June 24, 2020
| By: Szukang Hsien
Executive Business Manager, Automotive Business Unit
| and Reno Rossetti
Power Blogger; Maxim Integrated
New vehicle models are increasingly equipped with multiple displays (Figure 1) for different functions. These "infotainment" displays may include an instrument cluster, a central information display, mirror replacement displays, and multiple rear-seat entertainment displays. Since an instrument cluster display provides relevant safety information to the driver, its electronics must be robust enough to not only withstand a harsh automotive environment (cold/hot crank, load-dump, start/stop), but also incorporate additional diagnostics required to meet ASIL (Automotive Safety Integrity Level) safety standards, have low electromagnetic interference (EMI) to reduce interference with multiple RF receivers in the vehicle, and be small enough to fit additional electronics within the same space.
In this blog post, we'll review the main challenges of TFT-LCD backlight designs, with an emphasis on how they can meet ASIL safety standards. We'll also examine how they can operate without display flickering in the presence of low battery voltage (cold crank) while maintaining low EMI and a smaller size. Finally, we will introduce a TFT-LCD backlight driver IC that meets these criteria for automotive displays.
Figure 1. Multiple displays in the car.
Meeting Functional Safety (ASIL) Requirements
ISO (International Organization for Standardization)-26262 is one regulation that drives the requirements for functional safety. It addresses possible hazards caused by malfunctioning behavior of electrical safety-related systems, which includes the interaction of these systems. The level of functional safety required for a system is categorized by the system's ASIL rating, which ranges from level A to level D. Level D requires the most robust system. A system's ASIL rating is determined by the severity of potential injury, the controllability of failure, and the exposure to risks if a failure occurs. ASIL-compliant ICs include performance analysis. They are designed with tighter protections and higher accuracy, and have redundant references. They have fail-safe on open pins and supervisory circuitry to provide the detection, diagnosis, and validation necessary to make systems compliant. The integration of I2C communication capability into the IC facilitates control and diagnostics. Sample diagnostics include:
- Overvoltage/undervoltage detection on outputs
- Error correction on internal memory, if present
- Parity bit or CRC (cyclic redundancy check) on any interface for error detection/correction
The inclusion of these functions in a single IC enables the system to more easily reach the ASIL B level of integrity.
Cold crank occurs when the ambient temperature is too low, diminishing the battery's ability to deliver power, which causes the battery voltage to fall under the large current drawn to start the engine. The display's electronics operation under cold crank is essential to avoid flickering.
Figure 2 displays a typical cold-crank timing diagram from ISO 16750-2. It specifies battery operation down to 2.8V (worst case) for 15ms, before recovery to a value of 6V, which could remain for a couple of seconds. Since the instrument cluster display is already operational at cranking, it is important that the TFT-LCD backlight operates correctly at this precise point.
Figure 2. Cold crank detail per ISO 16750-2, Level III
Meeting this specification is challenging. The TFT-LCD backlight driver needs a continuous 5V to drive the external boost MOSFET, but during cold crank, the battery voltage is unable to support 5V operation. A typical solution has been to pass this problem over to somebody else.
In one solution, the backlight chip has a dedicated pin so that the system designer can feed an external 5V to the driver. If an external 5V rail is not available, another IC, for example an LDO with sufficient hold-up capacity, needs to be added to the BOM. This results in extra cost and a larger PCB area. If the external rail is present but is too low, an internal charge pump is provided at the cost of extra silicon and additional external capacitors that must perform the voltage boost. In all cases, the available solutions appear at best to meet typical 3V crank specification, not the 2.8V minimum requirement.
Demands for Small Size
A high level of integration is necessary to reduce component size to fit additional electronics within the same space and to reduce cost. Advanced monolithic processes help reduce the die size and enable the use of a smaller package. The integration of additional functions on-chip reduces the overall BOM while also minimizing PCB size and cost.
Backlight Driver IC That's Up to the Challenge
One device that's up for the challenges we've discussed is the MAX25024, a 4-channel backlight driver IC with a boost controller for automotive displays (Figure 3). The four integrated LED current outputs can sink up to 150mA each, allowing the IC to power an 8in. or 10in. backlight display. The output pin (BOOST) can sustain up to 52V. The device also accepts a wide input voltage range and withstands up to 40V automotive load-dump events.
