CN102510630B - Circuit for reducing mismatch of light emitting diode (LED) constant current controllers of channels - Google Patents

Abstract

The invention discloses a circuit for reducing mismatch of light emitting diode (LED) constant current controllers of a plurality of channels. An operational amplifier of each channel comprises an amplifier module and a switching control module, which are used for controlling the mismatch of LED constant current modules of the channel, wherein the amplifier module is provided with input P-channel metal oxide semiconductor (PMOS) geminate transistors P1 and P2, current mirror load N-channel metal oxide semiconductor (NMOS) transistors N1 and N2, a second-stage amplifier input NMOS transistor N3, two constant current sources I1 and I2, and eight switches S1 to S8. When the switching control module outputs a switching signal CP of which the duty ratio is 50 percent, positive and negative input ends are continuously changed by the geminate transistors P1 and P2, and an output end of the amplifier is continuously changed by the transistors N1 and N2, so that errors which are generated between two differential geminate transistors P1 and P2 and between the current mirror load NMOS transistors N1 and N2 can be counteracted and eliminated along with continuous change of the switching signal CP during continuous change of the operating state of P1 and P2 and N1 and N2.

Description

A kind of circuit that reduces a plurality of channel LED constant-current controller mismatches

Technical field

The present invention relates to a kind of circuit that reduces a plurality of channel LED constant-current controller mismatches, be mainly used in improving brightness uniformity and the glow color uniformity of liquid crystal display or LCD TV backlight.

Background technology

LED (light-emitting diode) is used widely in illumination and field of backlights at present.Because LED than traditional incandescent lamp, fluorescent lamp energy-conservation, light quality, the life-span, reliability, response speed, environmental protection all has superiority in aspect.Therefore, LED in, in the large liquid crystal display of molded dimension or the application backlight of LCD TV, replaced gradually other light sources.

LED belongs to current device, and the brightness of LED and color are very responsive to the variation of electric current.In order to keep applying a plurality of LED lamps, be brightness and solid colour, LED is conventionally together in series and can keeps electric current consistent.Yet in, in the application backlight of large-scale LCD TV, need tens, hundreds of, even thousands of LED lamps, and the normal working voltage of LED lamp, between 3 to 3.5V, therefore drive so many LED lamps just to need hundreds of volt even to go up the voltage of kilovolt.This is to be difficult to realize.Therefore just need tens LED of a kind of every road series connection as a passage, the structure that a plurality of passages are together in parallel realizes the application of more number LED-backlit.

Each LED channels in series needs to have a constant-current controller to make current stabilization.LED constant-current controller electric current between different paths must be consistent.Namely the matching precision between each passage wants high.Yet due to the error of actual process, add the reason of this body structure of galvanostat.Between each passage, be to have certain matching error (also referred to as mismatch).So will produce luminance difference and color distortion between each passage.Current mismatch between each passage is larger, and this differentiation also can be larger.Therefore, only has the matching error reducing as much as possible between each passage.Could realize the whole screen of LCD TV and there is higher brightness uniformity and colour consistency.Due to different LED passage all connect same reference voltage Vref, all use same precision resister R, but the offset voltage of the operational amplifier between different passages is different, so offset voltage Vos becomes the main cause that affects LED current mismatch.

Summary of the invention

The object of this invention is to provide a kind of circuit that reduces a plurality of channel LED constant-current controller mismatches, each channels designs a kind of operational amplifier of eliminating inefficacy voltage, and this operational amplifier is applied in multi-channel LED constant-current control circuit.Thereby reach the object of the currents match error that reduces each LED passage.

For achieving the above object, the technical solution used in the present invention is: a kind of circuit that reduces a plurality of channel LED constant-current controller mismatches, each passage is equipped with operational amplifier and LED constant-current controller, it is characterized in that: the operational amplifier of each passage comprises that an identical amplifier module and switch control module carry out mismatch control to the LED constant flow module of this passage, wherein:

LED constant-current controller comprises LED pipe, NMOS pipe N4 and the resistance of a plurality of series connection, and the positive pole of first LED pipe meets power supply VCC, and the negative electrode of an end LED pipe connects the leakage level of NMOS pipe N4, source class connecting resistance one end of NMOS pipe N4, resistance other end ground connection;

