Automotive electronic systems require low quiescent current

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Automotive electronic systems require low quiescent current [Copy link]

Today’s automotive electronic systems are becoming increasingly complex. At the same time, the automotive environment is a great challenge for any electronic product, as automotive electronic systems require a wide range of operating voltages and have large transient voltage and temperature variations. In addition, performance requirements are increasing, requiring multiple supply voltages to meet the different requirements of the system. A typical navigation system can have six or more different supply voltages, 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 smaller. Therefore, efficiency is a critical factor in situations where space and temperature requirements are very tight. At low output voltages, even at medium currents (above a few hundred mA), linear regulators cannot be expected to generate these system voltages. As a result, switching regulators have gradually replaced linear regulators over the past few years, mainly due to temperature limitations. The benefits of switching power supplies, such as improved efficiency and smaller pin footprints, offset their complexity and EMI considerations.
Given these constraints, the following features are required of a switching regulator:
- Wide input operating range
- High efficiency over a wide load range
- Low quiescent current in normal operation, standby, and shutdown
- Low thermal resistance
- Minimum noise and EMI.

The following are specific features:

Wide input operating range
Any switching regulator needs to be able to operate over an input voltage range of 3V to 60V and must be able to be used in 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 allows 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 over a load range of 10mA to 1.2A requires a 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 to the internal control circuitry can be provided through a bias pin that draws power from the output. This takes advantage of the power conversion efficiency of the buck converter. This bias current comes from the output rather than the input, reducing the input supply current required by the control circuitry by the ratio of the output to the input voltage. For example, a 100uA output current at 3.3V requires only a 30uA average input current 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 vehicle 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, and the regulator can go into a sleep state between pulses, with only some of the internal circuitry powered. Under light load current conditions, the switching regulator should automatically switch to burst mode operation. In this mode, the quiescent current requirement of the 12V to 3.3V converter should drop below 100uA. The built-in reference and power good circuitry is also enabled in the sleep state and can sense the output voltage. The quiescent 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 back of the device is exposed copper and 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 boards 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 close 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. Noise and EMI levels have proven to be manageable in many sensitive applications as long as the switching power conditions are predictable. 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 size and high operating frequency can provide a small and compact layout, 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 has high efficiency 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 topology to ensure good transient response and simple compensation, and a patented circuit can maintain 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 power good/reset, soft start, UVLO and other functions. This circuit provides a powerful, effective and small pin solution for automotive buck systems with currents up to 1.2A, allowing designers to design a direct and simple solution for complex automotive applications.