ROHM's New Automotive Monolithic LED Driver Ensures Stable Lighting Even During Battery Voltage Drops

Estimated reading time: 4 minutes

ROHM recently announced the development of an ultra-compact high output linear LED driver IC (BD18336NUF-M) that provides stable lighting even in the event of battery voltage drops on a single chip, making it ideal for a broad range of socket-type LED lamps used in vehicle systems, from DRLs (Daytime Running Lamps) and position lighting to rear lamps.  

In the automotive market, vehicle lamps are increasingly adopting LEDs due to their longer life and high-density mounting. Conventional onboard LEDs support various designs and functions by combining numerous electronic components on a modular substrate, and in recent years there is a growing demand for improved maintainability in addition to design. Socket-type LED lamps that can be replaced as easily as LED bulbs have been attracting attention by offering superior maintainability. However, it is difficult to decrease socket size, reducing design flexibility.

In response, ROHM developed a new LED driver IC that enables mounting in ultra-compact socket-type LED lamp circuits while ensuring stable lighting even during drop in battery voltage. 

The BD18336NUF-M adopts a current bypass function that prevents LED turn OFF and maintains a brightness of at least 30% at all times by switching the LED current path, even when the battery voltage decreases from 13V to 9V. In addition, a high output of 600mA is achieved in an ultra-compact 3.0mm square package, and mounting area is reduced by approx. 30% over conventional solutions (including the external circuit), making mounting possible on the 10mm square substrates required for small socket-type LED lamps. 

Conventional products, when configured without external circuitry, can cause the LED lamps to turn OFF when the battery voltage drops, possibly creating an unsafe situation during vehicle operation. However, ROHM’s new LED driver IC delivers stable lighting while contributing to the greater miniaturization of socket LED lamps. A function for suppressing LED heat generation is also built in that supports white LEDs that typically run hot, allowing them to be used in DRLs and other vehicle lighting systems utilizing white LEDs.

Key Features

1. Monolithic design ensures stable lighting during battery voltage drops 

The BD18336NUF-M is the first single-chip (ultra-compact 3.0mm square package) IC to incorporate a current bypass function that enables stable lighting during battery voltage drops. The following features (1-1, 1-2) contribute not only to stable lighting, but greater miniaturization as well in the latest socket-type LED lamps.

1-1. Maintains at least 30% brightness at all times even when the vehicle battery voltage decreases to 9V

Vehicle batteries normally supply a voltage of 13V (12V considering the backflow protection diode), but the output can vary due to various factors. With conventional products, when the voltage drops to as low as 9V, unless an external circuit is configured, the LEDs may turn OFF, which can cause problems by creating an unsafe situation when driving.

In contrast, ROHM's new IC includes a current bypass function at voltage drops that makes it possible to smoothly switch the LED lighting path through which current flows from 3 lamps to 2 without flickering when the supply voltage from the vehicle battery drops to 9V, contributing to stable lighting that prevents LED turn OFF while maintaining at least 30% brightness at all times (vs the normal ratio). This function can be configured using 3 external resistors (for the operating start voltage and operating interval), providing greater flexibility for a wide range of applications.

1-2. Reduces mounting area by 30% over conventional solutions, enabling support for ultra-compact socket-type LED lamps

The built-in current bypass function ensures stable driving during battery voltage drops that reduces the number of external parts by 7 over conventional designs, making it possible to decrease mounting area by up to 30% (including external circuitry). While conventional products may cause the LEDs to blink if configured without external circuitry (comprised of comparators, transistors, and other components) when the battery voltage drops, ROHM’s new IC makes it easy to stably light up LEDs with a single chip. This enables mounting on 10mm square LED module substrates, allowing the configuration of ultra-compact LED lamps that could not be achieved in the past.

2. Compatible with various lamp colors, including high brightness white LEDs

The BD18336NUF-M includes an output current derating function that limits the amount of heat generated by reducing the output of the LED driver when the temperature of the LEDs becomes too high. This function is designed for ultra-compact socket-type LED lamps while allowing the output current with respect to temperature to be adjusted by a single external thermistor.

Whereas conventional products are not suitable for driving white LEDs which are prone to high temperatures from the viewpoint of LED life, ROHM’s IC enables adjustment of the output current and supports various colors, making it ideal for a wide range of automotive lamp systems, including DRLs that use white LEDs along with rear and position lamps requiring red and yellow LEDs.