White LED driver solution for LCD lighting backlight of automotive lighting

LCD currently uses CCFL as the backlight light source more often. However, due to the complicated CCFL backlight driving circuit, high driving voltage and color rendering ability are required, and the light source of the backlight is the largest part of the power consumption in the system, so the power limit is becoming more Under severe circumstances, it has been gradually discussed by the industry that LEDs will be used instead.

In order to meet the needs of energy saving and environmental protection, for different applications and different power consumption ranges, various new energy consumption standards of many governments and energy agencies around the world have also been released. At the same time, stricter regulations are also being formulated. Reducing energy consumption has become an important issue that cannot be avoided, so higher requirements are placed on power management.

LED control forward current scheme

LED is a device driven by current, and its brightness is proportional to the forward current. There are two ways to control the forward current. The first method is to use the LEDV-I curve, generally using a voltage power supply and a rectifier resistor to determine the voltage required to provide the LED to produce the expected forward current. But this method has some disadvantages, such as any change in LED forward voltage will cause changes in LED current.

Assuming a fixed voltage of 3.6V and a current of 20mA, when the voltage becomes 4.0V, a specific voltage change caused by temperature or manufacturing changes, then the current may be reduced to 14mA. Therefore, when there is a large change in the forward voltage, it will cause a greater change in the forward current. In addition, the voltage drop and power consumption will also waste power and reduce the battery life. The second method is to use a fixed current to drive the LED. The fixed current can eliminate the current change caused by the forward voltage change, so it can produce a fixed LED brightness. Using a fixed current only requires adjusting the voltage through the current-sense resistor, rather than adjusting the output voltage of the power supply.

The power supply voltage and the current detection resistance value determine the LED current. When driving multiple LEDs, a fixed current can be achieved in each LED by simply connecting in series. When driving parallel LEDs, a rectifier resistor must be placed in each LED string, but this will result in reduced efficiency and current mismatch.

As in portable applications, the battery life is the key to the overall application. Therefore, the LED driver must achieve high efficiency. However, the efficiency measurement of LED drivers is somewhat different from that of typical power supplies. The definition of a typical power supply efficiency measurement is the output power divided by the input power. For LED drivers, the output power is not a relevant parameter, but the input power value required for LED brightness is expected to be the focus. At this point, the answer can be obtained by dividing the LED power by the input power.

Overvoltage protection

In the fixed current mode, the LED driver assembly must provide overvoltage protection. No matter what the load is, a fixed current can be generated. But if the load resistance increases, the output voltage of the relative power supply must also increase. When the power supply detects excessive load resistance, or the load is disconnected, the output voltage may exceed the maximum operating voltage range of the IC or other components. Therefore, overvoltage protection must be provided in the driver. For example, you can use a Zener diode in parallel with the LED. This way you can limit the output voltage to the Zener diode breakdown voltage and power supply. When an overvoltage occurs, the output voltage will increase to the breakdown point of the Zener diode and pass through the Zener diode and then to the grounded current detection resistor, so the Zener diode and the LED can provide an output current in parallel.

In addition, the output voltage can also be monitored to turn off the power supply before overvoltage is reached. When an overvoltage condition occurs, the LED driver can reduce power consumption and extend battery life.

PWM dimming

Many portable LCD backlight applications require limited brightness adjustment. In this part, two dimming methods can be used, that is, the analog or PWM method. Using analog dimming, as everyone is familiar with, increase the current by 50% on the LED, so that the brightness can be increased by 50%. But this method has disadvantages, that is, there will be LED color shift, and the need to use analog control signals, therefore, this mode is generally not much used. The key to using PWM to adjust brightness is to ensure that the user cannot see the PWM pulse phenomenon, the frequency of the PWM signal must be higher than 100 Hz, and the maximum PWM frequency depends on the power activation and response time.

Load disconnect

Load disconnection is a function often overlooked in LED drive power supplies, because when the power supply fails, the load can be disconnected to disconnect the LED from the power supply. This function is very important in the following two situations, that is, power off and PWM dimming. For example, during the power-off of the boost converter, the load is still connected to the input voltage through the inductor and diode. Because the input voltage is still connected to the LED, the total power supply has failed, and it will continue to generate a small current. When the leakage current occurs for a long time, the battery life will be shortened. In addition, the load disconnection is also very important when the PWM performs brightness control. Because during the period when the PWM is not operating, the power supply fails, but the output capacitor is still connected to the LED.

If no load is disconnected, the output capacitor will still provide LED power until the PWM power is turned on again. Because the capacitor will discharge at the beginning of each PWM cycle, the primary power supply must charge the output capacitor at the beginning of each PWM cycle, so a surge pulse will occur when each PWM cycle occurs. The inrush current will reduce the efficiency of the system, and a transient voltage will appear on the input bus. If there is a load disconnection function, the LED will be disconnected from the power supply, so that when the power supply fails, there will be no leakage current, and the output capacitors are full during the cycle of PWM brightness adjustment.

At present, major players in the world are actively developing white LED driver circuits with a more complete structure and higher backlight efficiency. Therefore, as mobile phones continue to develop in the direction of multifunctional intelligence, it is expected that the demand for LED drivers will continue to increase. For example: At present, ordinary mobile phones generally only use 2 to 4 LED drivers, but the more feature-rich dual-screen camera phone needs 7 to 9 LED drivers to meet the lighting requirements.

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