High-power LED constant current drive solution and examples

This article elaborates on the basic principles and practical cases of using DC/DC regulators to achieve constant voltage to constant current design, and explains the precautions for high-power LED driver design and heat dissipation design. It points out the new requirements for driver design for new high-power LED applications, and provides a guide to National Semiconductor's complete solutions. It helps electronic design engineers in the LED lighting industry to fully master the latest LED driver system design technology.
Although high-power LEDs cannot replace traditional lighting fixtures on a large scale, they are increasingly widely used in indoor and outdoor decoration and special lighting. Therefore, mastering the design technology of high-power LED constant current drivers is crucial to developing new applications for high-power LEDs. LEDs can be divided into high-power LEDs, high-brightness LEDs and ordinary LEDs according to power and luminous brightness. Generally speaking, the power of high-power LEDs is at least 1W, and the more common ones are 1W, 3W, 5W, 8W and 10W. 1W and 3W LEDs have been used in large quantities, while 5W, 8W and 10W LEDs are relatively less used. High-power LED lamps will be used in large quantities at the 2010 Shanghai World Expo, so the electronics and lighting industries are very concerned about the development and application of new LED lighting technologies.

Constant current drive and improving the optical efficiency of LED are two key issues in LED application design. This article introduces the application of high-power LED and the selection guide of its constant current drive solution. Then, taking the products of National Semiconductor (NS) as an example, it focuses on how to cleverly use the sampling resistor of the LED constant current drive circuit to improve the efficiency of high-power LED, and gives the precautions for the design and heat dissipation of high-power LED drivers.

Selection of driver chip

LED driver only accounts for a small part of the cost of LED lighting system, but it is related to the reliability of the performance of the entire system. At present, National Semiconductor's LED drive solution is mainly positioned in the mid-to-high-end LED lighting and lighting markets. Lighting is divided into indoor and outdoor types. Since the power supply environment used by indoor LED lights has two modes of AC/DC and DC/DC converters, the selection of driver chips should also be considered from these two aspects.

Figure 1: Basic variable current and variable voltage circuits

1. AC/DC converter

AC/DC is divided into 220V AC input and 12V AC input. 12V AC is the power source of halogen lamps widely used in hotels. Existing LEDs can be designed under the condition of retaining the existing AC 12V. For the design of replacing halogen lamps, the main advantages of National Semiconductor LM2734 are small size, high reliability, and output current up to 1A, which is just suitable for the small diameter of the halogen lamp mouth.

After replacing halogen lamps, LED lamps are generally made into 1W or 3W. Compared with halogen lamps, LED lamps have two major advantages: (1) The light source is more concentrated. The brightness obtained by 1W lighting is equivalent to the brightness of a halogen lamp of more than ten watts, so it is more energy-saving; (2) The life of LED lamps is longer than that of halogen lamps. The main

weakness of LED lamps is that the light angle is too narrow and the cost is relatively high. But in the long run, due to the longer life of LED lamps, it still has a very large cost advantage. 220V AC/DC converters (such as LM5021) are mainly targeted at the stage lamp and street lamp markets.

2. DC/DC converter

At present, LED flashlights account for the vast majority of the demand for DC/DC converters. The LED power used in flashlights is basically 1W, and the power supply methods include lithium batteries, nickel-zinc batteries, alkaline batteries, etc. There have always been some difficulties in the application of 3W flashlights, because the 3W LED lamp itself needs to dissipate heat, and the heat dissipation device is large, which to a certain extent weakens the advantage of the small size of the LED lamp. In addition, since the current of the 3W LED lamp is as high as 700mA, the battery life after a single charge is shortened. Despite this, National Semiconductor provides solutions such as LM3475, LM2623A and LM3485 for the above applications.

Mining lamps are also one of the main application areas of LED lamps. It belongs to the special lighting industry and requires professional certification standards. China has always attached great importance to the application of LED in the field of mining lamps. At present, the LED design industry has the problem of insufficient understanding of the needs of special industries, and some unrealistic and novel design solutions are often used in the design. For example, the LED lamp and battery are embedded in the helmet together, but the various needs of the special use environment of the mining lamp are not taken into account. This may be an important reason why the application of LED in the mining lamp market has not been opened up.

For the application of LED in mining lamps, National Semiconductor provides a wide range of DC/DC regulator products, including LM3485, LM3478 and LM5010. Some users have adopted a 1W LED lamp and placed 6 ordinary high-brightness LED lamps around it to form a mining lamp with special flashing function.

In short, LED lamps have broad development prospects in the lighting and special lighting industries, and National Semiconductor provides a complete new LED driver solution for this purpose.

