How to achieve ultra-fast power transient response in RF applications

How to achieve ultra-fast power transient response in RF applications

summary

This article shows a practical method for achieving ultra-fast power transient response in wireless applications, especially in the RF field. It aims to solve the problems and challenges of inefficient signal processing brought to system designers due to power transient blanking time. For different applications, ADI proposes corresponding example solutions and introduces Silent Switcher 3 monolithic power products to achieve optimal transient performance.

Introduction

Signal processing units and system-on-chip (SoC) units often have sudden load transient changes. Such load transient changes will disturb the power supply voltage, which is extremely important in radio frequency (RF) applications. This changing power supply voltage will highly affect the clock frequency, causing RF systems-on-chip (RFSoCs) to usually use blanking time during load transients. In 5G applications, the quality of information is highly correlated with the blanking time in the transition interval. Therefore, for any RF system-on-chip (RFSoC), there is an increasing need to reduce the load transient effects on the power supply side to improve system-level performance. This article will introduce several methods to achieve fast transient response of the power supply in RF applications.

Fast Transient Silent Switcher 3 Series for RF Applications

One of the most straightforward ways to achieve fast transient power rails is to select a regulator with fast transient performance. The Silent Switcher 3 family of ICs features very low frequency output noise, fast transient response, low EMI emissions, and high efficiency. It uses an ultra-high performance error amplifier design that provides additional stability even with aggressive compensation methods. The 4MHz maximum switching frequency enables the IC to push the bandwidth of the control loop into the 50kHz range in fixed frequency peak current control mode. Table 1 lists the Silent Switcher 3 ICs that designers can choose to achieve fast transient performance.

Figure 1 shows a typical 1V output power supply for 5G RFSoC based on LT8625SP, which needs to meet both fast transient response and low ripple/noise levels. The 1V load consists of transmit/receive related circuits as well as local oscillators (LOs) and voltage-controlled oscillators (VCOs). The transmit/receive load will experience sudden changes in load current during frequency division duplex (FDD) operation. At the same time, the LOs/VCOs load is constant but requires extremely high accuracy and extremely low noise. The high bandwidth characteristics of the LT8625SP enable designers to separate dynamic loads and static loads using a second inductor (L2), thereby powering two critical 1V load groups on a single IC. Figure 2 shows the output voltage response to a 4A to 6A dynamic load transient. The dynamic load can be recovered within 5us, and the peak voltage is less than 0.8%, which minimizes the impact on the static load side, with a peak voltage of only less than 0.1%. This circuit can be modified to accommodate other output combinations such as 0.8V and 1.8V, which can directly power the RFSoC without having to go to an LDO regulator stage due to the ultra-low noise, low voltage ripple and ultra-fast transient response in the low frequency range.

Figure 1. Typical application circuit of LT8625SP, dynamic/static RF load separation

Figure 2. Load transient response is fast, VOUT deviation is minimal, and does not affect static loads

In time division duplex (TDD) mode, the noise-sensitive LO/VCO loads and unloads as the transmit/receive mode changes. Therefore, the simplified circuit shown in Figure 3 can be used because all loads are considered dynamic loads and post filtering is more critical to maintain the low ripple/low noise characteristics of the LO/VCO. The 3-terminal capacitor in feed-through mode can achieve sufficient post filtering, and its minimized equivalent L can maintain the fast bandwidth of the load transient. The feed-through capacitor forms two additional LC filter circuits together with the remote output capacitor, and all Ls come from the equivalent series inductance (ESL) of the 3-terminal capacitor, which is extremely small and thus less harmful to the load transient. Figure 3 also illustrates the simple remote sense connection of the Silent Switcher 3 series. Due to the unique reference output and feedback technology, only the ground of the SET pin capacitor (C1) and the OUTS pin Kelvin connection are required to the desired remote feedback point. This connection does not require a level shifting circuit. Figure 4 shows the 1A load transient response waveform with a recovery time of less than 5us and an output voltage ripple of less than 1mV.

Figure 3. Typical application circuit of LT8625SP, dynamic/static RF load merging

Figure 4. Feedthrough capacitance improves transient response while maintaining minimum output voltage ripple.

