A design scheme for automobile taillight and brake controller

Light-emitting diodes ( LEDs ), long used in consumer electronics , have recently found their way into automotive lighting, providing signal functions, daytime running lights, and interior lighting. As this lighting technology becomes more popular, manufacturers are continually looking for new ways to use it to take advantage of the stylish and aesthetic benefits of LED headlights and taillights.

Today, red LEDs are widely used in taillights. Although cost remains an issue, factors such as safety, environmental protection, and styling flexibility all favor the use of LEDs. One of the most popular applications is the center brake light. This design idea shows how to use the same LED array for both the taillight and the brake light.

Figure 1: Running/brake light controller

The LED brightness is controlled by a simple switch, dimming when driving and brightening when braking. The functional block diagram is shown in Figure 1, including the LED driver/monitor AD8240 , PNP adjustment tube and PWM generator. AD8240 provides a constant voltage to drive the LED lamp. It also provides cost-effective LED lamp monitoring and short-circuit protection. When the battery voltage is between 12.5 V and 27 V, the output is stable at 12 V.

Light-emitting diodes (LEDs), long used in consumer electronics, have recently been used in automotive lighting to provide signal functions, daytime running lights, and interior lighting. As this lighting technology becomes more popular, manufacturers are also constantly exploring new ways to apply it in order to fully utilize the stylish and beautiful advantages of LED headlights and taillights.

Today, red LEDs are widely used in taillights. Although cost remains an issue, factors such as safety, environmental protection, and styling flexibility all favor the use of LEDs. One of the most popular applications is the center brake light. This design idea shows how to use the same LED array for both the taillight and the brake light.

Figure 1: Running/brake light controller

[page]

The LED brightness is controlled by a simple switch, dimming when driving and brightening when braking. The functional block diagram is shown in Figure 1, including the LED driver/monitor AD8240, PNP pass tube and PWM generator. AD8240 provides a constant voltage to drive the LED lamp. It also provides cost-effective LED lamp monitoring and short-circuit protection. When the battery voltage is between 12.5 V and 27 V, the output is stable at 12 V.

Figure 2 shows the PWM generator, which consists of two 555 timers. The PWM signal controls the LED brightness. When the PWM input is high, VO is turned on; when the PWM input is low, VO is turned off. The AD8240 is designed to operate at frequencies up to 500 Hz with a typical duty cycle range of 5% to 95%.

Figure 2: PWM signal generator

The AD8240 provides a low power, low cost, small package solution. An internal current sense amplifier measures the voltage across the external shunt resistor; when the current measurement drops below a preset threshold, an open LED condition is indicated. When this current reaches a level set by the external shunt resistor value, the output voltage is latched, limiting the output current. When the sense amplifier output exceeds 5 V, an internal comparator causes the driver to latch the output voltage. The latch is reset during the next PWM cycle. Overcurrent conditions can also be detected by measuring the sense amplifier output.

Costs are further reduced because no inductor is needed for switch designs; and LED lamps consume far less power than incandescent lamps, so no switch driver is required.

The LED is switched on and off by a digital voltage on the CMOS-compatible PWM pin (AD8240 pin 3). This voltage can be continuous for simple on/off control or PWM for dimming control. The PWM frequency should be less than 500 Hz, with a duty cycle range from 5% to 100%. Typical values ​​are 5% (when driving) and 95% (when braking). In Figure 2, the PWM frequency is determined by R1, R2, and C1 of timer A1. The pulse period is:

T = 0.693 (R1 + 2 R2) C1

When R1 = 49.9 kΩ, R2 = 10 kΩ, and C1 = 0.1 μF, the period is 4.84 ms, or approximately 206 Hz.

Timer A2 converts this signal into a pulse width modulated signal whose duty cycle is determined by R3, R4, R5 and C2. The pulse width is determined by the following equation:

Pulse width = 1.1 RC2

Where R is equal to R5, the parallel resistance of R3 and R5, or the parallel resistance of R4 and R5, depending on the switch position. When R3 = 2.37 kΩ, R4 = 45.7 kΩ, R5 = 42.4 kΩ, and C2 = 0.1 μF, the duty cycle is 5% (switch in position 1), 50% (switch in position 2), or 95% (switch in the OFF position).

Notice that the LED brightness increases as the duty cycle increases. When the brake is applied, the duty cycle is 95% and the LED array is brightest. During normal driving, the duty cycle is 5% and the LED array is dimmed. Sharing a single LED array for both operating states reduces costs.

If a short circuit or overload condition occurs, the voltage on Vsense (pin 1) drops to 0 V and the output shuts down. This circuit will reset during the next PWM cycle. If this condition persists, the AD8240 will attempt to drive the output to 12 V, shutting down and restarting after each PWM cycle.

This circuit provides a method to drive and monitor LEDs using a constant voltage with only two wires (power and ground). Many designs use a chassis or shared ground return, in which case the two wires can be reduced to one. Currently, these lights are controlled and driven by the body control ECU (Electronic Control Unit). When using this constant voltage architecture, the control and drive functions of the LEDs remain in the ECU with only minimal design modifications.