The taillights and brake lights of most cars use the same set of LEDs , which requires the LEDs to operate at two different brightness levels: full-brightness when braking, and 10% to 25% full brightness when running as a taillight (dimmable) ). The dimming method is best to choose pulse width modulation (PWM) to maintain the chromatogram of the LED over the entire brightness range. In addition, the LED driver with built-in 200Hz oscillator circuit can save the external PWM signal generator and simplify the design.
The taillights (adjustable LED brightness) and the brake lights (full light state) are controlled by the TAIL and STOP inputs of the LED driver . When the voltage is applied to the TAIL terminal, the taillight LED is driven to 10% to 25% of full brightness. When a voltage (brake) is applied to the STOP terminal, the LED is driven to the full brightness state (regardless of the state of the TAIL terminal input).
Taillight driver specifications
The input supply voltage (between STOP or TAIL and ground) is nominally 6V to 16V and may reach 45V under load.
The output voltage (VLED) is up to (VIN-1.4V).
The output current (LED current) during braking is 350mA, 240mA or 140mA, depending on the J1 setting.
The taillight LED current tube is regulated by 20% of full brightness over the PWM signal.
enter
STOP: Between STOP(+) and GND(-), the input voltage of 6V~16V is applied to drive the LED at 350mA, 240mA or 140mA continuous current. The current is set by J1.
TAIL: TAIL (+) and GND (-) between 6V ~ 16V input voltage, 350mA, 240mA or 140mA, 10% duty cycle drive LED, the current size is set by J1.
Output
LED+: Connected to the LED anode.
LED-: Connected to the LED cathode.
Circuit description
LED drivers for automotive taillights (STOP and TAIL modes) can be easily implemented with linear LED driver ICs (eg MAX16804) with very few external components. Figure 1 shows the corresponding circuit schematic, and Figure 2 shows the PCB layout. The maximum current flowing through the LED is set by R3 or R4, controlled by the connection mode of J1, and the brightness is controlled by the PWM signal, which is implemented inside the IC. The driver IC generates a 200Hz LED current regulation signal. The duty cycle depends on the voltage at the DIM terminal. For example, when the DIM terminal voltage is 0.78V, the taillight brightness is set at 20% of full brightness.
Figure 1: Schematic of the MAX16804 High-Brightness LED Driver
The STOP and TAIL inputs are connected to the IN pin through diodes D1 and D2. The IC can be powered from any input and the inputs do not interact with each other. D1 and D2 also provide voltage reverse protection when the vehicle's power bus has a spike. Capacitors C4 and C3 bypass any noise on the STOP line and provide protection for the DIM pin.
Taillight driving mode
A fixed 0.78V voltage is generated from the TAIL input to the DIM terminal (generated from the +5V regulated output), which converts the voltage to a 20% PWM duty cycle to regulate the LED current. The PWM duty cycle in TAIL mode is independent of the input voltage at the TAIL terminal. The DIM voltage in TAIL mode is set by the resistor divider R1/R2. If the duty cycle setting is not required to be equal to 20%, the corresponding DIM voltage (VDIM) can be calculated as follows.
Where D is the required duty cycle.
Select R1 and R2 according to Equation 2 to meet the DIM voltage requirements.
Where, 0.6V is the forward voltage of the diode, 5V is the regulator output, and VDIM is the required DIM pin voltage to meet the PWM duty cycle requirements. In order to avoid the DIM voltage being affected by the change of the input bias current of the DIM terminal, R2 is selected to be around 20kΩ.
Figure 2: PCB layout of the MAX16804 circuit  Â
Brake mode
In order to obtain a 100% duty cycle, the DIM voltage needs to be set above 3.1V. When the STOP terminal acts on the input voltage, the DIM terminal obtains sufficient driving voltage through D3 to ensure 100% PWM duty cycle. At this time, the LED will be driven at a continuous current of 350 mA regardless of the voltage input to the TAIL terminal.
LED current setting
The MAX16804 circuit shown in Figure 1 can be set to three different currents (140mA, 240mA, or 350mA) via jumper J1. As shown in Table 1:
Table 1: Jumper J1 sets three different current connections.
For different current settings, the current-sense resistor (R3 or R4) can be calculated as follows.
In the formula, 0.198V is the current-sense voltage, and IOUT is the required LED current, and the unit is A.
Power consumption
The device (MAX16804) dissipates 2.758W of power, which consumes the most power in STOP mode. Calculate the maximum power consumption according to the following formula:
VIN is the input voltage to the IN pin, VLED is the forward voltage of the LED string, and 350mA is the maximum LED current set by jumper J1. Most of the device power dissipation is dissipated through the exposed pad. To improve heat dissipation, the exposed pad should be soldered to the same area of ​​the board pad and multiple vias should be used to connect the exposed pad to the copper area of ​​the ground plane. .
The MAX16804 dissipates the maximum power rating at 25°C room temperature and reduces the power dissipated when the ambient temperature is high. In order to avoid the IC entering the thermal shutdown state, the power consumption should be appropriately reduced at high temperatures.
Circuit connection
The LED+ is connected to the anode of the LED, and the LED is connected to the cathode of the LED.
In order to get the LED current of 350mA, select pins 1 and 2 connected to J1.
In STOP mode, the supply voltage is connected between STOP(+) and GND(-). The minimum voltage is 6V, which is at least 1.4V higher than the LED forward voltage.
In TAIL mode, remove the STOP power supply and connect the power supply voltage between TAIL(+) and GND(-).
Test Results
During testing, the maximum LED current was set at 350mA and 240mA. In STOP mode, the LED is in full brightness. After the power supply voltage is applied between STOP and ground, the LED current will rise quickly to the set value (350mA or 240mA). The LED is driven with continuous current without overshoot.
In TAIL mode, the 200Hz, 20% duty cycle PWM control signal only allows current to flow through the LED in 1/5 cycles, thus reducing LED brightness. In STOP and TAIL modes, the LED current amplitude is the same, and the limited rise and fall times help to improve the EMI specification.
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