ROHM's latest driver technology for LED backlights
LED light sources have rapidly spread in many automotive applications. With its efficient LED light source drive technology, ROHM has built a broad lineup of LED drivers for taillights, backlights and headlamps. Here, you will be introduced to the LED driver for the backlight.
Development of LED driver for car backlight
In recent years, in the field of vehicle-mounted displays, in order to meet the requirements of hazardous substances, CCFL backlights using mercury are being rapidly replaced by LED backlights. In addition, various in-vehicle displays such as instrument panels, car navigation, audio display, and rear seat entertainment are developing in a diversified and large-scale direction. Under this trend, the demand for increasing the number of LED lamps and high brightness and high dimming rate is increasing. In order to meet this trend of increasing the number of LED lamps, ROHM will realize a high-voltage step-up DC/DC converter, a multi-lamp driving low-power LED, and a high-light-adjusting current driver circuit built into one. The chip expands the lineup of LED driver products.
Next, the LED driver BD81A34EFV-M for backlights developed by ROHM will be introduced.
The BD81A34EFV-M is roughly composed of three functional blocks: a DC/DC converter unit, a current driver unit, and a protection circuit unit (Fig. 1).
[Fig. 1] Block diagram of BD81A34EFV-M
As a driving of the backlight, first, a DC/DC converter generates a constant voltage. The output of the DC/DC converter is connected to the anode side of the LED of the panel, and a constant current is injected from the cathode side of the LED to the LED driver to cause the LED to emit light. To support low-power multi-lamp LED drivers, the number of LED channels (number of columns that can be connected) is designed to be four.
By controlling the switching duty cycle of the DC/DC converter, the output is level above the LED anode pin, which includes the portion of the LED segment that is linked to the current driver, that is, the VF generated by the LED, through the LED driver. The error amplifier performs feedback control so that the LED cathode pin (LED1~4 pins) connected to the IC is 1.0V. Through the above control, the current driver section can keep the LED current constant. As the brightness adjustment of the panel, the output current has PWM-dimming (PWM dimming) function. The duty cycle of the LED current can be varied synchronously with the external PWM signal input. Not only that, the BD81A34EFV-M is also equipped with LED open circuit and short circuit fault protection, LED ground fault protection, DC/DC converter output overcurrent and overvoltage protection. The perfect protection circuit is very helpful to improve the reliability of the panel.
The DC/DC converter circuit and the current driver circuit are introduced above. Next, the characteristics of the ROHM's on-board LED driver---anti-flicker circuit are introduced in order.
Buck-boost DC/DC converter
In the face of vehicle-specific battery voltage fluctuations and diversified LED lights, it is difficult to perform LED flicker control and platform design in boost mode and buck mode, to meet the high reliability required by the market and shorten the development cycle. The balance is not easy. Therefore, in order to stably supply the DC/DC converter output voltage regardless of the battery voltage, the ROHM adopts a unique buck-boost method called "REGSPIC structure". The following describes the REGSPIC structure with two advantages over the SEPIC structure used in the general buck-boost mode.
1 Reduce external components
Figure 2 shows the circuit configuration of SEPIC and REGSPIC. As can be seen from Fig. 2, in the REGSPIC structure, there are fewer coils having the highest area occupancy, and it is possible to achieve miniaturization and cost reduction. In addition, the inductance is reduced, and the efficiency corresponding to the loss portion of the coil can be improved.
[Figure 2] Circuit composition of SEPIC and REGSPIC
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