Transportation System Requirements
The input voltage to a transportation system may be as high as 14V (single-battery powered car), 28V (dual-battery powered truck, passenger car and airplane), or higher voltage, and its digital system requires one or more low voltage rails. Class system, you need to understand how to step-down from a very high input voltage easily, efficiently, and reliably. Figure 1 below shows that the input voltage in an automotive environment may change as its operating state changes, and its operating conditions may include Load dump changes to the cold car started a variety of circumstances, and even battery reverse connection.
NOMINAL 14V: nominal 14V120V LOAD DUMP: 120V load dump 6V CRANK: 6V cold start 85V NOISE: 85V noise 24V JUMP START: 24V Sudden start REVERSE BATTERY: battery reverse connection
Switching regulators are helpful when the application requires power conversion with very high efficiency to minimize the heat loss caused by power dissipation during the conversion Switching regulator is essentially a monolithic device, The MOSFETs are integrated on-chip with synchronous or asynchronous configuration. Alternatively, the switching regulator can also be composed of a switching controller that drives an external MOSFET in a single-stage or multi-stage topology (multiphase) to provide Tens of amps to hundreds of amps To meet such a large power range requirement, Analog Devices offers a wide range of switching regulator solutions that allow users to choose the most appropriate design for the final system according to specific design criteria Accordingly, our switching regulators have a very wide input voltage range (from 5V up to 150V) and output power levels from hundreds of milliamps up to over 1,000A.
An example of such a switching regulator is the LTC3895, a synchronous step-down converter with 150V input that can be configured for multi-phase operation, as shown in Figure 2.
VOUT FOLLOWS VIN WHEN VIN < 12V: VIN < 12V 时, VOUT 跟随 VINEFFICIENCY: 效率POWER LOSS: 功率损耗LOAD CURRENT: 负载电流
One common question in any transportation system is how to get a solution that has a high buck ratio and a small footprint without compromising performance and conversion efficiency. Until recently, no solution was available All key performance standards are achieved without sacrificing performance, but with the introduction of Analog Devices' single, 2MHz, synchronous buck converter LT86xx family, all required performance standards are immediately met.
A good example of this is the LT8609, a 2A, 42V input synchronous step-down switching regulator. The unique synchronous rectification topology provides 93% efficiency, while switching at 2MHz frees the designer from critical noise sensitivity Band, such as AM radio, while providing a very compact solution footprint. Burst Mode® operation maintains quiescent current below 2.5μA for no-load standby, making the device ideal for always-on Pass system. The LT8609's 3.0V to 42V input voltage range makes it ideal for automotive applications. Such applications must stabilize cold start and stop-on conditions with a minimum input voltage down to 3.0V and pass steady over 40V Load Transient. The internal 3.5A switch provides continuous output current up to 2A at peak load current of 3A. The schematic and the efficiency curve corresponding to the 2MHz switching frequency are shown in Figure 3.
Due to the fact that cold-crank and load dump conditions are common in single-cell or dual-battery vehicles, many transportation systems provide a wide range of input voltages and, further complicating matters, the required output voltage Beyond this already wide input voltage range.Therefore, the complex problem facing system designers is that the solution must be designed to allow a fixed output regardless of whether the input voltage is above, below, or equal to the output voltage.
A common solution to this problem is to use a SEPIC topology converter, but the design of this converter is complex and requires two inductors, and usually has low space efficiency and conversion efficiency. Therefore, Analog Devices designs a wide range of 4 switching buck-boost controllers that not only simplify the design but also provide high space efficiency and conversion efficiency with power losses between 5% and 7% (depending on the input to output voltage range ) The LT8705 shown in Figure 4 is an example of a 4V to 80V input buck-boost controller that provides a fixed 12V output common to the vehicle environment.