Amplifier module comprises two PMOS pipe P1 and P2, three NMOS pipe N1, N2 and N3, two constant-current source I1 and I2, 8 switch S 1-S8, one end of switch S 1 and S2 links together and connects reference voltage V ref, one end of switch S 3 and S4 links together and connects the source class of the pipe of NMOS in constant-current controller N4, the other end of switch S 1 and S4 links together and connects the grid that PMOS manages P2, the other end of switch S 2 and S3 links together and connects the grid that PMOS manages P1, the source class of PMOS pipe P1 and PMOS pipe P2 interconnects and connects the negative pole of constant-current source I1, the positive pole of constant-current source I1 meets power vd D, the leakage level of PMOS pipe P1 connects respectively the leakage level of NMOS pipe N1, one end of one end of switch S 5 and switch S 8, the leakage level of PMOS pipe P2 connects respectively the leakage level of NMOS pipe N2, one end of one end of switch S 7 and switch S 6, source class interconnection the ground connection of NMOS pipe N1 and NMOS pipe N2, the gate interconnection of NMOS pipe N1 and NMOS pipe N2 also links together with the other end of switch S 6 and the other end of switch S 5, the other end of switch S 7 and switch S 8 all connects the grid of NMOS pipe N3, the source ground of NMOS pipe N3, the leakage level of NMOS pipe N3 connects the negative pole of constant-current source I2 and the source class of NMOS pipe N4 and as the output end vo of amplifier module, constant-current source I2 positive pole meets power vd D,

Switch control module comprises an inverter, the input connecting valve signal CP of inverter as the control signal of switch S 1, S3, S5 and S7, and the output of inverter is as the control signal of switch S 2, S4, S6 and S8.

Advantage of the present invention and showing effect: the present invention uses into a kind of novel operational amplifier that can eliminate offset voltage instead to the operational amplifier of traditional LED constant-current control circuit.Thereby can greatly reduce the matching error between different LED passage.Make very high brightness uniformity and the colour consistency of maintenance between each channel LED lamp.

Accompanying drawing explanation

Fig. 1 theory diagram of the present invention;

Fig. 2 physical circuit figure of the present invention;

Fig. 3 switch S 1, S3, S5, the equivalent circuit diagram of amplifier when S7 is operated in closure state;

Fig. 4 switch S 2, S4, S6, the equivalent circuit diagram of amplifier when S8 is operated in closure state.

Embodiment

Referring to Fig. 1,2, the present invention proposes a kind of circuit that reduces a plurality of channel LED constant-current controller mismatches, each passage is equipped with operational amplifier and LED constant-current controller, and the operational amplifier of each passage comprises that an identical amplifier module and switch control module carry out mismatch control to the LED constant flow module of this passage.

LED constant-current controller comprises LED pipe, NMOS pipe N4 and the resistance of a plurality of series connection, and the positive pole of first LED pipe meets power supply VCC, and the negative electrode of an end LED pipe connects the leakage level of NMOS pipe N4, source class connecting resistance one end of NMOS pipe N4, resistance other end ground connection.

Amplifier module comprises that input is to pipe PMOS pipe P1 and P2, current mirror load NMOS pipe N1 and N2, second level amplifier input pipe NMOS pipe N3, two constant-current source I1 and I2 and 8 switch S 1 to S8.One end of switch S 1 and S2 links together and connects reference voltage V ref, one end of switch S 3 and S4 links together and connects the source class voltage Vn (being the offset voltage Vos into operational amplifier between reference voltage V ref and Vn) of the pipe of NMOS in constant-current controller N4, the other end of switch S 1 and S4 links together and connects the grid that PMOS manages P2, the other end of switch S 2 and S3 links together and connects the grid that PMOS manages P1, the source class of PMOS pipe P1 and PMOS pipe P2 interconnects and connects the negative pole of constant-current source I1, the positive pole of constant-current source I1 meets power vd D, the leakage level of PMOS pipe P1 connects respectively the leakage level of NMOS pipe N1, one end of one end of switch S 5 and switch S 8, the leakage level of PMOS pipe P2 connects respectively the leakage level of NMOS pipe N2, one end of one end of switch S 7 and switch S 6, source class interconnection the ground connection of NMOS pipe N1 and NMOS pipe N2, the gate interconnection of NMOS pipe N1 and NMOS pipe N2 also links together with the other end of switch S 6 and the other end of switch S 5, the other end of switch S 7 and switch S 8 all connects the grid of NMOS pipe N3, the source ground of NMOS pipe N3, the leakage level of NMOS pipe N3 connects the negative pole of constant-current source I2 and the source class of NMOS pipe N4 and as the output end vo of amplifier module, constant-current source I2 positive pole meets power vd D, switch control module comprises an inverter, the input connecting valve signal CP of inverter the direct control signal as switch S 1, S3, S5 and S7, the output of inverter is as the control signal of switch S 2, S4, S6 and S8, switch S 2, S4, S6, S8 is and S1, S3, S5, the signal of S7 single spin-echo.Because size and the threshold voltage of two differential pair tube P1 and P2 is not to equate completely, have certain error; Size and the threshold voltage of two current mirror NMOS pipe N1 and N2 are not to equate completely, have certain error; So amplifier can produce certain offset voltage.

