Linear voltage regulators can be considered as a low noise solution when the design needs to prioritize and minimize EMI (electromagnetic interference). However, due to the requirements of heat dissipation and efficiency, it is not suitable for this scenario. Instead, switching voltage regulators should be selected. Even in EMI sensitive applications, switching regulators are often the first active component on the input power bus, which significantly affects the EMI performance of the entire converter regardless of the downstream converter. So far, there is no clear way to ensure that EMI can be suppressed and efficiency requirements can be achieved through the selection of power IC. The LT8614 Silent Switcher™ voltage regulator now does just that.

 

The LT8614 reduces EMI by more than 20dB compared to current advanced switching regulators. In contrast, it reduces EMI by a factor of 10 in the frequency range above 30MHz without affecting the minimum on-off, off-off time or efficiency of the equivalent board area. It achieves this without additional components or shielding and represents a major breakthrough in the design of switching regulators.

 

 

A reliable way to solve EMI(electromagnetic interference) problems is to use a shield box for the entire circuit. Of course, this greatly increases the required board space, component, and assembly costs, as well as the complexity of thermal management and testing. Another way is to slow down the edge of the switch. However, this will lead to reduced efficiency, increase the minimum on-off time, turn-off time and related dead time, and affect the potential potential current control loop speed.

 

The LT8614 Silent Switcher regulator delivers the desired effect without the use of a shield box (see Figure 1). The LT8614 has an IQ as low as 2.5μA, and when adjusted without load, the device consumes a total supply current of 2.5μA, which is important for alway on systems.

 

                                                                                            

 

Its ultra-low pressure differential is limited only by the internal top switch. Unlike other solutions, the LT8614's VIN-VOUT (input-output voltage) limits are not limited by maximum duty cycle and minimum off time. As shown in Figure 6, the device will jump through the break period when there is a voltage drop to keep the internal top switch boost stage voltage stable.

 

At the same time, the typical minimum operating input voltage is only 2.9V (up to 3.4V), enabling it to provide a voltage of 3.3V in the step-down state. Due to its low overall switching resistance, the LT8614 is more efficient than similar devices under high current conditions.

 

The LT8614 can be synchronized with external operating frequencies ranging from 200kHz to 3MHz. Because of the low loss of AC switching, it can operate at high switching frequencies with minimal loss. A good balance is achieved in EMI-sensitive applications such as those commonly found in many automotive environments, and the LT8614 can operate below the AM band to achieve lower EMI, or above the AM band. In a setting with a 700kHz operating switching frequency, the standard LT8614 demo board did not exceed background noise in class 5 measurements of CISPR25.

 

FIG. 2 shows the results of measurement in an anechoic chamber with 12V input voltage, 3.3V output voltage, 2A current and a fixed switching frequency of 700kHz. To compare the LT8614 Silent Switcher technology with another current advanced switching regulator, the product was measured against the LT8610 (see Figure 3). In the GTEM chamber, standard demonstration boards for both devices were tested using the same load, input voltage, and the same inductance.

 

                                                                                                                         

 

As you can see, using the LT8614 Silent Switcher technology offers an additional 20dB improvement over the LT8610's already good EMI performance, especially in the more unmanageable high-frequency range.
In the time domain, the LT8614 performs well at the edge of the switch node, as shown in Figures 4 and 5. Even at 4ns/div, the LT8614Silent Switcher's voltage regulator ring is very small. As shown in Figure 4, the LT8610 has good ring suppression in comparison, but compared to the LT8614 (Figure 4), you can see that it stores a higher amount of energy in the thermal loop.

 

                                                                                                

                                                                                                                     

 

Figure 5 shows the switching node at 13.2V input voltage and how the LT8614 achieves a nearly ideal square wave at the switching node. All time-domain measurements in FIG. 4, 5, and 6 use a probe of 500MHz Tektronix P6139A with the probe tip shielded tightly connected to the GND plane of the PCB. Both devices use off-the-shelf demo boards.

