“In automotive lighting such as turn signals, brake lights, and tail lights, the LED circuit design usually uses discrete components, such as bipolar junction transistors (BJT). Discrete components stand out for several common reasons: they are simple, reliable, and inexpensive. However, as the number of LEDs and project requirements increase, it may be worthwhile to rethink discrete design. Let’s explore some common misunderstandings.
In automotive lighting such as turn signals, brake lights, and tail lights, the LED circuit design usually uses discrete components, such as bipolar junction transistors (BJT). Discrete components stand out for several common reasons: they are simple, reliable, and inexpensive. However, as the number of LEDs and project requirements increase, it may be worthwhile to rethink discrete design. Let’s explore some common misunderstandings.
Discrete design is simple
LED is a current-driven device. Using a transistor is a simple way to turn on the LED by adjusting the current. These transistor-based circuits serve as basic building blocks that can be replicated to drive any number of LED strings across projects, as shown in Figure 1.
Figure 1: Constant current discrete LED circuit
In projects with a large number of LEDs or challenging requirements, the circuit design not only looks crowded, but also requires complex analysis. Crowded with BJTs to drive multiple LEDs, the rear combination lamp (RCL) module may include the specifications listed in Table 1.
Specifications consider implementation
Function is used for the same LED group of taillights and parking lights to adjust the brightness level to indicate the mode analog current or pulse width modulation (PWM) dimming
Battery power supply denomination: 9 V to 16 V range: 6V to 40V stable output current, while withstanding voltage transients from start/stop, load dump, etc.
Constant current output circuit
Using a large number of components (such as the RCL example in Figure 2) will increase design and manufacturing risks, and we must carefully analyze the circuit.
Discrete design is reliable
Robust LED circuit design must consider fluctuations in voltage, current, and temperature. Once the forward voltage of the LED is reached, current will flow; the change in current changes the brightness proportionally. However, small changes beyond the forward voltage will cause the LED current to increase exponentially; excessive forward current will damage the LED.
We must also analyze power and heat dissipation. The factors that cause the temperature increase include the low power efficiency of the circuit, the on-time of the LED, and/or the warm environment. Poor heat dissipation can cause the LED to consume too much current, resulting in performance degradation.
The feedback circuit used to diagnose LED open circuit or short circuit faults can improve system reliability. Although this circuit adds more components, it still has advantages. For example, if the LED is damaged in the RCL, the brightness of the module no longer meets market regulations. The body control module (BCM) may have difficulty distinguishing the effective LED load from the lighting module from a single open circuit load, as shown in Figure 3. If the LED is damaged, implementing OFAF circuit will turn off all LEDs and make it easier to detect the open load. OFAF can also prevent further degradation of the LED.
Figure 3: BCM diagnoses faults from the LED drive module
The LED circuit must meet electromagnetic compatibility (EMC) emission and large current injection (BCI) immunity standards. If EMC is not taken into consideration, the LED driver module may interfere with or be affected by other applications, thus bringing a bad experience to the driver.
Discrete design is convenient
BJT is cheap commodity equipment. However, when tens or even hundreds are designed in one LED system, the number of components and system cost will increase. Considering the resources spent on design, debugging, and assembly, using an integrated LED driver solution can save time and money.
Linear LED driver integrated circuits (ICs) range from general-purpose single-channel to multi-channel devices with specific functions. As shown in Figure 4, an integrated solution like TPS92611-Q1 can replace multiple BJTs and other discrete components in the RCL circuit of Figure 2.
Figure 4: TPS92611-Q1 has a faulty bus connected to OFAF
In addition to reducing the number of components and system cost, the linear LED driver also provides a constant current output with low dropout, and provides adjustable brightness through analog current or PWM dimming. Thermal protection and short-circuit and open-circuit diagnosis provide reliable performance, and the OFAF fault bus can be connected across devices to ensure system reliability. Compared with discrete solutions, TPS92611-Q1 and other linear LED drivers also provide strong EMC performance.
The LED driver IC solution is a cost-effective alternative to discrete circuits, especially in applications with a large number of LEDs or complex requirements. With simple design, reliable performance and cost competitiveness, now stop using discrete circuits and switch to integrated linear LED drivers.