When CP (duty ratio is 50% switching signal) is high level, switch S 1, S3, S5, S7 is opening, switch S 2, S4, S6, S8 is closed condition.Now Vref termination PMOS manages the grid of P2, the grid of Vn termination PMOS pipe P1, and the grid of NMOS pipe N1 and N2 is connected the drain electrode of P1 and N1.The drain electrode of P2 and N2 connects the grid of NMOS pipe N3.The positive input terminal that Vref is amplifier like this, the negative input end that Vn is amplifier.The leakage level that is output as NMOS pipe N2 and PMOS pipe P2 of primary amplifier, they receive the input of secondary amplifier---the grid of NMOS pipe N3.Its equivalent electric circuit as shown in Figure 3.

When CP is low level, switch S 1, S3, S5, S7 is closed condition, switch S 2, S4, S6, S8 is for closing opening.Now Vref termination PMOS manages the grid of P1, the grid of Vn termination PMOS pipe P2, and the grid of NMOS pipe N1 and N2 is connected the drain electrode of P2 and N2.The drain electrode of P1 and N1 connects the grid of NMOS pipe N3.Same or the positive input terminal of amplifier of Vref like this, Vn also or the negative input end of amplifier.The leakage level that is output as NMOS pipe N1 and PMOS pipe P1 of primary amplifier, receives input---the grid of NMOS pipe N3 of secondary amplifier.Its equivalent electric circuit as shown in Figure 4.

According to above-mentioned two kinds of operating states, can reach a conclusion, no matter CP is high level or low level, the positive-negative input end of amplifier does not all have to change, what change is the annexation of primary amplifier inside, be that PMOS pipe P1 and P2 constantly convert positive-negative input end, NMOS pipe N1 and N2 are also constantly converting the output of primary amplifier simultaneously.Along with CP signal becomes low level from high level, the drain terminal of former cause P1 and N1 becomes the output as primary amplifier by the drain terminal of P2 and N2 as the output of primary amplifier.The operating state that is to say P1 and N1 and P2 and N2 is along with the height variation of CP signal and mutual conversion.Like this by between two differential pair tube P1 and P2 and the error producing between current mirror load N1 and N2 can be along with the continuous variation of CP signal, at P1 and P2, N1 and N2 constantly convert and in operating state, are cancelled and eliminate.Suppose that the input offset voltage of amplifier when CP is high level is Vos, the input offset voltage of amplifier when CP is low level is exactly-Vos so.So along with the continuous variation of CP signal, total offset voltage of amplifier is Vos+ (Vos)=0.

Below further illustrate operation principle of the present invention:

As shown in Figure 2, LED constant-current control circuit is a kind of current/charge-voltage convertor.Amplifier and power NMOS pipe N4 and resistance R form a negative feedback structure.This structure is the reference voltage V ref of fixed voltage value to be converted to the LED electric current I of fixed current value _lED, wherein Vref is the reference voltage in system, the offset voltage that Vos is operational amplifier.

In fact, in multi-channel LED constant-current system, every passage is all just like a galvanostat of this structure of Fig. 2.With first passage and second channel, give an example, the electric current of two passes is respectively:

$I_{\mathrm{<figure-callout\; id="1"\; label="LED"\; filenames="111107183017.png"\; state="\{\{state\}\}">LED</figure-callout>}1}=\frac{\mathrm{Vref}-\mathrm{Vos}1}{R1}---\left(1\right)$

$I_{\mathrm{LED}2}=\frac{\mathrm{Vref}-\mathrm{Vos}2}{R2}---\left(2\right)$

Mismatch between two paths (being matching error) is defined as:

$\mathrm{\&epsiv;}=\&PlusMinus;\frac{I_{\mathrm{LED}1}-I_{\mathrm{LED}2}}{I_{\mathrm{<figure-callout\; id="1"\; label="LED"\; filenames="111107183017.png"\; state="\{\{state\}\}">LED</figure-callout>}1}+I_{\mathrm{LED}2}}---\left(3\right)$

