1. Introduction
Automotive technology is undergoing a major transformation. For example, it is a well-known fact that electronic components enable the safety features and sensing functions required for autonomous driving technology.
The exterior lights of cars, represented by headlights and taillights, have also evolved from traditional bulb-type exterior lights to LED (Light Emitting Diode) lights, and light source technology is making great progress. In recent years, products that not only allow LEDs to play the role of lighting, but also improve safety by controlling LED lights are becoming increasingly popular. In addition, the technology and product updates of two-wheeled vehicles are also changing with each passing day, and the quality requirements are very high, just like cars.
In this context, ROHM is developing small, highly reliable LEDs for automotive applications and LED driver ICs that are essential for controlling LED light, and is providing solutions for automotive lighting. In this article, we will introduce ROHM's unique technologies and products.
Figure 1. Comparison of power consumption characteristics
2. Market demand for LED driver ICs
2-1. Market demand for LED lights
Exterior lights have evolved from bulb-type to LED lights, which allow light sources to become smaller and thinner, and more and more manufacturers are committed to developing LED lights with excellent design flexibility.
To improve the design flexibility of LED lights, it is necessary to increase the output power of LEDs, reduce the power consumption of the control circuit that drives the LEDs, and achieve miniaturization. In addition, it is also necessary to extend the service life. The design premise of bulb-type exterior lights is that "they may break after a certain life and need to be replaced"; while LED lights are usually modular products that integrate LEDs and control circuits, so it is not easy to replace them. Therefore, the design premise is "no damage", so reliability must be ensured at the component level.
The need to reduce costs cannot be ignored. For example, in ASEAN and India, where two-wheeled vehicles are the main means of transportation, the price of two-wheeled vehicles is very low. Therefore, the cost requirements for each module are also very strict, even for LED lights.
2-2. Differences between resistor circuit and LED driver IC circuit
Until now, the circuit for controlling LEDs has mainly adopted a resistor circuit that controls the current through a resistor due to cost advantages. Since the resistor circuit can light the LED with a simple structure similar to the conventional bulb-type exterior lamp, the cost is very low. However, there are problems such as reduced efficiency due to heat loss in the circuit and the inability to detect LED failures.
Circuits using LED driver ICs, which have attracted much attention in recent years (hereinafter referred to as "LED driver IC circuits"), have advantages such as lower power consumption and the ability to detect LED failures through built-in protection functions to ensure reliability, but there is a problem of increased component costs.
The differences between them are explained in detail below.
① Power consumption
When the battery voltage, which is the power input of the driving circuit, rises, the LED current characteristics controlled by the resistor circuit and the LED driver IC circuit are very different. In the case of the resistor circuit, the LED current increases as the battery voltage rises. In the case of the LED driver IC circuit, even if the battery voltage rises, constant current drive can be performed according to the pre-set current value. For example, taking the current value when the battery voltage is 13V as an example, the power consumption of the LED driver IC circuit can be reduced by 50% compared to the resistor circuit. This shows that the LED driver IC circuit has an advantage in low power consumption. (Figure 1)
② Reliability
LED driver IC circuits are also more advantageous in terms of reliability. This is because the number of components actually installed is smaller, so the possibility of component failure in the control circuit board is also lower. In addition, for open-circuit and short-circuit failures of LED lamps, LED driver ICs can detect LED abnormalities and output abnormal signals to notify the outside. This allows unsafe conditions such as reduced LED lamp brightness caused by LED problems to be discovered as early as possible, and countermeasures can be taken as early as possible.
③ Cost
The resistor circuit has an advantage in terms of cost. For example, in Figure 1, assuming that 9 LEDs (3 columns of LEDs × 3 segments in series, about 150mA/column) are driven, at this time, at least 10 1W resistors are required in the resistor circuit, while the LED driver IC circuit only requires about 4 ICs (the specific number varies depending on the package). The resistor circuit seems to have a large number of components and a high cost, but the cost can be reduced by using multiple high-power resistors that are much cheaper than ICs. In the LED driver IC circuit, the more LEDs that need to be controlled, the more ICs are required, and the cost increases compared to the resistor circuit.
