LED constant current source control circuit quickly reaching steady state

Abstract

The application relates to a LED constant current source control circuit capable of quickly reaching a steady state and belongs to the technical field of circuits. When a PWM signal is high, the LED constant current source control circuit capable of quickly reaching a steady state generates a transient high pulse with a time of about 20 ns through a One Shot monostable circuit, the signal turns on switches S1 and S2, a bias current I B The gate voltage of the NMOS tube MN is quickly pulled up to the vicinity of a steady state voltage, so that the delay of LED conduction time is greatly shortened, the LED dimming precision is greatly improved, the circuit does not generate extra power consumption, the structure is simple, and the application cost is very low.

Description

Technical Field

[0001] This invention relates to the field of circuit technology, and particularly to the field of LED driver circuit structure technology, specifically to an LED constant current source control circuit that quickly reaches a steady state. Background Technology

[0002] In LED driver circuits, to improve PWM dimming accuracy, it is necessary to quickly turn on the LED's constant current source and bring the current of the constant current source to a steady state within a very short time.

[0003] Traditional LED driver circuits, such as Figure 1 As shown. The LED current changes as follows when different reference voltages are set: (1) When the PWM signal is high, the circuit starts working, and the gate voltage of the NMOS transistor MN begins to rise. Due to power consumption limitations and the parasitic gate capacitance C of MN, the time t required for the gate voltage of MN to reach steady state from zero is: (2) Capacitor C is the parasitic capacitance of the gate of MN, V GS MN represents the voltage difference between the gate and source, and I represents the current that the op-amp output stage can provide.

[0004] Typically, the gate parasitic capacitance C of an NMOS transistor MN is approximately 10pF. If V REF Let's set it to 1V, V GS The voltage is 1V, and the current supplied by the op-amp output stage is 10uA. Therefore, the time required for the NMOS transistor MN to reach steady state can be calculated using equation (2) to be approximately 2us. This 2us delay significantly affects the accuracy of PWM dimming. For example, if the PWM signal is 100KHz and the duty cycle is 50%, the theoretical conduction time of the LED constant current source within one cycle is 5us. However, due to the 2us delay, the actual conduction time for the LED to maintain the theoretical current value within one cycle is only 3us.

[0005] Therefore, how to provide a control circuit that can quickly reach a steady state has become an urgent problem to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide an LED control circuit that can quickly reach a steady state.

[0007] To achieve the above objectives, the LED constant current source control circuit of the present invention for rapidly reaching steady state includes an operational amplifier AMP and an NMOS transistor MN. Several LEDs are connected in series between the input voltage VIN and the drain of the NMOS transistor MN. The non-inverting input terminal of the operational amplifier AMP is connected to a reference voltage V. REF Its inverting input is connected to the source of the NMOS transistor, its signal input is connected to the PWM signal, its output stage is connected to the gate of the NMOS transistor MN, and it also includes a constant current source I. B A monostable circuit and a PMOS transistor MP are used. The input of the monostable circuit is connected to the PWM signal, and the output of the monostable circuit is connected to and controls switches S1 and S2. When the PWM signal is high, the monostable circuit generates a momentary high pulse and controls switches S1 and S2 to connect. The constant current source I... B The front end is connected to the power supply VDD, and the constant current source I... B The rear end is connected to the gate of the NMOS transistor MN via the switch S1; the gate of the PMOS transistor MP is connected to the reference voltage V. REF Its source is connected to the gate of the NMOS transistor MN through the switch S2, and the drain of the PMOS transistor MP is grounded.

[0008] In this LED constant current source control circuit that quickly reaches steady state, the duration t of an instantaneous high pulse generated by the monostable circuit is 20ns.

[0009] In this LED constant current source control circuit that quickly reaches steady state, the constant current source I... B Determined according to the following formula: Where C is the gate parasitic capacitance of the NMOS transistor MN, and V GS This is the gate-source voltage of the PMOS transistor MP.

[0010] In this LED constant current source control circuit that quickly reaches steady state, the PWM signal is a 100KHz PWM dimming signal with a duty cycle of 50%, and the LED conduction time is 4.98us in one cycle.

[0011] The LED constant current source control circuit of this invention, which achieves steady state quickly, generates a transient high pulse of approximately 20ns when the PWM signal is high. This signal turns on switches S1 and S2, and the bias current I... BThe gate voltage of the NMOS transistor MN is quickly pulled up to near the steady-state voltage, thereby greatly shortening the LED conduction time delay and greatly improving the LED dimming accuracy. Moreover, the circuit of this invention does not generate additional static power consumption, has a simple structure, and is very inexpensive to apply. Attached Figure Description

[0012] Figure 1 It is a traditional LED driver circuit; Figure 2 This invention provides an LED constant current source control circuit for rapidly reaching a steady state. Detailed Implementation

[0013] To better understand the technical content of this invention, the following embodiments are provided for detailed explanation.

[0014] Please see Figure 2 As shown, this is the LED constant current source control circuit for quickly reaching steady state according to the present invention.

