Changing exterior lamps from bulbs to LEDs makes it possible to achieve smaller, thinner light sources, allowing more manufacturers to improve lamp design.
To improve design, it is necessary to increase LED output while reducing the power consumption of the LED drive control circuit along with size.
At the same time, longer life is needed.
In addition, since LEDs are not expected to break, the tendency is to require high reliability at the component level.
At the same time, the demand to reduce costs cannot be ignored.
The difference between resistor and driver IC circuits
Until recently, resistor circuits were typically used to control LED current due to cost advantages.
However, disadvantages include lower efficiency due to circuit heat loss and the inability to detect LED failures.
In contrast, circuits using LED driver ICs that have been attracting attention in recent years provide a number of advantages, i
ncluding lower power consumption while ensuring high reliability by incorporating protection circuits that can detect LED failures.
However, at the same time, these circuits also increase the cost of components.
1) Power Consumption
Resistance and LED driver circuits utilize very different methods of controlling LED current when the battery voltage (that is, the supply input of the drive circuit) rises.
In the case of a resistor circuit, the LED current increases along with the battery voltage.
But with LED drive circuits, constant current drive is possible at a preset value even if the battery voltage increases.
This makes LED driver IC circuits advantageous in terms of power consumption.
LED driver IC circuits are also beneficial in terms of reliability.
This is because the number of mounted parts is small, reducing the likelihood of component failure on the control board.
In addition, LED driver ICs can detect errors such as LED open/short failures and provide external notification.
Resistor circuits are generally more cost efficient.
So while it seems that the more components used in resistor circuits should result in higher costs,
it is possible to significantly reduce costs by adopting multiple high power resistors, which are much cheaper than ICs.
Conversely, LED driver ICs require more ICs as the number of LEDs increases, leading to higher costs compared with resistor circuits.
As a result, conventional resistor and LED driver IC circuits only satisfy either the requirements for low power consumption, high reliability, or low cost.
Therefore, in order to achieve the increased adoption of LED lamps, it is necessary to develop an LED driver IC that balances these 3 demands.
ROHM’s New LED Driver IC
In response to the recent market demands as mentioned above, ROHM developed the BD183x7EFVM series of 4ch LED driver ICs
that utilize a new control method called Energy Sharing in which power consumption is distributed from within the LED driver IC to external resistors.
Challenges in reducing LED driver IC power consumption
The Fig below shows a general driver IC, comprised of a constant current circuit that supplies current to the LEDs,
an input that connects to the battery power supply, and an output that connects to that connects to the LEDs.
When Power Supply A to which the input voltage from the battery is connected rises to some extent,
the constant current circuit within the LED driver IC can output a constant LED current.
So as a result, the output terminal voltage is equivalent to the forward voltage characteristics of the connected LEDs.
Since the power consumption of the LED driver IC is the product of the input-output voltage difference of the constant current circuit and LED current,
the power consumption will increase as the input voltage from the battery rises.
Therefore, to decrease LED driver IC power consumption it is necessary to reduce either the input-output voltage difference of the constant current circuit or LED current.
However, customer requirements and other factors make it difficult to change the LED current,so ROHM developed a method for controlling
the voltage between the input and output of the constant current circuit.
ROHM’s Energy Sharing control method reduces costs by decreasing IC power consumption
The Fig below shows ROHM’s new Energy Sharing control method that achieves lower costs by reducing LED driver IC power consumption.
The voltage between the input and output of the constant current circuit is controlled by passing a part of the LED current through the external resistor of the LED driver IC
to suppress heat generation.
At the same time, a newly added block monitors the output pin voltage to control Power Supply A to a constant voltage.
The current flowing through the resistor is represented by the external resistor R and the voltage difference between the battery voltage generated at both
ends of the resistor and Power Supply A voltage (Battery Voltage - Power Supply A Voltage).
Power Supply A voltage is controlled to a constant value by increasing the resistance current as the battery voltage rises.
Using this control method allows most of the power previously consumed by the LED driver IC itself to be consumed by the external resistor R,
reducing LED driver IC power consumption by approx 75% vs conventional solutions.
So by sharing power consumption between the LED driver and external resistor, the power achieved by 4 conventional ICs can be handled with a single IC and high power resistor.
Although circuits equipped with ROHM’s new LED driver IC are slightly more expensive than resistor circuits, a cost savings of approx. 40% can be realized over conventional LED driver circuits.
As a result, in addition to lower power consumption and greater reliability, lower cost on par with resistor circuits is possible by pairing with an external resistor.
In addition, ROHM allows making this function available by only adding the input terminal with one pin to a conventional LED driver IC.