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
2 achieve high reliability
In the SEPIC structure of Figure 2, C1 operates on the output voltage like a charge, so Q1 needs to reach the DC/DC converter output voltage.
(VOUT) + The withstand voltage level of the battery voltage. On the other hand, in the REGSPIC structure, since the withstand voltage is higher than the DC/DC converter output voltage and the battery voltage, the REGSPIC structure is composed of a low-voltage component and is easier to control.
In addition, Q2 is not only used for buck-boost control, but also can be used as a switch for disconnecting the battery from the battery when it is short-circuited to an external component such as an LED anode or a diode. Therefore, it can protect the external components when an abnormality occurs, which helps to achieve more. High reliability. In the SEPIC structure, Q3 is used only as a switch to cut off the path to the battery.
<High dimming current driver>
In order to meet the trend of the development of automotive panels in the direction of high brightness, ROHM has completed the technical development of the high-definition LED driver BD81A34EFV-M. The following explains why a higher brightness of the panel requires a higher dimming rate. Although the panel brightness can be higher, the minimum brightness level required is almost unchanged. Considering the low brightness of the output in the dark, such as the human eye does not feel tired, if the highest brightness (dimming rate 100%) is lower, even low dimming rate can output low brightness, but in recent years, the panel specification is generally the highest brightness. They are all very high, so high dimming is required for low brightness output.
In order to achieve high dimming rate, the BD81A34EFV-M uses ROHM's unique technology to improve the response performance of the current driver output LED. The LED current is switched on and off according to the external PWM input duty cycle. At this time, the current driver circuit is turned off when the PWM signal is low, and the current driver circuit is turned on when the level is high, and the LED current is adjusted according to the time ratio of the ON/OFF section. It is ideal to have the input PWM and the output current fully synchronized and the timing is consistent. As long as this can be achieved, high brightness can be achieved. In fact, a circuit delay is generated from the input of the PWM signal to the current output, and due to this delay, it is impossible to generate a pulse within the time width.
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