                                                                                                             

 

The LT861x series has an absolute maximum input voltage rating of 42V, which is important for automotive and industrial environments. In automotive applications, voltage drop performance is as important as its maximum input voltage rating. In general, the critical 3.3V logic power supply must be supported in cold startup situations. In this case, the LT8614 Silent Switcher regulator maintains the LT861x series' near-ideal performance. As shown in Figure 6, the LT8610/11/14 device does not employ the higher undervoltage locking voltage and maximum duty ratio clamping features of the replacement component, but instead drops to 3.4V operation and begins cycle skipping when necessary. This results in a good voltage drop, as shown in Figure 7.

                                                                                                 

 

                                                                                                       

 

The LT8614 has a minimum on-off time of 30ns, enabling a large step-down ratio even at high switching frequencies. As a result, it can provide logical core voltage in a single step-down at input voltages up to 42V.

 

As we all know, in order to pass the electromagnetic interference test when the system is designed, the electromagnetic interference problem should be carefully considered in the initial design stage of the converter. The LT8614 Silent Switcher regulator ensures successful and feasible implementation of this requirement by selecting a power IC. The LT8614 reduces the electromagnetic interference generated by current advanced switching regulators by more than 20dB, even improving conversion efficiency without requiring additional components or additional shielding.

 

 

The layout design of printed circuit boards determines the function, electromagnetic interference and thermal behavior of a power supply, and determines the success or failure of each power supply design. While switching power supply layout is not a magic, it is often overlooked in the initial design process. Since functional and EMI requirements must be met, factors that favor the functional stability of a power supply are usually also favorable for EMI radiation. Know that having a good layout from the start not only doesn't add cost, it actually saves money and eliminates the need to change EMI filters, mechanical shielding, EMI test times, and PC boards.

 

There are two types of EMI emission: conduction and radiation. Conducted emission is connected to the product by wires and wiring. Because the noise is limited to a specific terminal or connector in the design, compliance with the conducted radiation requirements can usually be ensured by good layout and filter design early in the development process.

 

Radiation emission, however, is another matter. Anything carrying current on a circuit board radiates electromagnetic fields. Each line on the circuit board is an antenna, and each copper layer is a mirror. Anything other than a pure sine wave or DC voltage produces a wide spectrum of signals. Even with careful design, developers don't really know how bad the radiation will be until the system is tested. Radiation emission tests can only be formally conducted after the design is substantially complete.

 

Filters usually attenuate signal strength at or within a certain frequency range to reduce electromagnetic interference. A portion of the energy of space (radiation) can be attenuated by adding a metal plate as a magnetic shield. By adding ferrite beads and other filters to suppress the energy of the low frequency portion of the PCB's wiring (conduction). EMI cannot be eliminated, but it can be attenuated to a level acceptable to other communications and digital devices. In addition, some regulators ensure compliance by enforcing standards.

 

Modern input filters using surface-mounted technology have better performance than through-hole devices. However, this improvement has not kept pace with the increasing frequency of switching regulators. Higher efficiency, shorter on-off and off-off times, and faster switching results in higher harmonic content. When all other parameters (such as switching capacity and switching time) are held constant, EMI deteriorates by 6dB for every doubling of switching frequency. If the switching frequency is increased by a factor of 10, the broadband EMI performance will be like that of a first-order high-pass filter, with an increase in radiation of 20dB.
Smart PCB designers will make the thermal loop smaller and keep the shield layer as close to the active layer as possible. Nevertheless, the requirements of the device pin arrangement, package structure, heat dissipation design, and package size required to store sufficient energy in the decoupling element all require that thermal loops be minimized as much as possible. A further complication in typical flat PCB designs is that magnetic or transformer-type coupling higher than 30 MHz between lines diminishes the effect of all filters, since poor magnetic coupling or antenna coupling is more significant at higher frequencies.

 

Potential interference and noise problems may become more serious when multiple DC/DC switching mode regulators are connected in parallel to achieve current sharing and higher output power. If all regulators operate at a similar frequency (switching), then the combined energy produced by the multiple regulators in the circuit is concentrated on that frequency and its harmonics. The existence of this energy can be a difficult problem to solve for other ics and other system boards on the PC board. The devices are very close to each other and are susceptible to this radiation energy. This problem is particularly worrisome in automotive systems, where components are dense and often close to audio, RF, CAN buses, and various receiving systems.