all ended, reach the purpose of close current mirror 33; Be not coupled to ground connection and work as transistor seconds Q22 control end, transistor seconds Q22 and each the first transistor Q1i are all worked, reach the purpose of firing current mirror 33.

Voltage compensating circuit 34 comprises a plurality of first diode D11～D1m, the second bipolar transistor Q2, the 3rd resistance to the eight resistance R, 3～R8 and the first capacitor C 1, the cathode terminal of each the first diode D1i is coupled to the second end of corresponding lamp string 1i, the anode tap of each the first diode D1i all is coupled to the first end of the 3rd resistance R 3 and the first end of the 4th resistance R 4, the reference edge that the second end of the 3rd resistance R 3 is coupled to adjustable shunt regulator TL1 is used for receiving supply voltage VEE, and the second end of the 3rd resistance R 3 also is coupled to the first end of the 5th resistance R 5, the second end of the second end of the 4th resistance R 4 and the 5th resistance R 5 is coupled to respectively base terminal and the collector terminal of the second bipolar transistor Q2, the first end of the 6th resistance R 6 and the second end are coupled to respectively emitter terminal and the ground connection of the second bipolar transistor Q2, the first end of the 7th resistance R 7 is coupled to the first end of collector terminal and the first capacitor C 1 of the second bipolar transistor Q2, the second end of the 7th resistance R 7 is coupled to the first end of the 8th resistance R 8, the second end of the 8th resistance R 8 is coupled to the second end and the ground connection of the first capacitor C 1, the first end output compensating signal Vcomp of the 8th resistance R 8, compensating signal Vcomp is used for adjusting lamp string and presses the VBUS value.

Suppose that each lamp string 1i includes 13 light-emitting diode D1～D13, ideally lamp string 1i forward conduction voltage drop be 42.9V, and suppose this moment the first transistor Q1i of current mirror 33 and the pressure drop of resistance R 1i be 1V, so ideally lamp string presses VBUS to should be 43.9V and lamp string the second terminal voltage should be 1V.But, in fact each lamp string 11～1m has forward conduction voltage drop of difference, exist part lamp string (such as lamp string 11) to be slightly larger than 42.9V, also exist part lamp string (such as lamp string 12) to be slightly less than 42.9V, this moment is in order all can to light each lamp string 11～1m, the lamp string pressure VBUS that applies when lighting a lamp must be slightly larger than 43.9V is lighted lamp string 11, but but cause the second terminal voltage of lamp string 12 too high like this, make in the current mirror 33 corresponding the first transistor Q11 and resistance R 11 pressure drops too high, be under the current balance type at lamp string 11～1m, the first transistor Q11 in the current mirror 33 and resistance R 11 can produce more power losss and shorten useful life, but so the embodiment of the invention adjust by voltage compensating circuit 34 that lamp strings are pressed VBUS values so that in the power loss of guaranteeing the equal ignition condition decline low current mirror 33 of each lamp string.

Because ideally the second terminal voltage of lamp string is 1V, and suppose the first diode D1i forward conduction voltage drop be 0.7V, therefore ideally the anode tap voltage of the first diode D1i is 1.7V, when the anode tap voltage of the first diode D1i during greater than 1.7V, the second bipolar transistor Q2 conducting, force the voltage drop that is stored on the capacitor C 1, the pressure drop of capacitor C 1 by resistance R 7 and R8 dividing potential drop with output compensating signal Vcomp; When the anode tap voltage of the first diode D1i during less than 1.7V, the second bipolar transistor Q2 cut-off, supply voltage VEE directly by resistance R 5, R7 and R8 dividing potential drop with output compensating signal Vcomp.Ideally compensating signal Vcomp magnitude of voltage is about 1V, and when compensating signal Vcomp magnitude of voltage during less than 1V, DC/DC transducer 31 is turned down the lamp string of output and pressed the VBUS value; When compensating signal Vcomp magnitude of voltage during greater than 1V, DC/DC transducer 31 is heightened the lamp string of output and is pressed the VBUS value.Because supply voltage VCC value is greater than the constant current threshold value, supply voltage VEE is constant voltage, suppose the second bipolar transistor Q2 conducting, this moment, the anode tap voltage of the first diode D1i was (VEE-Vbe2) * [R4+ (1+ β) * R6]/[R3+R4+ (1+ β) * R6], wherein, base terminal was to the emitter terminal pressure drop when Vbe2 was the second bipolar transistor Q2 conducting, and β is the second bipolar transistor Q2 cascode current gain.Therefore, the anode tap voltage of the first diode D1i can be set by the resistance value that resistance R 3, R4 and R6 are set, for example greater than 1.7V the time, make the second bipolar transistor Q2 conducting, and less than 1.7V the time, make the second bipolar transistor Q2 cut-off.

