High-precision LED constant current driving circuit with ultra-low voltage margin

Abstract

The application discloses a high-precision LED constant current driving circuit with ultra-low voltage margin, comprising an NMOS tube M0, an NMOS tube M1, an NMOS tube M2, an operational amplifier, a current source and a compensation capacitor C0. The application introduces an operational amplifier and the NMOS tube M2 as a compensation tube. If the phase margin of the newly-added feedback loop is insufficient, the compensation capacitor C0 can be additionally added to improve the phase margin. The operational amplifier is used to make the potential of the output side and the reference side real-time forcedly equal, so that the high-precision current proportion is realized under the ultra-low voltage difference. The circuit of the application can automatically adjust the working point of the NMOS tube M1 according to the potential demand of the reference end, so that the drain-source voltage Vds1 of the NMOS tube M1 does not need additional margin, and the power consumption is significantly reduced. In addition, the frequency compensation network is used to ensure the closed-loop stability under the ultra-low voltage difference driving. Since the drain-source voltage Vds of the NMOS tube M0 and the drain-source voltage Vds of the NMOS tube M1 are perfectly matched, the current error problem caused by the different Vds of the traditional current mirror under the low voltage is solved.

Description

Technical Field

[0001] This invention relates to the field of analog circuit technology, and in particular to a high-precision LED constant current drive circuit with ultra-low voltage margin. Background Technology

[0002] In LED driver circuit design, NMOS transistor current mirrors are often used as driver circuits, such as... Figure 1 As shown in the diagram. In this circuit, the current I from the current source is amplified N times due to the current mirror, used to provide a constant current to the LED. This circuit may produce the following situations: the LED DVDD voltage is relatively low, a short-wavelength LED is used (in which case the LED's on-state voltage drop will be relatively large), and the LED current is relatively large. When any of the above situations occur, the LED's cathode voltage will be relatively low, easily causing the M1 transistor to enter the linear region, which in turn leads to a decrease in current. At the same time, the impedance to ground decreases, introducing a worse PSRR (power supply rejection ratio) problem caused by the poor LED DVDD.

[0003] The gate-source voltage Vgs of transistors M0 and M1 are the same. When their drain-source voltages Vds are different, due to the channel modulation effect, the current difference between M0 and M1 is relatively large. For M0, Vds = Vgs and must be greater than the threshold voltage Vth + Vsat (the minimum saturation voltage). When the Vds of M1 is compressed to Vsat, the mismatch between the Vds of M0 and M1 will cause a current deviation on both sides.

[0004] To reduce the channel modulation effect of M0 and M1 transistors, the commonly used method is to use a cascode circuit to form a cascode current mirror, but this will further compress the cathode voltage of the LED. Summary of the Invention

[0005] The purpose of this invention is to provide a high-precision LED constant current driving circuit with ultra-low voltage margin to solve the problems in the background art.

[0006] To solve the above technical problems, the present invention provides a high-precision LED constant current driving circuit with ultra-low voltage margin, including a current source, a MOSFET M0 and a MOSFET M1; the gate terminals of MOSFET M0 and MOSFET M1 are connected to the current source, the source terminal of MOSFET M0 is connected to the source terminal of MOSFET M1; the drain terminal of MOSFET M1 is connected to the cathode of the LED, and the anode of the LED is connected to the voltage LEDVDD. The high-precision LED constant current drive circuit also includes: an operational amplifier and a compensation transistor; The non-inverting input terminal of the operational amplifier is connected to both the cathode of the LED and the drain of the MOSFET M1, the negative-inverting input terminal is connected to both the source of the compensation transistor and the drain of the MOSFET M0, and the output terminal is connected to the gate of the compensation transistor; the drain of the compensation transistor is connected to a current source.

[0007] In one feasible implementation, the high-precision LED constant current driving circuit further includes a compensation capacitor, the first end of which is connected between the output terminal of the operational amplifier and the gate terminal of the compensation transistor, and the second end of which is grounded.

[0008] In one feasible implementation, the MOS transistor M0, the MOS transistor M1, and the compensation transistor are all NMOS transistors.

[0009] This invention provides a high-precision LED constant current drive circuit with ultra-low voltage margin, which can automatically adjust the operating point of NMOS transistor M1 according to the potential requirement of the reference terminal, so that the drain-source voltage Vds1 of NMOS transistor M1 does not require additional margin, thereby significantly reducing power consumption. In addition, a frequency compensation network (compensation capacitor C0) ensures closed-loop stability under ultra-low dropout driving. Since the drain-source voltage Vds of NMOS transistor M0 and NMOS transistor M1 are perfectly matched, the current error problem caused by the difference in Vds at low voltages in traditional current mirrors is solved. Attached Figure Description

[0010] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0011] Figure 1 This is a schematic diagram of a common NMOS current mirror used as a constant current drive circuit for LEDs.

[0012] Figure 2 This is a schematic diagram of the high-precision LED constant current driving circuit provided by the present invention. Detailed Implementation

[0013] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0014] This invention provides a high-precision LED constant current driving circuit, which employs a method of decoupling the gate and source terminals of an NMOS transistor MOSFET MOSFET. The specific circuit is shown below. Figure 2 As shown, it includes NMOS transistors M0, M1, and M2, an operational amplifier, a current source, and a compensation capacitor C0.

[0015] The gate of NMOS transistor M0 is connected to the gate of NMOS transistor M1 and then to a current source. The source of NMOS transistor M0 is connected to the source of NMOS transistor M1. The drain of NMOS transistor M1 is connected to the cathode of the LED, and the anode of the LED is connected to the voltage LEDVDD.