In summary, the previous resistor circuits and LED driver IC circuits can only meet part of the market's requirements for "low power consumption", "high reliability" and "low cost". In order to further popularize LED lights in the future, it is necessary to develop LED driver ICs that meet all three requirements.
Figure 2. Circuit structure and characteristics of a common LED driver IC
3. ROHM's new LED driver IC
ROHM provides LED driver ICs for odometer indicator light sources, CID (Center Information Display) and white backlights for LCD instrument panels, as well as headlights and taillights. It has a wide range of technologies and products for controlling various LEDs used in automobiles and two-wheeled vehicles.
In response to the latest market needs mentioned above, ROHM has established a new control method "Energy Sharing" that can distribute the power consumption from the LED driver IC to the external resistor, and developed the MOSFET built-in 4-channel linear LED driver IC "BD183x7EFV-M" (BD18337EFV-M / BD18347EFV-M) that adopts this control method. It is suitable for the increasingly popular LED lights (brake lights, tail lights, fog lights, turn signals, etc.) in two-wheeled and four-wheeled vehicles.
The following is an introduction to the “Energy Sharing” control method.
3-1. Issues in reducing power consumption of LED driver ICs
First, the circuit structure and characteristic diagram of a common LED driver IC are given in Figure 2. There is a constant current circuit inside the LED driver IC that provides current to the LED. The battery power supply is connected to the input end of the IC, and the LED is connected to the output end of the IC. When the power supply A connected to the input voltage of the battery power supply rises to a certain level, the constant current circuit inside the LED driver IC can output the LED current constantly. Therefore, the voltage of the output pin is equivalent to the forward voltage characteristic of the connected LED and is constant.
The power consumption of LED driver IC is the product of the voltage difference between the input and output of the constant current circuit and the LED current, so the power consumption increases as the battery input voltage increases. It can be seen that in order to reduce the power consumption of LED driver IC, it is necessary to reduce the voltage difference between the input and output of the constant current circuit or the LED current. Since the LED current is determined according to customer requirements and is difficult to change, ROHM has developed a method to control the voltage between the input and output of the constant current circuit.
3-2. “Energy Sharing” control method that reduces costs by reducing IC power consumption
Next, Figure 3 shows the circuit structure of the new control method "Energy Sharing" developed by ROHM, which achieves lower costs by reducing the power consumption of the LED driver IC. By diverting part of the LED current to the resistor R outside the driver IC, the input-output voltage of the constant current circuit is controlled, and the heat generation of the LED driver IC is suppressed. The voltage of the power supply A is controlled to a constant voltage by monitoring the output pin voltage through the newly added module. The current flowing through the resistor is represented by the voltage difference between the battery voltage and the power supply A voltage (battery voltage - power supply A voltage) generated across the resistor, and the external resistor R. The power supply A voltage is controlled to a constant voltage by increasing the resistor current as the battery voltage increases. With this control method, most of the power consumed by the LED driver IC itself can be consumed by the external resistor R, reducing the power consumption of the LED driver IC by about 75% compared to the past. In this way, this structure in which the power consumption is shared by the LED driver IC and the external resistor R respectively makes it possible to achieve the power that was previously achieved by four ICs with only one IC and a high-power resistor.
Although the circuit cost of the new product equipped with LED driver IC is slightly higher than that of the resistor circuit, it can be reduced by about 40% compared with the previous LED driver IC circuit. In addition to "low power consumption" and "high reliability", the "low cost" of the resistor circuit can also be achieved by combining it with an external resistor. ROHM can achieve this function by simply adding one pin to the input pin end of the previous LED driver IC. In addition, it also supports the on/off light mode unique to two-wheeled vehicles, and most of the necessary functions can be met by only IC.
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