Over-voltage detection circuit 35 is coupled to the second end of each lamp string 1i, is used for output error signal Vault when the second terminal voltage that detects arbitrary lamp string surpasses overvoltage threshold, and it is zero that rub-out signal Vault is used for making lamp string to press the VBUS value.

In the present embodiment, over-voltage detection circuit 35 comprises a plurality of second diode D21～D2m, Zener diode ZD1, the 9th resistance R 9, the tenth resistance R 10 and the second capacitor C 2.The anode tap of each the second diode D2i is coupled to the second end of corresponding lamp string 1i, the cathode terminal of each the second diode D2i all is coupled to the cathode terminal of Zener diode ZD1, the anode tap of Zener diode ZD1 is coupled to the first end of the 9th resistance R 9, the second end of the 9th resistance R 9 is coupled to the first end of the tenth resistance R 10 and the first end of the second capacitor C 2, the second end of the tenth resistance R 10 is coupled to the second end and the ground connection of the second capacitor C 2, the first end output error signal Vfault of the tenth resistance R 10.When the second terminal voltage of arbitrary lamp string (such as lamp string 11) among lamp string 11～1m surpasses overvoltage threshold and so that during Zener diode ZD1 collapse, the pressure drop that the second terminal voltage of lamp string 11 deducts behind diode D21 forward voltage drop and the Zener diode ZD1 breakdown voltage will drop on the 9th resistance R 9 and the tenth resistance R 10, setting make the tenth resistance R 10 minutes voltage (i.e. the first end voltage of the tenth resistance R 10) be high level, can represent output error signal Vfault.When the second terminal voltage of arbitrary lamp string among lamp string 11～1m can't not make Zener diode ZD1 collapse above overvoltage threshold, to there be drop of pressure in the 9th resistance R 9 and the tenth resistance R 10, the tenth resistance R 10 minutes voltage be zero or low level, can represent not output error signal Vfault.In addition, can design by the Zener diode with different breakdown voltages different overvoltage thresholds, ability output error signal Vfault when what light-emitting diodes tube short circuits are arranged in the lamp string namely is set.

In supply voltage VCC value during greater than the constant current threshold value, the reference current Iref of reference current generator 32 output constant currents, so that current mirror 33 according to reference current Iref produce current drawn I1～Im is constant current, therefore will force the electric current that flows through lamp string 11～1m when current mirror 33 is opened is constant current, be that lamp string 11～1m provides changeless brightness, need this moment to adopt digital dimming to realize adjusting the function of led light source brightness.Digital dimming namely is by opening and closing current mirror 33 alternately, make lamp string 11～1m luminous (bright) once not luminous (secretly) once alternately, if bright dark switching frequency is more than 100Hz, human eye will be because of the imperceptible bright dark variation of the impact of the persistence of vision, can only feel the mean value of this variation, namely human eye can only be experienced mean flow rate and this mean flow rate is directly proportional with bright dark ratio.Therefore, by the time scale of adjustment current mirror 33 opening and closing, and then adjust the bright dark ratio of lamp string 11～1m, can realize adjusting in the digital dimming mode function of led light source brightness.