[0015] In one embodiment, the LED constant current source control circuit for rapidly reaching steady state includes an operational amplifier AMP and an NMOS transistor MN, with several LEDs connected in series between the input voltage VIN and the drain of the NMOS transistor MN, and the non-inverting input terminal of the operational amplifier AMP connected to a reference voltage V. REF Its inverting input is connected to the source of the NMOS transistor, its signal input is connected to the PWM signal, and its output stage is connected to the gate of the NMOS transistor MN.

[0016] The LED constant current source control circuit that quickly reaches steady state also includes constant current source I. B A monostable circuit and a PMOS transistor MP are used. The input of the monostable circuit is connected to the PWM signal, and the output of the monostable circuit is connected to and controls switches S1 and S2. When the PWM signal is high, the monostable circuit generates a momentary high pulse and controls switches S1 and S2 to connect. The constant current source I... B The front end is connected to the power supply VDD, and the constant current source I... B The rear end is connected to the gate of the NMOS transistor MN via the switch S1; the gate of the PMOS transistor MP is connected to the reference voltage V. REF Its source is connected to the gate of the NMOS transistor MN through the switch S2, and the drain of the PMOS transistor MP is grounded.

[0017] In a preferred embodiment, the duration t of a transient high pulse generated by the monostable circuit is 20 ns. The constant current source I... B Determined according to the following formula: Where C is the gate parasitic capacitance of the NMOS transistor MN, V GS This is the gate-source voltage of the PMOS transistor MP. The PWM signal is a 100kHz PWM dimming signal with a 50% duty cycle, so the LED conduction time in one cycle is 4.98µs.

[0018] In the application of this invention, the LED constant current source control circuit that quickly reaches steady state can be based on the gate parasitic capacitance C of the NMOS transistor MN and the reference voltage V. REF Set bias current I B The value of V allows the gate voltage of the NMOS transistor MN to charge to near its steady-state point within 20ns. REF +V SG2 , where V SG2 This represents the source-gate voltage of the PMOS transistor MP. After the 20ns high pulse, both switches S1 and S2 are in the off state. Compared to conventional circuits, this invention does not generate additional power consumption. Because this invention significantly shortens the LED turn-on time delay, the dimming accuracy is also greatly improved. Similarly, with a 100kHz PWM dimming signal and a 50% duty cycle, the LED current's theoretical turn-on time is approximately 4.98µs within one cycle.

[0019] Under the same conditions, a conventional circuit takes approximately 2µs to reach steady state from power-on, while the control circuit of this invention only takes approximately 20ns. This significant reduction in steady-state time improves the accuracy of PWM dimming. Furthermore, in steady state, this invention does not increase power consumption compared to conventional circuits.

[0020] The LED constant current source control circuit of this invention, which achieves steady state quickly, generates a transient high pulse of approximately 20ns when the PWM signal is high. This signal turns on switches S1 and S2, and the bias current I... B The gate voltage of the NMOS transistor MN is quickly pulled up to near the steady-state voltage, thereby greatly shortening the LED conduction time delay and greatly improving the LED dimming accuracy. Moreover, the circuit of this invention does not generate additional static power consumption, has a simple structure, and is very inexpensive to apply.

[0021] In this specification, the invention has been described with reference to specific embodiments thereof. However, it will be apparent that various modifications and variations can be made without departing from the spirit and scope of the invention. Therefore, the specification and drawings should be considered illustrative rather than restrictive.

Claims

1. A fast steady-state LED constant current source control circuit, comprising an operational amplifier AMP and an NMOS transistor MN, wherein a plurality of LEDs are connected in series between an input voltage VIN and the drain of the NMOS transistor MN, and the non-inverting input terminal of the operational amplifier AMP is connected to a reference voltage V. REF Its inverting input is connected to the source of the NMOS transistor, its signal input is connected to the PWM signal, and its output stage is connected to the gate of the NMOS transistor MN. Its features are, Also includes constant current source I B A monostable circuit and a PMOS transistor MP are used. The input of the monostable circuit is connected to the PWM signal, and the output of the monostable circuit is connected to and controls switches S1 and S2. When the PWM signal is high, the monostable circuit generates a momentary high pulse and controls switches S1 and S2 to connect. The constant current source I... B The front end is connected to the power supply VDD, and the constant current source I... B The rear end is connected to the gate of the NMOS transistor MN via the switch S1; the gate of the PMOS transistor MP is connected to the reference voltage V. REF Its source is connected to the gate of the NMOS transistor MN through the switch S2, and the drain of the PMOS transistor MP is grounded.

2. The LED constant current source control circuit for rapidly reaching steady state according to claim 1, characterized in that, The duration t of a momentary high pulse generated by the monostable circuit is 20 ns.

3. The LED constant current source control circuit for rapidly reaching steady state according to claim 2, characterized in that, The constant current source I B Determined according to the following formula: Where C is the gate parasitic capacitance of the NMOS transistor MN, and V GS This is the gate-source voltage of the PMOS transistor MP.

4. The LED constant current source control circuit for rapidly reaching steady state according to claim 2, characterized in that, The PWM signal is a 100kHz PWM dimming signal with a 50% duty cycle, and the LED conduction time is 4.98µs in one cycle.

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