Question

In the constant current drive solution, how to achieve constant current?

Answer 1

A constant current drive solution aims to ensure that the current through a load (such as an LED) remains constant, regardless of input voltage fluctuations or load variations. There are several methods for achieving constant current, and the following are some common ones:

1. Linear Regulator: Using a linear regulator is a simple way to achieve constant current, especially when lower output current is required. This method adjusts the conduction level of an internal transistor to maintain a constant current. However, its efficiency is relatively low, especially when there is a large difference between the input and output voltages.

2. Switched-Mode Power Supply (SMPS): This is a more efficient constant current solution, particularly suitable for high-power applications. It uses fast switching elements (such as MOSFETs) to regulate the output current. Because the switching element is either fully off or fully on, energy losses are reduced, improving efficiency. Furthermore, SMPSs can continuously adjust the switching frequency or duty cycle through feedback circuitry to maintain a stable output current.

3. Constant Current Diode: This component automatically adjusts its resistance value to maintain a fixed current, eliminating the need for additional control circuitry. While they are well-suited for low-current applications, their cost and size increase rapidly as the required current increases.

4. Using an operational amplifier and MOSFET feedback loop: In this approach, the operational amplifier senses the voltage across the load and adjusts the conduction level of the MOSFET based on a set reference voltage, thereby maintaining a constant current through the load. This approach can be precisely tailored to specific needs and is suitable for a variety of application scenarios.

5. Dedicated constant current ICs: There are many integrated circuits (ICs) on the market specifically designed to provide constant current functionality. These ICs typically integrate one or more of the aforementioned technologies, providing easy-to-use solutions with high efficiency and stability. Selecting the right constant current IC can simplify the design process while meeting specific application requirements.

The choice of approach depends on specific project requirements, budget constraints, and requirements for efficiency, size, and cost.

Answer 2

In a constant current drive solution, the key to achieving constant current is to dynamically adjust circuit parameters through a negative feedback mechanism to maintain a stable output current, unaffected by load changes, input voltage fluctuations, or temperature drift. The following are specific implementation methods and key technologies:

1. Linear Constant Current Drive (Linear Current Regulator)

Principle: Utilizes a transistor or field-effect transistor (such as a MOSFET) operating in its linear region, controlling current by adjusting its on-resistance.

Implementation:

Sampling Resistor: A small sampling resistor (such as 0.1Ω) is connected in series with the load circuit to convert the current into a voltage signal.

Error Amplifier: Compares the sampled voltage with a reference voltage (such as a stable voltage provided by a bandgap reference) and outputs an error signal.

Pass Transistor: The error signal controls the gate/base voltage of a pass transistor (such as an NPN transistor or PMOS transistor), dynamically changing its on-resistance to maintain a constant current.

Features:

Advantages: Simple circuit, low cost, and no EMI.

Disadvantages: The pass transistor consumes a lot of power (low efficiency, especially at high voltage differentials), requiring additional heat sinking. Applications: Low-power LED driving, precision control of low currents.

2. Switching Constant Current Driver (DC-DC Converter)

Principle: Energy conversion is achieved through high-frequency switching (such as MOSFETs) and inductive energy storage, combined with negative feedback to control the output current.

Common Topologies:

Buck: Suitable for scenarios where the input voltage is higher than the output voltage.

Boost: Suitable for scenarios where the input voltage is lower than the output voltage.

Buck-Boost: The input voltage can be higher or lower than the output voltage.

Implementation:

Current Sampling: Connect a sampling resistor in series with the inductor or switch branch, or use inductor current slope detection (such as valley/peak current mode).

PWM Controller: Adjusts the switch duty cycle based on the error between the sampled current and the reference current, dynamically controlling the inductor charge and discharge time to stabilize the output current.

Compensation Network: Optimizes the phase margin and gain of the feedback loop to ensure system stability.

Features:

Advantages: High efficiency (over 90%), suitable for high-power scenarios. Disadvantages: Complex circuitry, requires design of energy storage components such as inductors and capacitors, and may generate EMI.

Applications: High-power LED drivers, battery charging, and motor control.

3. Dedicated Constant Current Driver ICs

Principle: Integrates the above functions (such as sampling, comparison, and pass transistors) into a single chip, simplifying the design.

Typical ICs:

Linear constant current ICs: Examples include the LM317 (configurable as a constant current source) and the AMS1117 (fixed output current).

Switching constant current ICs: Examples include the PT4115 (Buck-type LED driver) and the XL4015 (Buck-Boost type).

Features:

Advantages: Simple peripheral circuitry, high reliability, and short development cycle.

Disadvantages: Limited flexibility (fixed parameters) and potentially higher cost than discrete designs.

Applications: Rapid prototyping and standardized products (such as LED light strips and mobile phone flashlights).

4. Key Design Points

Sampling Resistor Selection: The resistance value should balance power dissipation (I²R) and accuracy (typically 0.01Ω to 1Ω).

Reference Voltage Stability: Use a bandgap reference (such as the TL431) to provide a reference voltage with a low temperature coefficient.

Feedback Loop Compensation: Switching drivers require a compensation network (such as an RC filter) to prevent oscillation.

Protection: Add overvoltage, overcurrent, and overtemperature protection circuits to improve reliability.

Example: LED Constant Current Drive Circuit

Linear Solution: Sampling resistor → op amp comparator → MOSFET adjustment to achieve constant LED current.

Switching Solution: PT4115 chip + inductor + diode to regulate LED current via PWM.

Using the above approach, constant current drive solutions can flexibly adapt to different power, efficiency, and cost requirements and are widely used in lighting, power supplies, industrial control, and other fields.