In the present embodiment, by light adjusting circuit 36 opening and closing current mirror 33 alternately.Light adjusting circuit 36 is coupled to reference current generator 32 and/or current mirror 33, receive PWM (Pulse-Width Modulation from light modulation end DIM, pulse-width modulation) signal Vpwm, and according to pwm signal Vpwm alternately opening and closing reference current generator 32 and/or current mirror 33, and adjust the time scale of current mirror 33 opening and closing by the pulsewidth (or duty ratio) of adjusting pwm signal Vpwm.Therefore, namely be pwm signal Vpwm by pulse width variability as dim signal input light adjusting circuit 36, with by light adjusting circuit 36 alternately opening and closing current mirror 33 realize the digital dimming function.

In the present embodiment, light adjusting circuit 36 comprises transistor switch Q3～Q6 and current-limiting resistance R31～R34.When pwm signal Vpwm is low level, transistor switch Q5 cut-off, Q6 conducting, the transistor Q11～Q1m of current mirror 33 and Q22 base terminal all are coupled to ground connection and end, and no longer produce current drawn I11～I1m, and expression current mirror 33 is closed.When pwm signal Vpwm is high level, transistor switch Q5 conducting, Q6 cut-off, light adjusting circuit 36 does not affect the running of current mirror 33, and expression current mirror 33 is unlocked.In addition, when power switch signal Von-off is low level, transistor switch Q3 cut-off, Q4 conducting, the base terminal of the first bipolar transistor Q1 of reference current generator 32 is coupled to ground connection and ends, and the cathode terminal of adjustable shunt regulator TL1 and anode tap all are coupled to ground connection and make reference current Iref and supply voltage VEE be zero, and expression reference current generator 32 is closed.When power switch signal Von-off is high level, transistor switch Q3 conducting, Q4 cut-off, light adjusting circuit 36 does not affect the running of reference current generator 32, and expression reference current generator 32 is unlocked.

Fig. 6 A and Fig. 6 B are the signal mode graphoid of LED current balance circuit shown in Figure 4, and wherein to drive 6 lamp strings 11～16 and the electric current I 1～I6 that flows through lamp string 11～16 is set be 20mA to the LED current balance circuit.Please first referring to Fig. 6 A, be under 50% the condition in the duty ratio of pwm signal Vpwm, can find out from analog waveform that current drawn I1～I6 value all equates or (namely reach current balance type) in the certain error scope, the brightness that lamp string 11～16 is provided equates or in the certain error scope.Please again referring to Fig. 6 B, if existing disturbance, supply voltage VCC between 4V～15V, can find out from analog waveform that current drawn I1～I6 value all less than changing, proves that the LED current balance circuit that the embodiment of the invention provides has strong anti-electric source disturbance characteristic.

Fig. 7 is the actual signal survey map of LED current balance circuit shown in Figure 4, and wherein, the LED current balance circuit drives 6 lamp strings 11～16 and the electric current I 1～I6 that flows through lamp string 11～16 is set is 20mA.See also Fig. 7, it is the current waveform that a certain lamp string is flow through in actual measurement under 1%, 25% and 50% condition for the duty ratio at pwm signal Vpwm, and wherein the every lattice unit of abscissa is 20 μ s, and the every lattice unit of ordinate is 20mA.As seen when the pulsewidth of pwm signal Vpwm changes, the electric current that flows through the lamp string can change according to pulsewidth, and current value all can maintain under the 20mA condition in the at every turn change process, and therefore (when being equivalent to the duty ratio change) still has goodish linear effective current adjustment when pulsewidth changes.In addition, electric current I 1～I6 that lamp string 11～16 is flow through in actual measurement is respectively 19.8mA, 19.8mA, 19.9mA, 19.9mA, 20.1 and 20.0mA, so the LED current balance circuit that provides of the embodiment of the invention can provide the current error regulation near 1.5%.

For the dimming mode that the embodiment of the invention provides, above-mentioned for adopting digital dimming (or being called the PWM light modulation), yet also can adopting, the embodiment of the invention simulates light modulation (or being called the direct current light modulation).Because in supply voltage VCC value during less than the constant current threshold value, the first bipolar transistor Q1 cut-off, the reference current Iref value of reference current generator 32 outputs is determined by supply voltage VCC value, therefore can realize simulating light modulation by variable supply voltage VCC as dim signal.When adopting the simulation light modulation, the circuit that is used for digital dimming in the light adjusting circuit 36 must make it lose efficacy, and the pwm signal Vpwm that for example light modulation end DIM is received arranges the permanent high level that is.