INCREASED VOUT RIPPLE FOR VIN> 60V: VOUT ripple increases when VIN> 60V
Another solution to the problem of a car's cold crank is to use a step-up converter followed by a step-down converter in which the output from the cell-provided step-up converter Which is several volts above the nominal battery voltage and is then stepped down with a buck converter to reach the operating voltage required for downstream electronic components.Although this approach requires two converters, the ADI The company has developed a device that combines a step-up controller and a step-down controller that can be used standalone or as a step-up / step-down follower. The LTC7813 in Figure 5 illustrates this The working principle of the device.
8V TO 60V DOWN TO 2.2V AFTER START-UP: 8V to 60V, drops to 2.2V after startup
Low noise power management
Electromagnetic Emissions (EMR), Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC) These terms relate to the energy of charged particles and related magnetic fields that may affect circuit performance and interfere with signal transmission. With the proliferation of wireless communications, a large number of Emerging and growing use of communications equipment, occupying more and more frequency spectrum (some bands overlap), electromagnetic interference has become a lingering fact.In order to reduce the impact of electromagnetic interference, many government agencies and regulatory agencies Have set the communications equipment and instrumentation electromagnetic radiation limits.
It is therefore clear that low emissions are a key requirement for many automotive and transportation equipment manufacturers.How can system designers meet the stringent requirements of CISPR 25, Class 5 (shown in Figure 6), while still maintaining high efficiency and Small solution size?
AMPLITUDE: Amplitude VERTICAL POLARIZATION: Vertical Polarization PEAK DETECTOR: Peak Detector CLASS 5 PEAK LIMIT: Class 5 Peak Limit FIXED FREQUENCY MODE: Fixed Frequency Mode SPREAD SPECTRUM MODE: Extended Spectrum Mode FREQUENCY: Frequency
One answer may be ADI's Silent Switcher® family of devices, the LT8614, for example, as a 42V input, 4A output, monolithic buck converter with switching frequencies above 2MHz and 94% conversion efficiency Switching due to its short 30ns minimum on-time to meet the 16V input to 1.8V output buck ratio.In addition, thanks to its patented Silent Switcher technology, so the device can surpass CISPR 25 and CISPR 22 Class B radiation requirements, as shown in Figure 7.
NOISE FLOOR: Noise Floor FREQUENCY: Frequency RADIATED NOISE LEVEL: Radiated Noise CISPR 22 RADIATED CLASS B LIMITS: CISPR 22 Class B Radiation Limits
Low quiescent current is also a key requirement
In transportation electronics systems, there are many applications that require continuous power, even after the vehicle has been parked, such as remote keyless entry, security and even personal infotainment systems, which often include navigation, GPS positioning and emergency call systems. It is difficult to understand why these systems must remain connected even when the vehicle is not in operation, although it is understandable that the GPS system must "always be on" to cope with emergencies and ensure safety. This is necessary in order to Use basic control functions by external operation when needed.
A key requirement for this type of application is low quiescent current to extend battery life. Since 2010, Analog Devices has been manufacturing switching regulators with standby quiescent current of less than 10μA. The standby quiescent current of some of our recently introduced products Have been below 2μ A. Therefore, these products have been applied to many automotive electronic systems are well prepared.
in conclusion
Analog Devices offers a wealth of switching regulator products that meet all the requirements of switching regulators and are uniquely suited to a wide range of transportation systems with features that include:
• Wide input voltage range of 2.xV to 150V • Low quiescent current in standby mode: typically less than 10μA in standby mode • Minimal output noise and very low EMI / EMC radiation • Extended temperature range: 150 ° C Guaranteed Operation at Ambient Temperature and Junction Temperature • High Efficiency: Up to 97% at Full Load and Up to 80% at Light Load • Low Thermal Resistance Package: Down to 10 ° C / W (θjc) • High Step-Down, High switching frequency operation up to 4MHz • High current density: Up to 5A continuous output current in 3mm x 5mm MSOP package • Industry-leading FIT rate: typically less than 0.2