Low quiescent current meets automotive electronic system requirements

frozenviolet

Low quiescent current meets automotive electronic system requirements [Copy link]

Today's automotive electronic systems are becoming more and more complex. At the same time, the automotive environment is a great challenge for any electronic product, because automotive electronic systems require a wide operating voltage range and have large transient voltage and temperature changes. In addition, performance requirements are becoming increasingly higher, requiring multiple supply voltages to meet the different requirements of the system. A typical navigation system can have six or more different power supplies, including 8V, 5V, 3.3V, 2.5V, 1.8V, and 1.5V.

At the same time, while the number of components has increased, space has become increasingly limited. Therefore, efficiency is a critical factor in situations where space and temperature requirements are very high. At low output voltages, and even at medium currents (above a few hundred mA), linear regulators cannot be relied upon to generate these system voltages. As a result, switching regulators have gradually replaced linear regulators over the past few years, primarily due to temperature limitations. The benefits of switching power supplies, such as increased efficiency and a smaller footprint, offset their complexity and EMI considerations.

Considering these limitations, the following features are required for a switching regulator:

Input operating range should be wide

High efficiency over a wide load range

Low quiescent current in normal operation, standby and shutdown

Thermal resistance should be low

Noise and EMI should be minimized.

The following is a detailed description of the above features:

Input operating range should be wide

Any switching regulator needs to be set up to operate over an input voltage range of 3V to 60V and must be able to be used in either 14V or 42V automotive systems. The 60V rating provides good headroom for 14V systems, which are typically in the 36V to 40V range. In addition, the 60V rating enables the device to be used in future 42V systems. This means that designs made today for 14V systems can be upgraded to 42V systems without much redesign.

efficiency

High efficiency power conversion over a wide load range is critical in most automotive systems. For example, a 5V output with a load range of 10mA to 1.2A requires power conversion efficiency of around 85%. At high currents, the internal switches need to have good saturation, typically 0.2 at 1A. To improve low load efficiency, the drive current needs to be reduced or adjusted to be proportional to the load current. Alternatively, power for the internal control circuitry can be provided through a bias pin that is powered from the output. This takes advantage of the power conversion efficiency of the buck converter. The bias current is taken from the output rather than the input, reducing the input supply current required by the control circuitry by the ratio of the output to input voltage. For example, a 100uA output current at 3.3V requires only an average input current of 30uA at 12V. This minimizes the input current to the control circuitry while also improving light load efficiency.

Low quiescent current

There are many applications in automotive systems that require continuous power, even when the car is parked. The most important requirement for these applications is low quiescent current. The device can operate in normal continuous switching mode until the output current drops to about 100mA. Below this level, the switching regulator must skip pulses to maintain regulation. The regulator can enter a sleep state between pulses, when only part of the built-in circuit is powered. Under light load current conditions, the switching regulator needs to automatically switch to burst mode operation. In this mode, the 12V to 3.3V converter requires that the quiescent current should drop below 100uA. The built-in reference and power good status circuits are also enabled in the sleep state and can detect the output voltage. The static circuit should be less than 1uA in the shutdown state.

Low thermal resistance

Ideally, the junction-to-frame thermal resistance should be low. If the device has exposed copper on the backside and is soldered to the surface of the PCB, the PCB can be used to dissipate heat away from the device. Most current four-layer boards with built-in power planes can achieve thermal resistances in the 40 C/W range. High ambient temperature applications that can dissipate heat well into the metal case can achieve junction-to-frame thermal resistances closer to the typical 10 C/W. This helps extend the useful operating temperature range.

Noise and EMI Considerations

Although switching regulators generate more noise than linear regulators, they are much more efficient. As long as the switching power supply conditions are predictable, noise and EMI levels have proven to be manageable in many sensitive applications. EMI can be minimized if the switching regulator switches at a constant frequency under normal conditions, and the switching edges are clean and predictable, without overshoot or high-frequency ringing. Small package sizes and high operating frequencies can provide small and compact layouts, thereby minimizing EMI. In addition, if the regulator can be used with low-ESR ceramic capacitors, both input and output voltage ripple can be minimized, which are another source of noise in the system.

The LT1976 from Linear Technology is the latest in a family of 60V monolithic step-down switching regulators. The device meets several important requirements for automotive applications. The LT1976 has a wide input voltage range of 3.3V to 60V. It is highly efficient at load currents up to 1.2A. The reference accuracy is +/- 2% over a wide range of line, load and temperature conditions. Due to its burst mode feature, the quiescent current is 90uA in 12V to 3.3V applications. The device is packaged in a small, flat TSSOP package with very low thermal resistance, allowing for a very small footprint. Finally, it uses a current mode layout to ensure good transient response and simple compensation, and a patented circuit maintains a constant peak switch current for all duty cycles. The switching frequency is a constant 200kHz, and the device can be synchronized to higher frequencies.

It provides accurate regulation over the automotive temperature range and includes features such as power good/reset, soft-start, UVLO, etc. This circuit provides a robust, effective and small pin solution for automotive buck systems with currents up to 1.2A, allowing designers to design a straightforward and simple solution for complex automotive applications.

chewang

Thank you for your hard work.

aiqinghai

Literacy post!