In addition, the embodiment of the invention also can adopt numeral and simulation mixing light modulation to reach higher brightness contrast, and instead be coupled to light modulation end DIM with supply voltage VCC this moment, and receive pwm signal Vpwm by light modulation end DIM.In the duty ratio (such as 50%) of pwm signal Vpwm during greater than duty cycle threshold (such as 20%), pwm signal Vpwm level during the high level activation is constant voltage and greater than the constant current threshold value, is equivalent to only adopt digital dimming this moment.In the duty ratio (such as 10%) of pwm signal Vpwm during less than duty cycle threshold (such as 20%), pwm signal Vpwm level during the high level activation is variable and less than the constant current threshold value, and along with pwm signal Vpwm duty ratio is lower, level can be lower during the high level activation, and finer and smoother brightness contrast is arranged when being equivalent in further employing simulation light modulation under the situation that adopts digital dimming so that in low-light level.

In sum, the LED current balance circuit that the embodiment of the invention provides provides the reference current with strong anti-supply voltage disturbance characteristic to produce current drawn to each lamp string for current mirror because of the reference current generator that adopts, and the framework that current mirror adopts is not affected by the transistor gain ratio, therefore current mirror can be stablized the electric current that each lamp string is flow through in strangulation, and the adding voltage compensating circuit detects each lamp string end-point voltage, so that the forward conduction voltage drop to each lamp string compensates, different forward each lamp string of conduction voltage drop all can be lighted except guaranteeing to have, also can make the electric current balance more that flows through each lamp string, and because not adopting special-purpose led controller and cost quite cheap and have more competitiveness.

The above only is preferred embodiment of the present invention, and is in order to limit the present invention, within the spirit and principles in the present invention not all, any modification of doing, is equal to replacement, improvement etc., all should be included within protection scope of the present invention.

Claims

1. A light-emitting diode current balancing circuit for driving a plurality of light strings, each light string comprising a plurality of series-coupled light-emitting diodes, the first end of each light string being coupled to a light string voltage, It is characterized in that the LED current balance circuit includes:

A current mirror for generating a plurality of sink currents according to a reference current when turned on, each sink current is provided to the second end of a corresponding light string, so that the current flowing through each light string is balanced, and when turned off Make the current value flowing through each light string zero;

A reference current generator comprising a first bipolar transistor, an adjustable shunt regulator, a first resistor and a second resistor, the adjustable shunt regulator has a cathode terminal, an anode terminal and a reference terminal, The collector terminal of the first bipolar transistor is coupled to a power supply voltage and the first terminal of the first resistor, and the base terminal of the first bipolar transistor is coupled to the second terminal of the first resistor and the first terminal of the first resistor. The cathode terminal of the adjustable parallel voltage regulator, the emitter terminal of the first bipolar transistor is coupled to the reference terminal of the adjustable parallel voltage regulator and the first end of the second resistor, the adjustable parallel voltage regulator The anode terminal of the second resistor is coupled to a ground, and the second terminal of the second resistor outputs the reference current; and

A voltage compensation circuit, including a plurality of first diodes, a second bipolar transistor, a third resistor, a fourth resistor, a fifth resistor, a sixth resistor, a seventh resistor, and an eighth resistor resistor and a first capacitor, the cathode terminal of each first diode is coupled to the second terminal of a corresponding light string, and the anode terminal of each first diode is coupled to the first terminal of the third resistor One terminal and the first terminal of the fourth resistor, the second terminal of the third resistor is coupled to the reference terminal of the adjustable shunt regulator and the first terminal of the fifth resistor, the second terminal of the fourth resistor terminal and the second terminal of the fifth resistor are respectively coupled to the base terminal and the collector terminal of the second bipolar transistor, and the first terminal and the second terminal of the sixth resistor are respectively coupled to the second bipolar transistor The emitter terminal of the bipolar transistor and the ground, the first terminal of the seventh resistor is coupled to the collector terminal of the second bipolar transistor and the first terminal of the first capacitor, and the second terminal of the seventh resistor is coupled to connected to the first end of the eighth resistor, the second end of the eighth resistor is coupled to the second end of the first capacitor and the ground, the first end of the eighth resistor outputs a compensation signal, and the compensation signal It is used to adjust the voltage value of the light string.

