Driving Daytime Running Lights LEDs With Thermal Foldback Reference Design
1 Overview
The TIDA-01382 implements thermal foldback for derating current through the LEDs without using a microcontroller. This TI Design uses the following devices: the TPS92691-Q1 multi-topology LED driver in boost configuration to control the LEDs, the TLC555-Q1 LinCMOS™ timer together with the OPA2377-Q1 operational amplifier to measure and generate the accurate PWM signal by applying a feedback loop and a precision shunt regulator for setting the accurate duty cycle, and the LMT87-Q1 to implement thermal foldback. The input and output stage of the design is EMI- and EMC-filtered and can be directly supplied by a car battery.
2 Resources
| TIDA-01382 | Design Folder |
| TPS92691-Q1 | Product Folder |
| LMT87-Q1 | Product Folder |
| TLC555-Q1 | Product Folder |
| OPA2377-Q1 | Product Folder |
| TL431A-Q1 | Product Folder |
3 Features
- Thermal Foldback
- Precision PWM Dimming
- Efficiency-Optimized Design
- Operation Through Cold Crank
- Load Dump Tolerant
4 Applications
- Automotive Front Lighting
- Automotive Daytime Running Lights
- Automotive Tail and Brake Lights
5 Design Images
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An IMPORTANT NOTICE at the end of this TI reference design addresses authorized use, intellectual property matters and other important disclaimers and information. |
6 System Overview
6.1 System Description
This system has been designed to be a solution to precision pulse width modulation (PWM) dimming daytime running lights and implement thermal foldback without the necessity of using a microcontroller (MCU). The design includes key peripherals like electromagnetic interference (EMI) and electromagnetic compatibility (EMC) filtering voltage conditioning (shunt regulator), thermal foldback, precision clock generation, and LED drive.
The TIDA-01382 has been designed with the following points in consideration:
- The design must be able to generate a precision PWM signal in the range of 5% to 50% duty cycle.
- The design must be able to implement thermal foldback.
- The design must satisfy power requirements for one TPS92691 device driving a string of 1 to 12 LEDs for daytime running lights
- The design operate over the full range of automotive battery conditions:
- VIN(min) down to 5 V simulating a cold-cranking condition (ISO 7637-2:2004 pulse 4)
- VIN(max) up to 18 V simulating the upper range of normal battery operation
- The design must survive and continue operation through:
- Load dump (ISO 7637-2:2004 pulses 5a)
- Double battery condition
- The output must be protected against short-to-battery and GND voltage.
- The design must optimize the individual blocks for smallest power dissipation and highest efficiency.
- The layout of the board must be set up in such a way to minimize the footprint of the solution while maintaining high performance.
- The design must provide a flexible board interface to either mate to custom board through screw terminals
- The design provide power for the TLC555-Q1, OPA2377, and LMT87-Q1.
6.2 Key System Specifications
Table 1. Electrical Characteristics
| PARAMETER | COMMENTS | MIN | TYP | MAX | UNIT | |
|---|---|---|---|---|---|---|
| SYSTEM INPUT AND OUTPUT | ||||||
| VIN | Operating input voltage | Battery-voltage range; outputs are functional | 5 | 14 | 18 | V |
| VUVLO | Input UVLO setting | Undervoltage lockout (UVLO) | — | 4.5 | — | — |
| VSWMax | Vmax switch | Maximum switch node voltage | — | — | 100 | V |
| VOUT | Output voltage | LED+ to LED– (Boost) | 21 | — | 60 | V |
| VOUT | Output voltage | LED + to VIN (Boost-to-Battery) | 3 | — | 36 | — |
| VTR | Transient immunity | Load dump (ISO7637-2) | — | — | 60 | V |
| VIN_MIN | Minimum input voltage | Cold crank (ISO 7637-2) | 5 | — | — | V |
| IIN | Input current | Output at full load | — | 2 | — | A |
| IOUT | Output current | Maximum current per string | — | 350 | 365 | mA |
| Maximum output power | — | — | — | — | 25 | W |
| PWM dimming range | 240-Hz PWM frequency | — | — | 20:1 | — | — |
| LEDOpen and short detect | LED open and short detection | — | — | Yes | — | — |
| LED Single short detect | LED single-short detection | — | — | No | — | — |
| ONBOARD VOLTAGES | ||||||
| V 5V5 | Auxiliary supply, shunt regulator | TLC555, LMT87-Q1, op amp supply, and reference generation | — | 5 | — | V |
| VCC | Bias | Supply shunt regulator
(TLE431-Q1) |
— | 5 | — | V |
| VPREC_PWM | TLC555 out | Amplitude TLC555 clock at output | — | 5 | — | V |
| CLOCKS | ||||||
| fPREC_PWM | Square wave frequency | Frequency at TLC555-Q1 out, U1 | 240 | — | — | Hz |
| DOUT | Square wave duty cycle | Duty cycle of fPREC_PWM | 5 | — | 50 | % |
| DACC | Duty cycle accuracy | Accuracy of DOUT | 2% | — | — | — |
| fOSCL | Oscillator frequency | LED driver, TPS92691-Q1 | — | 390 | — | kHz |
| THERMAL | ||||||
| TA | Temperature range | Operating and ambient temperature | –40 | — | 125 | °C |
| PULSE TOLERANCE | ||||||
| Cold crank | Operational | |||||
| Jump start | Operational | |||||
| Jump start | Operational | |||||
| BASEBOARD | ||||||
| Number of layers | Two layers, single-side populated | |||||
| Form factor | 71 mm × 51 mm | |||||
6.3 Block Diagram
Figure 1. TIDA-01382 Block Diagram 6.4 Highlighted Products
6.4.1 LMT87-Q1
The LMT87-Q1 is a precision CMOS integrated-circuit temperature sensor with an analog output voltage that is linearly and inversely proportional to temperature. Its features make it suitable for many general temperature sensing applications. It can operate down to a 2.7-V supply with a 5.4-μA power consumption. Package options including a through-hole TO-92 package allows the LMT87-Q1 to be mounted onboard, off-board, to a heat sink, or on multiple unique locations in the same application. A class-AB output structure gives the LMT87-Q1 a strong output source and sink current capability that can directly drive up to 1.1-nF capacitive loads. This means it is well suited to drive an analog-to-digital converter sample-and-hold input with its transient load requirements. It has accuracy specified in the operating range of −50°C to 150°C. The accuracy, three-lead package options, and other features also make the LMT87-Q1 an alternative to NTC or PTC thermistors
Figure 2. LMT87-Q1 Functional Block Diagram 