Step-Down Switching Regulators and LDOs for Industrial and Automotive Applications

Autonomous control is extremely common in industrial and automotive applications, and much of the autonomous control relies on current loops. In industrial applications, for example, current loops are common in control loops operating in both directions. The loop transmits measured values from the sensor to the programmable logic controller PLC, which transmits control outputs from the PLC to the process modulation device.

In such systems, the power supply for the current loop is, as a rule, supplied through a linear regulator. Although this has traditionally been the case, linear regulators have drawbacks, with relatively low efficiency and limited current capacity. In general, if the input and output voltage are close, the linear regulator can also achieve high efficiency. Still, if not close, the voltage drop is too large, and the energy consumed in the linear regulator will increase quite a bit, which then easily affects the efficiency. Once the efficiency becomes low, it will bring various control system function problems.

Step-down switching regulators instead of linear regulators

Compared to linear regulators, step-down regulators offer higher current capacity, wider input range, and thus improved system efficiency, so replacing linear regulators in many current loop systems with step-down regulators is a good way to improve performance.

In a standard 4 mA to 20 mA current loop, the circuit is typically designed to draw power directly from the current loop without needing an additional power supply. If no additional features or functionality are added, then a conventional linear regulator is perfectly capable of doing the job. But linear regulators that cannot provide additional current capability become limited when various features are added to the system. High load capacity buck switching regulators do not have this limitation, and higher efficiency can reduce the thermal burden of the loop system design.

On the other hand, in industrial and automotive grade applications, the voltage at the sensor side may go very high in transient devices, which is the situation we mentioned above where the input and output voltage are not close. This situation is not uncommon in industrial as well as automotive grade applications. Only a buck switching regulator with a wide enough voltage range can simplify the demanding design requirements of automotive and industrial applications where large voltage transients may occur.

Low quiescent current and high-efficiency buck regulators close the current loop.

In 4 mA to 20 mA current loop applications, low quiescent current, high efficiency, and wide input range are all essential if a step-down switching regulator is to be used in place of an LDO. At the same time, the components must operate reliably with high accuracy and low power consumption over the extended industrial range of -40°C to +125°C.

The diagram above shows a loop built with ADI's LT8618 series. The LT8618 series of the high-speed synchronous monolithic buck switching regulators can efficiently deliver up to 100mA to the output at a constant frequency (even up to 2.2MHz). Unlike LDOs, buck regulators can multiply the input current provided to the load. The additional load capability greatly increases the additional design space and enlightenment, simplifying part of the system design work. All necessary circuits in these devices are generally equipped with top and bottom power switches to reduce the need for external components, which is a clear advantage in simplifying design difficulties.

Also, in this series, for example, the buck regulator consumes only 2.5μA of quiescent current in transient mode over a wide input voltage range of up to 65V (transient conditions only, 60V under continuous operation), and the ultra-low quiescent current keeps the output ripple well under control. Under transient conditions, LDO regulators are very inefficient, and buck regulators are also very efficient at high step-down ratios. Synchronous buck efficiency like the LT8618 series can reach more than 90% (under ideal conditions).

Low on-state and ripple limiting

The reduced on-time helps to obtain a larger buck ratio, and a low on-time buck regulator means that direct buck conversion from different voltage level inputs to lower voltage rails is supported, thus reducing system complexity and cost. The performance benefits of devices that consistently maintain low on-time is more pronounced at high switching frequencies.

On the other hand, surge and ripple are important considerations when using such devices. To suppress ripple, the input capacitance of the buck regulator is critical. The best choice is the smallest input capacitance that meets the ripple current and ripple voltage requirements. Some will also be optimized for EMI requirements, including adaptive gate drivers controlled by the swing rate, and the spread spectrum can reduce peak emission.

Summary

There is nothing wrong with replacing LDO regulators with high-efficiency buck regulators in industrial and automotive systems regarding efficiency and performance. Size-wise, LDOs require few external components, typically only one or two bypass capacitors. If the high-efficiency buck regulator can be made very compact, it is also acceptable in size.