Silent Switcher 3 Series ICs Driven by Precharge Signal for Fast Transient Response

In some cases, if the signal processing unit is powerful, has enough pins (GPIO), and the signal processing method is reasonable, the transient can be predicted. This usually happens in some FPGA power supply designs, where a pre-charge signal can be generated to assist the transient response of the driving power supply. Figure 5 shows a typical application circuit that uses a pre-charge signal generated by the FPGA to provide a bias before the actual load transition occurs, so that the LT8625SP can have extra time to adapt to the load disturbance without excessive output voltage (VOUT) deviation and recovery time. Because the pre-charge signal interferes with the feedback, the tuning circuit from the FPGA pin (GPIO) to the inverter input is omitted. The level control is 35mV. In addition, in order to avoid the influence of the pre-charge signal on the steady state, a high-pass filter is set between the pre-charge signal and OUTS. Figure 6 shows the 1.7A to 4.2A load transient response waveform. The pre-charge signal is applied to the feedback terminal (OUTS) before the actual load transient, and its recovery time is less than 5us.

Figure 5. The T8625SP feeds a precharge signal into the OUTS pin for fast transient response

Figure 6. Precharge signal and load transient simultaneously impact the LT8625SP, achieving fast recovery time

Circuit actively steps down voltage for ultra-fast recovery transients

In beamformer applications, the power supply voltage is constantly changing to accommodate different power levels. Therefore, the accuracy requirement for the power supply voltage is usually in the range of 5% to 10%. In this application, stability is more important than voltage accuracy, and minimizing the recovery time during load transients will maximize data processing efficiency. Buck circuits are well suited for this application because dropping the voltage can reduce or even eliminate the recovery time. The schematic diagram of the active buck circuit of the LT8627SP is shown in Figure 7. An additional drop resistor is added between the negative input (OUTS) and the output (VC) of the error amplifier to maintain a steady-state error in the feedback control loop during transients. The dropped voltage can be expressed as:

Figure 7. An active drop resistor is placed between OUTS and VC of the LT8627SP to achieve fast transient recovery time

ΔVOUT is the initial voltage change caused by the load transient, ΔIOUT is the load transient current, and g is the VC pin used to switch the current gain. When designing the buck circuit shown in Figure 7, the following points need to be considered in particular:

► The falling current should not exceed the VC pin current limit. For the error amplifier output of the LT8627SP, it is best to limit the current below 200µA to avoid saturation, which can be achieved by changing the values ​​of R7 and R8.

►The droop voltage needs to accommodate the output capacitor so that the voltage deviation during the transient is roughly close to the droop voltage, thus achieving the shortest recovery time during the transient.

Figure 8 shows typical waveforms for the above circuit during a 1A to 16A to 1A load transient. Note that now the 16A to 1A load transient speed is no longer limited by bandwidth, but by the regulator minimum on-time.

Figure 8. Buck transient response can be implemented to minimize the transient recovery time of the LT8627SP

in conclusion

The wireless RF field is becoming increasingly computationally dependent and sensitive to transient response times due to the time-critical nature of high-speed signal processing. System design engineers are faced with the challenge of improving the power supply transient response speed to minimize blanking time. The Silent Switcher 3 family is a next-generation monolithic regulator optimized for noise-sensitive, high-dynamic load transient solutions in the wireless, industrial and healthcare fields. Depending on the load conditions, special techniques and circuits can be applied to further improve transient response.

About Analog Devices

Analog Devices, Inc. (NASDAQ: ADI) plays an important role at the center of the modern digital economy, transforming real-world phenomena into actionable insights with its broad range of analog and mixed-signal, power management, RF, digital and sensor technologies. ADI serves 125,000 customers worldwide and offers more than 75,000 products in the industrial, communications, automotive and consumer markets. ADI is headquartered in Wilmington, Massachusetts.

About the Author

Xinyu Liang is an application engineering manager for the Industrial and Multi-Market Division at Analog Devices, focusing on power products. He received his Ph.D. in electrical engineering from North Carolina State University in 2018 and started his career at Analog Devices in 2019 after graduation.