Driver-monitoring systems need a new breed of IR-LED drivers

By Nazzareno (Reno) Rossetti and Yin Wu, Maxim Integrated

Driver-monitoring systems (DMS) are becoming common in modern automobiles. Infrared (IR) cameras, utilizing an IR-LED in combination with a photo sensor, help recognize the hazardous microsleep that can affect motorists. DMS is also an enabling technology for the advancement of autonomous vehicle (AV) driving. In situations in which the driver needs to take back control from the car, the monitoring system will give the driver adequate time to react.

All of these functions and their associated electronics must fit seamlessly inside the car, which creates the need for flexible, small, and efficient solutions. They must also be able to cope with harsh automobile electrical environments.

In this design solution, we review an IR camera system and discuss the shortcomings of a typical solution. Subsequently, we present an IR-LED driver IC that is flexible, compact, and efficient while interfacing directly to the car battery.

Fig. 1: Driver-monitoring system in action.

The infrared advantage
Some key advantages of infrared light are its invisibility to the human eye and its ability to work day and night. In addition to DMS, IR cameras can also detect and classify pedestrians in darkness, through most fog conditions, and are unaffected by sun glare, delivering improved situational awareness that results in more robust, reliable, and safe advanced driver-assistance systems (ADAS) and AV solutions. Other ADAS applications include seat-occupancy recognition, night-vision systems, short-range detection of surroundings, and monitoring drivers’ blind spots.

Infrared camera
Fig. 2 shows the main elements of an infrared camera. The IR-LED illuminates the target. The reflected light is collected by the image sensor (CCD or CMOS photodiode) and processed by the vision processor to determine the response to the situation at hand.

Fig. 2: IR-LED camera for DMS.

Buck LED driver for DMS
The LED driver controls the IR light intensity and strobes it at the right frequency and duty cycle. Ideally, it must work directly off of the 12-V battery and withstand the harsh automotive environment.

Vehicles that employ start/stop technology experience large battery voltage dips when the engine starts, causing the battery voltage to drop well below the typical 12 V. Starting from cold conditions (cold-crank), the battery can dip as low as 4.5 V. Disconnecting the battery from the alternator during operation results in large voltage transients (dump) up to 60 V.

The automotive environment is also subject to electromagnetic interference (EMI) due to both external and internal sources. The “arc and spark” noise that comes from ignition components, motors, and similar pulse-type systems affects the supply voltage rails by producing disruptive undervoltages or overvoltages. The IR-LED buck converter, with its fast switching waveforms, should be able to mitigate any contribution to this noisy environment.

Given the typical forward voltage for an IR-LED diode of 2.4 V and a forward current of 1 A, a well-designed buck LED driver has enough headroom to be directly connected to the battery without the need for voltage boosting. It must also withstand dump voltage and introduce minimum electromagnetic noise.

Typical high-power buck IR-LED driver solution
A typical buck IR-LED driver solution is shown in Fig. 3 . It utilizes an n-channel transistor (typical R_DS(ON) = 0.3 Ω) and a non-synchronous architecture that relies on the Schottky diode (D) for current recirculation. The latter is a sure sign of an inefficient implementation.

Fig. 3: Typical non-synchronous buck IR-LED driver.

Consider the typical automotive case in which the input voltage is the car’s 12-V battery and the output is the forward voltage of an IR-LED diode (2.4 V at 1 A). Here, the buck converter duty cycle is only 20%. This means that the MOSFET in Fig. 3 conducts for only 20% of the time (0.3 Ω at 1 A = 0.3 W), while the Schottky (0.5 V at 1 A = 0.5 W) conducts for 80% of the time. The total power dissipated in the power train is 0.46 W, mostly due to the Schottky diode. After accounting for switching and other losses, this solution barely reaches an efficiency of 80%.

Integrated synchronous rectification solution
As an example, the MAX20050 synchronous buck LED driver is an ideal solution (Fig. 4 ). The device includes a unique spread-spectrum mode that reduces EMI at the switching frequency and its harmonics. With its 4.5-V to 65-V input supply range, the IC can easily operate under start/stop conditions and cold-crank. It can withstand battery load dump, making it ideal as the front-end buck converter connecting the IR-LED driver directly to the car battery.

Fig. 4: IR-LED driver integrated, synchronous solution.

High efficiency
The fully synchronous, 2-A step-down converter integrates two low-R_DS(ON) 0.14-Ω (typ) n-channel MOSFETs, ensuring minimum ohmic losses. Here, 0.14-Ω R_DS(ON) resistances will produce losses of only 140 mW, one-third of the previous case. This solution can easily achieve high efficiency. In Fig. 5 , the synchronous solution achieves peak efficiency of 86% at 2.1 MHz and 92% at 400 kHz! Increasing the frequency to 2.1 MHz reduces the bill-of-materials (BOM) size at the expense of a few percentage points of efficiency while avoiding interference within the AM band.

Fig. 5: Efficiency vs. LED current.

Small size
The high level of integration of this solution yields minimal PCB area occupation. Fig. 6 shows a non-synchronous buck converter IC requiring an external Schottky diode that occupies a PCB area almost double (78%) that of the single-chip solution.

Fig. 6: Size comparison of non-synchronous vs. synchronous buck ICs.

Flexibility
For maximum flexibility, a family of IR-LED drivers (Table 1 ) offers two operational frequencies to address efficiency-versus-size trade-offs and to provide internal-versus-external loop compensation for dynamic response optimization.

Table 1: IR-LED driver family.

The devices are specified for operation over the full –40°C to 125°C temperature range and are available in thermally enhanced 12-pin (3 × 3-mm) TDFN and 14-pin (5 × 4.4-mm) TSSOP packages with an exposed pad.

Conclusion
Driver-monitoring systems are appearing more frequently in modern automobiles. They must fit seamlessly within the automotive electronic system, creating the need for flexible, small, and efficient solutions. They must also cope with harsh automobile environments.

About the Authors:
Nazzareno (Reno) Rossetti is an Analog and Power Management expert at Maxim Integrated. He is a published author who holds several patents in this field. He has a doctorate in electrical engineering from Politecnico di Torino, Italy.

Yin Wu, MBA, MSEE at Maxim Integrated, is a semiconductor business professional. He holds a master’s in business administration from Santa Clara University and a master’s degree in electrical engineering from San Jose State University.