Figure 3. 4-channel backlight driver with boost controller
The backlight driver IC features I2C-controlled pulse-width-modulation (PWM) dimming and hybrid dimming. In either case, the minimum pulse width is 500ns. Phase-shifted dimming of the strings and spread spectrum are incorporated for lower EMI. The IC is available in a 24-pin TQFN or 24-pin side-wettable TQFN (SWTQFN) package and operates over the -40°C to +125°C temperature range.
Meeting ASIL B Specifications
Comprehensive diagnostic information is available through the MAX25024 I2C interface to ease integration in systems that require ASIL B compliance. Features for ASIL B systems include voltage reference redundancy, individual LED current measurement on each string, boost input current measurement, boost output voltage measurement, LED open/short detection and protection, along with boost output undervoltage and overvoltage.
Exceeding Cold-Crank Specification
The MAX25024 accepts a wide 2.5V to 36V input voltage range that meets and exceeds the 2.8V minimum cold-crank specification. A unique circuit architecture (see Figure 4) allows the IC to achieve this level of performance. An on-board LDO, normally powered by the battery (IN pin), provides 5V to the driver (DR). When an undervoltage spike is sensed, the LDO input is switched to the output (BOOST pin). The output capacitor (COUT) now can sustain the LDO operation during the 15ms specified cold crank.
Figure 4. 5V LDO bias during cold crank
Figure 5 shows a 2.5V, 100ms cold-crank spike (see yellow curve) that doesn't affect the output voltage (VBOOST) and LED current. Accordingly, the fault flag (FLTB) remains de-asserted.
Figure 5. Cold-crank test: 9 LEDs/string, 100mA/string x 4, 400kHz, VIN = 2.5V (100ms)
The on-board LDO consumes minimal power, and the package's low thermal resistance assures a very low junction temperature. In Figure 6, the picture on the left shows an IC temperature of 39.8°C before the crank, with a 12V battery voltage. The picture on the right shows an IC temperature of 40.3°C immediately after a 100ms crank down to 2.5V, for a net temperature rise of less than 1°C.
Figure 6. IC temperatures with 9 LEDs/string, 100mA/string x 4, 400kHz before and after a 100ms crank
The MAX25024 is also tested for electromagnetic radiation according to EMI CISPR 25 Class 5 specifications. As an example, Figure 7 shows one of many EMI tests that were performed with a 200MHz to 1GHz log-periodic (horizontal) antenna. The IC's emission levels are well below the limit.
Figure 7. Meeting the CISPR 25 Class 5 EMI specification
Reduced Size and Cost
With its high level of integration, the MAX25024 provides multiple functions in a small 4mm x 4mm TQFN package. By comparison, competitor devices are housed in a 7mm x 7mm package, which is a 3x increase in PCB size occupancy. The external NMOS switch disconnects the battery from the output in the case of a shorted output. The NMOS is turned on by means of an integrated charge pump, avoiding the use of extra pins or external capacitors. A small and cost-effective NMOSFET, as opposed to the large PMOSFET found in competitor solutions, is used in the MAX25024 to minimize BOM size and cost. These advantages yield a PCB size that is 45% smaller than other solutions.
TFT-LCD displays are ubiquitous in modern automobiles. Their backlight must operate under harsh conditions, meet functional safety requirements, and be small in size. The MAX25024 backlight driver IC not only meets ASIL B functionality, but exceeds cold-crank specification and has low EMI, all in a small package.
Take the Next Step
To see how the MAX25024 can enhance your automotive display design, check out the MAX25024EVKIT evaluation kit. The kit operates from a DC supply voltage between 2.5V and 36V. Its switching frequency can be set at 2.2MHz or 400kHz and it operates in I2C mode.
About the Authors
Szukang Hsien is the executive business manager for automotive display power and gesture solutions at Maxim Integrated. Prior to Maxim, he served in roles including mixed-signal designer, automotive infotainment systems marketing engineer, and strategic marketing manager for high-voltage DC-DC products. He has six US patents and has published eight technical/conference papers. "Redefining the possible" is his motto.
Nazzareno (Reno) Rossetti is an analog and power management expert at Maxim Integrated. He is a published author and holds several patents in this field. Reno holds a doctorate in Electrical Engineering from Politecnico di Torino, Italy.