“Under the combined action of wide operating voltage requirements, large transient voltages, and large temperature drifts in the automotive environment, Electronic systems are facing severe conditions. This article describes how to design multiple power supply voltages under conditions where performance requirements become more and more demanding. Meet the requirements of different parts of the automotive electronic system.
Under the combined action of wide operating voltage requirements, large transient voltages, and large temperature drifts in the automotive environment, electronic systems are facing severe conditions. This article describes how to design multiple power supply voltages under conditions where performance requirements become more and more demanding. Meet the requirements of different parts of the automotive electronic system.
Most mid-to-high-end cars currently produced are equipped with DVD-based GPS navigation systems as standard equipment (Figure 1). However, it can be confirmed that if you want to design a power supply for controlling different voltage rails in such a system, the complexity is no less than that of designing a power supply system for a notebook computer. A standard car navigation system may have 6 or more power supplies, including 8V, 5V, 3.3V, 2.5V, 1.5V and 1.2V. The 8V power supply is used to power the DVD motor that rotates the disc; this often requires up to 2A of peak current. The 5V and 3.3V power rails are usually the system bus, and generally require a current of 2A to 3A each. The 2.5V power rail is used for memory and I/O, so a current of 1A ~ 2A is sufficient. The 1.5V and 1.2V power rails are used to provide the CPU core and DSP core voltages, respectively. The power levels of these two power rails are generally between 3W and 5W.
Figure 1: Most mid-to-high-end cars are equipped with DVD-based GPS navigation systems as standard equipment.
At the same time, as the number of components in these systems increases, the available space is getting narrower. Therefore, in view of the fact that all actual radiators are so large that they are inconvenient to install, the importance of conversion efficiency has become more prominent due to space constraints and requirements for operating temperature ranges. Under conditions of low output voltages and medium current levels higher than a few hundred milliamps, it is no longer feasible to simply use a linear regulator to generate these system voltages. Therefore, in the past few years, mainly due to heat dissipation limitations, switching regulators have been gradually replacing linear regulators. The advantages of switching power supplies include higher efficiency and smaller footprint, which makes the increase in complexity and EMI issues less important.
If you consider the constraints of switching regulators in car navigation systems, they will need to have the following characteristics and characteristics:
Wide input working range
Good efficiency in a wide load range
Low quiescent current in normal operation, standby and shutdown conditions
Low thermal resistance
Lowest noise and EMI radiation
Let’s study these basic abilities in more detail:
1. Wide input working range
Any switching regulator needs to be specified to work within a wide input voltage range of 3V to 60V to ensure that it can meet the conditions of “cold start” and “load dump”. It also has the additional advantage of enabling these automotive systems to operate at 14V or 42V. Moreover, the rated voltage of 60V also provides a good margin for the 14V system that is usually clamped at 36V to 40V. In addition, the rated voltage of 60V also enables the device to be used in future 42V systems. This means that a current design for a 14V system can be easily upgraded to the requirements of a 42V system without any major redesign work.
In most automotive systems, it is essential to achieve high-efficiency power conversion within a wide load range. For example, in the load range of 10mA to 2.5A, the power conversion efficiency of a 5V output is required to reach about 85%. Under high current conditions, the internal switch needs to have good saturation, usually 0.1Ω at 3A current. In order to improve the light load efficiency, it is necessary to reduce the drive current or make it proportional to the load current. Moreover, the power for the internal control circuit can be provided through a bias pin, which can be powered by the output. This benefits from the power conversion efficiency of a buck converter. Since the bias current is absorbed from the output (rather than the input), the input power supply current required by the control circuit is reduced, and the drop is the ratio of the output to the input voltage. For example, an output current of 100μA at 3.3V only requires an average input current of 30μA at 12V. This minimizes the input current required by the control circuit and improves the efficiency level at light loads.
Table 1: High voltage, low quiescent current step-down DC/DC converter
3. Low quiescent current
There are many applications in automotive systems that require continuous power supply even when the vehicle is at a standstill. A key requirement for these applications is low quiescent current. Before the output current drops below about 100mA, the device will operate in a normal continuous switching mode. Below this current level, the switching regulator must skip several pulses in order to maintain a regulated state. The regulator can enter sleep mode between pulses, at this time only part of the internal circuit power supply. Under light load current conditions, the switching regulator needs to automatically switch to burst mode operation. In this mode, for a 12V to 3.3V converter, the quiescent current should drop below 100μA. In the sleep mode, the internal reference and the power good circuit will remain in operation to monitor the output voltage. In shutdown mode, the quiescent current should be less than 1μA.
4. Low thermal resistance
Ideally, the junction-to-case thermal resistance should be very low. If the back of the device is a bare copper surface and is soldered to the surface of a PC board, the PC board can be used to conduct heat away from the device. At present, four-layer circuit boards with internal power planes are commonly used to achieve a thermal resistance of about 40°C/W. High ambient temperature applications with good heat conduction to the metal case can obtain a typical junction-to-case thermal resistance value close to 10°C/W. This helps to extend the operating temperature range.
5. Concerning noise and EMI considerations
Although switching regulators generate more noise than linear regulators, their efficiency is much higher than the latter. In many sensitive applications, it has been proven that as long as the switching power supply operates in a predictable manner, the noise and EMI levels can be controlled. If the switching regulator operates at a constant frequency in normal mode, and the switching pulse edges are clean and predictable (no overshoot or high-frequency ringing), EMI will be suppressed to the greatest extent. The small size package and high operating frequency can achieve a compact and compact layout, which can minimize EMI radiation. In addition, if the regulator can be used with low ESR ceramic capacitors, it can minimize input and output ripples (they are additional noise sources in the system).
Obviously, the design and development of this type of switching regulator is not simple. However, in the past few years, Linear Technology has been committed to the work of this high-voltage DC/DC converter, and has a product library specifically designed to meet these requirements and with an increasing number of models ( Table 1).
The LT3434 is an example of this type of DC/DC converter recently launched. It belongs to a family of monolithic step-down switching regulators that are growing and can handle 60V. This device can solve many of the problems faced by the aforementioned car navigation applications. The LT3434 can operate in a wide input voltage range from 3.3V to 60V (Figure 2). It can provide high efficiency under load current conditions up to 2.5A. The reference accuracy is ±2% under all voltage, load and temperature conditions.
Because the device has a BurstMode operation function, its quiescent current is less than 100μA for 12V to 3.3V applications. The device adopts a small profile and flat TSSOP package with very low thermal resistance to achieve a small footprint design. Finally, it uses a current mode topology designed to achieve excellent transient response and easy compensation, and uses a patented circuit for maintaining a constant peak switching current under all duty cycle conditions. The switching frequency is a constant 200kHz, and the device can be synchronized to a higher frequency. It can provide strict voltage regulation within the automotive temperature range, and has power good/reset, soft start and UVLO (under voltage lockout) functions. At current levels up to 2.5A, the circuit provides a rugged, efficient, and small footprint solution.