Start/Stop and Cold Crank Applications

Other Parts Discussed in Post: LM26003, LM26420, LM3481, PMP9477

Written By: Martin Moss and Xiang Fang

The heavy use of cars in high populated areas has cities looking into alternative transportation such as trains, buses or e-bikes.  To keep up with environmental regulations and concerns, the automotive industry continues to evolve in using technology to help make cars more efficient. This has been further advanced by government policy and installing regulations and standards for emissions. More popular in Europe, the trend is becoming a global standard as more manufacturing are adopting the technology.

Cold Crank Simulator: 

There are many integrated circuits for the automotive environment. To show the performance of these devices, we have designed a "Cranking Test Pulse" simulator that generates a test pulse to show the battery voltage collapse during the cranking of the engine. As the car battery voltage can dip down below 5.0V, this can lead to problems with the system’s circuits. TI offers solutions to specifically handle these conditions. In order to address this problem, in most cases, a boost is needed such as the LM3481. This device is placed before the converter circuit to ensure the voltage droop does not cause unacceptable behavior during this very short cranking period, for example in navigation,  infotainment systems or the instrument cluster. Vehicle occupants expect these systems to continue operating during cold cranking. In developing such a solution, the voltage drop must be tested to ensure the solution reacts fast enough during the cranking operation.

Each automotive manufacturer has unique cranking profiles to simulate such a condition in their vehicles. Figure 1 represents an outline of our cranking simulator and some of the test pulses.

Figure 1

Typical cold crank profile

The standard power supply in a conventional vehicle is typically a lead-acid-type battery which is constructed from six 2.1V galvanic cells in series providing a nominal 12V (actually 12.6V). In normal operation, the vehicle’s battery voltage typically varies between 9 and 16V. 

Texas Instruments Cranking Simulator

 Cranking Reference Design

The reference design is a conducted EMI-optimized multi-output power supply for automotive driver information units. Its power supply has a pre-boost stage for cold crank and start-stop operation, and two buck regulators. Demonstrating compliance to an EMC standard shows that the devices first do not generate a high amount of noise in the system but principally it shows the designer will have fewer issues with system integration. This is getting tougher now as more electronics are integrated into the modern automobile.

The pre-boost stage features the LM3481 boost controller, and will start to operate and regulate a 9.5V output when the input voltage drops below 9.5V. In normal or higher input voltage conditions, the LM3481 is in standby mode without switching bypassing the input to the following buck stage. The design has two buck regulators to generate three output voltages: the first stage buck generates a 5V, 2A output using the LM26003 and the second stage generates two outputs of 2.8V, 2A and 1.8V, 2A using the LM26420 with 15W max output power. The LM26003 is a Wide Vin non-synchronous buck regulator, and the LM26420 is a 5V input dual 2A, high frequency synchronous buck regulator. All three ICs are qualified in AEC-Q100 Grade 1. They also demonstrate a highly compact, robust, reliable and low noise solution to overcome dips on the battery voltages.

The board layout is optimized for improved conducted EMI performance. The power inductor in the pre-boost stage is utilized as an input EMI filter, and thus no additional filter components are required. The board is tested under the automotive EMC standard, CISPR 25, and its conducted emissions are in compliance with the CISPR 25 Class 5 requirements.

The design’s input voltage range is 4.5V to 38V, making it suitable for 12V battery systems. When the input voltage goes as low as 4.5V, the output power should be limited to 10W in total, which is a current limit of the LM3481 boost controller. There’s a PFET in the input power path to control the turn on/off of the power supply via pin jumper. If the function is not desired, the PFET can be bypassed and the minimum input voltage will be further extended to 3.5V.

PMP9477.1 Cold Crank Reference Design using LM3481, LM26003 and LM26420

Lower Conducted Emissions Assist System Integration

The tested board has the common mode choke and the filter inductor bypassed. The conducted emissions are tested under the CISPR 25 standards. The frequency band examined spans from 150 kHz to 108 MHz covering the AM, FM radio bands, VHF band, and TV band specified in the CISPR 25.

The test results are shown in Figure 2. The limit lines in red are the Class 5 limits for conducted disturbances specified in the standard. The figures below show the conducted EMI noise from 150 kHz to 30 MHz using peak detector and the limit lines are the Class 5 average limits. The peak measurement result is well below the average limits. The Figures also show the noise scan result from 30 MHz to 108 MHz using peak and average detector, with the Class 5 peak and average limits respectively. Therefore, the power supply board is in compliance with the CISPR 25 Class 5.

Figure 2 - Conducted EMI noise from 150 kHz to 30 MHz


 Figure 3 - Conducted EMI Noise for 30 MHz to 108 MHz

  • "There’s a PFET in the input power path to control the turn on/off of the power supply via pin jumper. If the function is not desired, the PFET can be bypassed and the minimum input voltage will be further extended to 3.5V."

    Could you share more Information about this?

    From the battery input Vbat+ towards to the LM3481 input is only Si7465DP P-MOSFET.  It has Rds ON about 0.08Ohm at 25°C. At max. load (9.5V/1.5A) and with worst case efficiency about 80%  the input current will be around 5A (9.5V x 1.5A = 14.25W / 0.8 = 17.3W => 17.3/3.5W ~ 5A).

    Voltage drop on the MOSFET will be then ~0.4V.  Where will be the residual 0.6V voltage drop used?

  • Dear, Sir.

    Thank you so much this kind of valuable information.

    My customer is considering to realize pre-boost during cranking phase.

    So it is quite helpful for us.

    I would like to just know the bottle-neck why the minimum input voltage is limitted to 3.5V

    even if without external PFET. Because Vin min. for LM3481 is 2.97V.

    Best Regards,

    H. Sakai