2. A light-emitting diode current balancing circuit according to claim 1, wherein the current mirror comprises a plurality of matched first transistors and a second transistor, each of the first transistor and the second transistor has The first terminal, the second terminal and the control terminal, the first terminal of each first transistor is coupled to the second terminal of a corresponding light string to provide a corresponding sink current, the first terminal of the second transistor is coupled to to the control terminal and coupled to the second terminal of the second resistor to receive the reference current, the second terminals of each first transistor and the second transistor are coupled to the ground, each first transistor and the second transistor The control terminals of the two transistors are coupled to each other.

3. The LED current balancing circuit as claimed in claim 2, wherein each of the first transistor and the second transistor is a bipolar transistor or a field effect transistor.

4. The LED current balancing circuit as claimed in claim 2, wherein the second terminals of each of the first transistor and the second transistor are coupled to the ground through a corresponding resistor.

5. The light-emitting diode current balancing circuit according to claim 2, wherein the current mirror is turned on when the control terminal of the second transistor is not coupled to the ground, and the current mirror is turned on at the control terminal of the second transistor Closed when coupled to this ground.

6. The LED current balance circuit according to claim 1, further comprising an overvoltage detection circuit, the overvoltage detection circuit is coupled to the second LED of each light string. The terminal is used for outputting an error signal when it is detected that the voltage of the second terminal of any light string exceeds an overvoltage threshold, and the error signal is used to make the voltage value of the light string zero.

7. A light-emitting diode current balancing circuit as claimed in claim 6, wherein the overvoltage detection circuit comprises a plurality of second diodes, a Zener diode, a ninth resistor, a tenth resistor and a The second capacitor, the anode terminal of each second diode is coupled to the second terminal of a corresponding light string, the cathode terminal of each second diode is coupled to the cathode terminal of the Zener diode, the The anode terminal of the Zener diode is coupled to the first terminal of the ninth resistor, the second terminal of the ninth resistor is coupled to the first terminal of the tenth resistor and the first terminal of the second capacitor, and the tenth resistor The second terminal of the resistor is coupled to the second terminal of the second capacitor and the ground, and the first terminal of the tenth resistor outputs the error signal.

8. The LED current balancing circuit according to claim 1, wherein when the power supply voltage is less than a constant current threshold, the first bipolar transistor is turned off, and the reference current generator outputs the The reference current value is determined by the power supply voltage, and the variable power supply voltage is used as a dimming signal to realize analog dimming.

9. The LED current balancing circuit according to claim 1, wherein when the power supply voltage is greater than a constant current threshold, the first bipolar transistor is turned on and the adjustable shunt regulator In normal operation, the reference current generator outputs a constant current of the reference current, and the LED current balancing circuit further includes a dimming circuit, the dimming circuit is coupled to the reference current generator and/or the current mirror for Receive a pulse width modulation signal from a dimming terminal and turn on and off the reference current generator and/or the current mirror alternately according to the pulse width modulation signal, by using the pulse width modulation signal with variable pulse width as dimming signal for digital dimming.

10. A light-emitting diode current balancing circuit according to claim 9, wherein the power supply voltage is changed to be coupled to the dimming terminal, and when the duty cycle of the pulse width modulation signal is greater than a duty cycle threshold When the pulse width modulation signal is at a constant voltage and greater than the constant current threshold during the high-level enabling period, and when the duty cycle of the pulse width modulation signal is less than the duty cycle threshold, the pulse width modulation signal During the high-level enabling period, the level is variable and less than the constant current threshold.