[0016] The operational amplifier's non-inverting input is connected to the drain of NMOS transistor M1, its negative input to the drain of NMOS transistor M0, and its output to the gate of NMOS transistor M2. The source of NMOS transistor M2 is connected to the drain of NMOS transistor M0, and the drain of NMOS transistor M2 is connected to a current source. The first terminal of the compensation capacitor C0 is connected between the operational amplifier's output and the gate of NMOS transistor M2, and the second terminal is grounded.

[0017] The connection point of the operational amplifier output terminal, the first terminal of the compensation capacitor C0, and the gate terminal of NMOS transistor M2 is node 1 (net1). The connection point of the current source, the drain terminal of NMOS transistor M2, the gate terminal of NMOS transistor M0, and the gate terminal of NMOS transistor M1 is node 2 (net2). The connection point of the operational amplifier negative input terminal, the source terminal of NMOS transistor M2, and the drain terminal of NMOS transistor M0 is node 3 (net3). The connection point of the operational amplifier positive input terminal, the drain terminal of NMOS transistor M1, and the cathode of the LED is node 4 (net4).

[0018] The working principle of this invention is as follows: Voltage matching (Drain Matching): The negative input of the operational amplifier is connected to the drain of NMOS transistor M0 (node ​​3), and the non-inverting input is connected to the cathode of the LED and the drain of NMOS transistor M1 (node ​​4). This creates two negative feedback loops: when the voltage at node 3 increases, the voltage at node 1 decreases, leading to a decrease in the voltage at node 3; when the voltage at node 4 increases, the voltage at node 1 increases, leading to an increase in the voltage at node 3, which in turn leads to an increase in the voltage at node 2, resulting in a decrease in the voltage at node 4. These two negative feedback loops can cause the voltage at node 4 to be approximately equal to that at node 3. The degree of their approximation is limited by the amplifier gain A, specifically: Vnet4 - Vnet3 = Vnet1 / A; where Vnet4, Vnet3, and Vnet1 are the voltages at nodes 4, 3, and 1, respectively.

[0019] The operational amplifier controls the gate of NMOS transistor M2 (i.e., node 1) through feedback, thereby forcing the voltage of the LED cathode to be equal to the voltage of the reference branch node 2.

[0020] The principle of reducing margin: In the high-precision LED constant current drive circuit of this invention, the source voltage and gate voltage of NMOS transistor M0 can be decoupled. Node 3 can be set low enough to approach the saturation voltage drop Vsat of NMOS transistor M0; under this condition, the cathode voltage of the LED (node ​​4) can be synchronously "suppressed" to the same low level, approaching the saturation voltage drop Vsat of NMOS transistor M1, and the drain-source voltage Vds of NMOS transistor M1 can be kept completely consistent with the drain-source voltage Vds of NMOS transistor M0, eliminating the current mismatch caused by the channel length modulation effect, resulting in extremely high precision.

[0021] This invention introduces an operational amplifier and an NMOS transistor M2 as a compensation transistor. If the phase margin of the newly added feedback loop is insufficient, an additional compensation capacitor C0 can be added to improve the phase margin. The operational amplifier forces the potentials of the output side (LED cathode) and the reference side (node ​​2) to be equal in real time, thereby achieving a high-precision current ratio of 1:N under extremely low voltage drop, where N is a positive integer greater than 1, and this N is obtained by comparing the ratio of the drive current of the output LED to the input current. The circuit of this invention can automatically adjust the operating point of the NMOS transistor M1 according to the potential requirements of the reference terminal, so that the drain-source voltage Vds1 of the NMOS transistor M1 does not require an additional margin, thus significantly reducing power consumption.

[0022] In addition, a frequency compensation network (compensation capacitor C0) ensures closed-loop stability under ultra-low dropout driving. Since the drain-source voltage Vds of NMOS transistor M0 and NMOS transistor M1 are perfectly matched, the current error problem caused by the difference in Vds at low voltage in traditional current mirrors is solved.

[0023] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0024] The above embodiments merely illustrate several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A high-precision LED constant current drive circuit with ultra-low voltage margin, characterized in that, It includes a current source, MOSFET M0 and MOSFET M1; the gate terminals of MOSFET M0 and MOSFET M1 are connected to the current source, and the source terminal of MOSFET M0 is connected to the source terminal of MOSFET M1; the drain terminal of MOSFET M1 is connected to the cathode of the LED, and the anode of the LED is connected to the voltage LEDVDD. The high-precision LED constant current driving circuit is characterized in that it further includes: an operational amplifier and a compensation transistor; The non-inverting input terminal of the operational amplifier is connected to both the cathode of the LED and the drain of the MOSFET M1, the negative-inverting input terminal is connected to both the source of the compensation transistor and the drain of the MOSFET M0, and the output terminal is connected to the gate of the compensation transistor; the drain of the compensation transistor is connected to a current source.

2. The high-precision LED constant current drive circuit with ultra-low voltage margin as described in claim 1, characterized in that, The high-precision LED constant current driving circuit also includes a compensation capacitor. The first end of the compensation capacitor is connected between the output terminal of the operational amplifier and the gate terminal of the compensation transistor, and the second end is grounded.

3. The high-precision LED constant current drive circuit with ultra-low voltage margin as described in claim 1, characterized in that, The MOS transistors M0 and M1, as well as the compensation transistor, are all NMOS transistors.

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