【Introduction】Signal processing units and system-on-chip (SoC) units often have abrupt load transients. Such load transients will interfere with the supply Voltage, which is extremely important in radio frequency (RF) applications because the changing supply voltage can highly affect the clock frequency. Therefore, radio frequency system-on-chip (RFSoC) typically uses blanking time during load transients. In 5G applications, the information quality is highly correlated with the blanking time in the transition interval. Therefore, for any RF system-on-chip, there is a growing need to reduce the effects of load transients on the power supply side to improve system-level performance.
Fast Transient Silent Switcher 3 Series for RF Applications
One of the most straightforward ways to implement fast transient power rails is to choose a regulator with fast transient performance. The Silent Switcher 3 series ICs feature very low frequency output noise, fast transient response, low EMI emissions and high efficiency. It is designed with an ultra-high performance error amplifier to provide additional stability even with aggressive compensation methods. The maximum switching frequency of 4MHz enables the IC to push the bandwidth of the control loop to 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.
Table 1. Silent Switcher 3 Series Parameters
Figure 1 shows a typical 1 V output power supply for a 5G RFSoC based on the LT8625SP that requires both fast transient response and low ripple/noise levels. The 1 V load consists of the transmit/receive related circuits as well as the local oscillator (LO) and voltage controlled oscillator (VCO). In frequency division duplex (FDD) operation, the transmit/receive load experiences sudden changes in load current. At the same time, the LO/VCO load is constant, but high accuracy and low noise are critical. The high bandwidth of the LT8625SP enables designers to separate dynamic and static loads with a second Inductor (L2), allowing a single IC to power two critical 1 V load groups. Figure 2 shows the output voltage response with a 4 A to 6 A dynamic load transient. Dynamic loads recover within 5 µs with less than 0.8% peak-to-peak voltage, which has minimal impact on the static load side with less than 0.1% peak-to-peak voltage. This circuit can be modified to support other output combinations such as 0.8 V and 1.8 V; all of them can directly power the RFSoC load without requiring LDO stabilization due to ultra-low noise in the low frequency range, low voltage ripple, and ultra-fast transient response. voltage stage.
Figure 1. Typical Application Circuit of LT8625SP, Dynamic/Static RF Load Separation
Figure 2. Fast Load Transient Response, Minimal VOUT Deviation, Does Not Affect Static Loads
In time division duplex (TDD) mode, the noise critical LO/VCO is loaded and unloaded as the transmit/receive mode changes. Therefore, the simplified circuit shown in Figure 3 can be used, as all loads are considered dynamic loads, while more critical post-filtering is required to maintain the low ripple/low noise characteristics of the LO/VCO. A 3-terminal capacitor in feedthrough mode can be used to achieve sufficient post-filtering with a minimized equivalent L to maintain fast bandwidth for load transients. The feedthrough capacitors together with the far end output capacitors form the other two LC filter stages, while all L comes from the ESL of the 3-terminal capacitor, which is very small and less harmful to load transients. Figure 3 also shows a simple remote detection connection for the Silent Switcher 3 series. Due to the unique reference generation and feedback technique, simply connect the ground of the SET pin capacitor (C1) and the OUTS pin Kelvin to the desired remote feedback point. This connection does not require level shifting circuitry. Figure 4 shows the 1 A load transient response waveform with less than 5 μs recovery time and less than 1 mV output voltage ripple.
Figure 3. Typical Application Circuit of LT8625SP, Combined Dynamic/Static RF Load
Figure 4. Feedthrough Capacitor Improves Transient Response While Minimizing Output Voltage Ripple
Precharge Signal Drives Silent Switcher 3 Series for Fast Transient Response
In some cases, the signal processing unit is powerful, with enough GPIO, and the signal processing is arranged properly because transient events can be known in advance. This typically occurs in some FPGA power supply designs, where a precharge signal can be generated to help drive the power supply transient response. Figure 5 shows a typical application circuit that uses an FPGA-generated precharge signal to provide bias before the actual load transition occurs so that the LT8625SP has additional time to accommodate load disturbances without significant VOUT skew and recovery time . Since the precharge signal interferes with the feedback, the tuning circuit from the FPGA’s GPIO to the Inverter input is omitted. Level control is 35 mV. In addition, in order to avoid the influence of the precharge signal on the steady state, a high-pass filter is set between the precharge signal and OUTS. Figure 6 shows the 1.7 A to 4.2 A load transient response waveform. The precharge signal is applied to the feedback (OUTS) prior to the actual load transient with a recovery time of less than 5 µs.
Figure 5. T8625SP feeds precharge signal to OUTS pin for fast transient response
Figure 6. Precharge Signal and Load Transient Simultaneously Affect the LT8625SP for Fast Recovery Time
Circuit Active Bucks for Ultra-Fast Recovery Transients
In a beamformer application, the supply voltage varies from time to time to accommodate different power levels. Therefore, the accuracy requirements for the supply voltage are usually in the range of 5% to 10%. In this application, stability is more important than voltage accuracy, as minimizing recovery time during load transients will maximize data processing efficiency. A buck circuit is ideal for this application because the drop voltage reduces or even eliminates the recovery time. The schematic diagram of the active buck circuit of the LT8627SP is shown in Figure 7. An additional step-down resistor is added between the negative input (OUTS) and output (VC) of the error amplifier to maintain steady-state error in the feedback control loop during transients. The drop voltage can be expressed as:
Figure 7. Place an active step-down resistor between OUTS and VC of the LT8627SP for fast transient recovery time
ΔVOUTis the initial voltage change due to the load transient, ΔIOUT is the load transient current, and g is the VC pin used to switch the current gain. When designing the step-down circuit shown in Figure 7, the following points need special consideration:
● 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 drooping voltage needs to accommodate the output capacitor so that the voltage deviation during the transient is approximately close to the drooping voltage, resulting in the shortest recovery time during the transient.
Figure 8 shows typical waveforms for the above circuit during a 1 A to 16 A to 1 A load transient. It is worth noting that the 16 A to 1 A load transient speed is now no longer limited by bandwidth, but by the minimum regulator on-time.
Figure 8. Buck transient response can be implemented to minimize the transient recovery time of the LT8627SP
in conclusion
Due to the time-critical nature of high-speed signal processing, the wireless RF field has become increasingly computationally dependent and sensitive to transient response times. The challenge for system design engineers is to improve the power supply transient response speed to minimize blanking time. The Silent Switcher 3 family is a new generation of monolithic regulators optimized for noise-sensitive, high dynamic load transient solutions in wireless, industrial, defense and healthcare. Depending on load conditions, special techniques and circuits can be applied to further improve transient response.
The Links: LB104S01(TL)(02) LQ121S1LG81