Figure 3: Constant current drive circuit based on LM2734

The basic circuit of the constant voltage power supply of the efficient constant current drive circuit

(Figure 1 left) uses feedback resistors RFB1 and RFB2. When the load current changes, VFB also changes accordingly. The DC/DC regulator maintains the output voltage at a fixed level by sensing the change of VFB: V0=(VFB*(RFB1+RFB2))/RFB1 (1)

In the circuit on the right side of Figure 1, the FB of the DC/DC regulator is a high-impedance input terminal, and the current IF flowing through the LED is: IF=VFB/RFB (2)

To keep IF constant, the DC/DC regulator senses VFB and then adjusts the positive terminal voltage of the LED to keep the current flowing through the LED constant. This is the principle of using the FB feedback terminal of the DC/DC regulator to achieve constant voltage to constant current conversion.

Generally speaking, the DC/DC regulator has a range of sensing changes in VFB. Once the LED is selected, the size of its operating current IF is also determined. The selected resistor must ensure that VFB falls within the allowable range of the DC/DC regulator.

Taking VFB equal to 1.25V as an example, assuming that IF is 15mA, 350mA and 700mA respectively, the power consumption of the sampling resistor will be less than 20mW, 400mW and 800mW respectively. For a 1W LED, the power consumption of the sampling resistor accounts for 2%, 40% and 80% of the total power consumption respectively. Therefore, the design of the sampling resistor is crucial to improving the efficacy of the LED, and it should be selected as small as possible.

Since directly connecting RFB to the FB terminal will cause RFB to consume too much power, an operational amplifier is placed between the FB terminal and RFB to amplify the voltage VTAP collected by RFB (Figure 2).

IF=VTAP/RFB=(VFB/RFB)*(1+RF/RI) (3)

Usually, the typical operating current of a 1W high-power LED is 350mA. If RFB is selected to be equal to 1 ohm, the power consumption of RFB is:
PRFB=I2*R=0.352*1=0.12W (4)

Considering the power consumption of the operational amplifier itself, the power consumption of RFB and its auxiliary circuits is about 12% of the 1W LED power. In this way, while ensuring that the LED is powered by a constant current, the power consumption of RFB can be reduced to an acceptable level, so that the voltage across the LED is as large as possible and the current flowing through it is as large as possible. National Semiconductor's voltage regulators that work according to this principle include LM2736 and LM2734.

Figure 4 Design of directly obtaining feedback voltage from sampling resistor

In addition, the optical efficiency of the LED lighting system depends not only on the LED constant current drive solution, but also on the heat dissipation design of the entire system. In order to reduce the volume, some LED constant current drive systems design the LED drive circuit close to the heat dissipation part, which can easily affect reliability.

Generally speaking, the heat source of the LED lighting system is basically the heat source of the LED lamp itself. If the heat source is too concentrated, it will cause heat loss. Therefore, the LED drive circuit cannot be close to the heat dissipation system. It is recommended to take the following heat dissipation measures: LED lamps use aluminum substrates for heat dissipation; power devices are evenly arranged; avoid

designing the LED drive circuit close to the heat dissipation part as much as possible; suppress the thermal impedance of the package to the printed circuit substrate; improve the heat dissipation smoothness of the LED chip to reduce the thermal impedance.

Table 1: High-power LEDs have a great advantage in lifespan

New application requirements for drivers

High-power LEDs are called "green light sources". They will develop in the direction of large LED current (300mA to 1.4A), high efficiency (60 to 120 lumens/watt), and adjustable brightness.

Since high-power LEDs have great advantages in life (Table 1), they have a very broad development prospect. The most promising lighting applications are automobiles, medical equipment, instrumentation, and other special lighting environments. However, these applications also put forward new requirements for LED drive system design, including: the input voltage range is generally required to be 6V to 24V; with impact load protection, reverse phase and overvoltage protection; very low standby power consumption; low bandgap reference to reduce current detection loss and PWM brightness adjustment function.

In response to these needs, National Semiconductor provides a full range of LED driver design solutions (see Table 2), which can provide users with comprehensive LED driver solutions.

LED lighting systems require constant current power supply. The current mainstream constant current drive design scheme is to use linear or switching DC/DC regulators combined with specific feedback circuits to provide constant current power supply for LEDs. According to the differences in the peripheral circuit design of DC/DC regulators, it can be divided into inductive LED drivers and switched

capacitor LED drivers. The advantages of the inductive boost driver scheme are higher driving current, lower LED terminal voltage, lower power consumption, and unchanged efficiency. It is particularly suitable for applications that drive multiple LEDs. In the design of high-power LED drivers, the switched capacitor LED driver scheme is mainly used. Its advantages are higher voltage across the LED and larger current flowing through, thereby obtaining higher efficacy and optical efficiency. Advanced switched capacitor technology can also improve efficiency, so it is widely used in high-power LED drivers.

Table 2: National Semiconductor's LED driver solutions list