After bringing formula (1) and formula (2) into formula (3), draw:

$\mathrm{\&epsiv;}=\&PlusMinus;\frac{I_{\mathrm{LED}1}-I_{\mathrm{LED}2}}{I_{\mathrm{<figure-callout\; id="1"\; label="LED"\; filenames="111107183017.png"\; state="\{\{state\}\}">LED</figure-callout>}1}+I_{\mathrm{LED}2}}=\frac{\mathrm{Vref}(R2-R1)-R2\&CenterDot;\mathrm{Vos}1+\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="111107183017.png"\; state="\{\{state\}\}">R</figure-callout>}1\&CenterDot;\mathrm{Vos}2}{\mathrm{Vref}(R2+R1)-R2\&CenterDot;\mathrm{Vos}1-\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="111107183017.png"\; state="\{\{state\}\}">R</figure-callout>}1\&CenterDot;\mathrm{Vos}2}---\left(4\right)$

Because the resistance between leading to is all made the form of mutual coupling in actual plate-making, especially high-precision POLY resistance or injection resistance, in the situation that matching each other, precision can reach 0.4%, and the error that this error ratio Vos produces is little a lot.Therefore, after negligible resistance matching error.Mismatch between passage 1 and passage 2 is:

$\mathrm{\&epsiv;}=\&PlusMinus;\frac{\mathrm{Vos}2-\mathrm{Vos}1}{2\mathrm{Vref}-\mathrm{Vos}2-\mathrm{Vos}1}---\left(5\right)$

By formula (5), can be found out, the offset voltage that reduces operational amplifier in galvanostat can effectively reduce the mismatch between the different passages of LED.For example, in order to improve circuit efficiency, in galvanostat resistance general smaller be 5 Ω.Resistors match error is 0.4%.Reference voltage is 1V, and generally the offset voltage of amplifier is 2mV, and LED electric current is 50mA.Mismatch ε=1.6% of system in this case; If after offset voltage is eliminated, mismatch ε=0.4%.This mismatch can meet the requirement of high accuracy LED-backlit.Therefore, the offset voltage of elimination operational amplifier has important function to improving multi-channel LED currents match degree.

Claims (1)

1. a circuit that reduces a plurality of channel LED constant-current controller mismatches, each passage is equipped with operational amplifier and LED constant-current controller, it is characterized in that: the operational amplifier of each passage comprises that amplifier module and switch control module carry out mismatch control to the LED constant flow module of this passage, wherein:

LED constant-current controller comprises LED pipe, NMOS pipe N4 and the resistance of a plurality of series connection, and the positive pole of first LED pipe meets power supply VCC, and the negative electrode of an end LED pipe connects the leakage level of NMOS pipe N4, source class connecting resistance one end of NMOS pipe N4, resistance other end ground connection;

Amplifier module comprises two PMOS pipe P1 and P2, three NMOS pipe N1, N2 and N3, two constant-current source I1 and I2, 8 switch S 1-S8, one end of switch S 1 and S2 links together and connects reference voltage V ref, one end of switch S 3 and S4 links together and connects the source class of the pipe of NMOS in constant-current controller N4, the other end of switch S 1 and S4 links together and connects the grid that PMOS manages P2, the other end of switch S 2 and S3 links together and connects the grid that PMOS manages P1, the source class of PMOS pipe P1 and PMOS pipe P2 interconnects and connects the negative pole of constant-current source I1, the positive pole of constant-current source I1 meets power vd D, the leakage level of PMOS pipe P1 connects respectively the leakage level of NMOS pipe N1, one end of one end of switch S 5 and switch S 8, the leakage level of PMOS pipe P2 connects respectively the leakage level of NMOS pipe N2, one end of one end of switch S 7 and switch S 6, source class interconnection the ground connection of NMOS pipe N1 and NMOS pipe N2, the gate interconnection of NMOS pipe N1 and NMOS pipe N2 also links together with the other end of switch S 6 and the other end of switch S 5, the other end of switch S 7 and switch S 8 all connects the grid of NMOS pipe N3, the source ground of NMOS pipe N3, the leakage level of NMOS pipe N3 connects the negative pole of constant-current source I2 and the source class of NMOS pipe N4 and as the output end vo of amplifier module, constant-current source I2 positive pole meets power vd D,

Switch control module comprises an inverter, and the input connecting valve signal CP of inverter is also connected with the control signal end of switch S 1, S3, S5 and S7, and the output of inverter is connected with the control signal end of switch S 2, S4, S6 and S8.

Images