Feature: RF Design
Figure 4. Feedthrough capacitor boosts transient response while maintaining minimised output voltage ripple.
while a more critical postfiltering is required to maintain the low ripple/low noise feature for the LOs/VCOs. A 3-terminal capacitor in feedthrough mode can be used to achieve enough post-filtering with a minimised equivalent L that maintains a fast bandwidth for the load transients. Te feedthrough capacitor together with the remote side output capacitors forms two more LC filter stages while all the Ls come from ESLs of the 3-terminal capacitor, which is very small and less harmful to the load transient. Figure 3 also illustrates an easy remote sensing connection for the Silent Switcher 3 family. Due to the unique reference generation and feedback technology, one only needs to Kelvin connect the SET pin capacitor’s (C1) ground and the OUTS pin to the desired remote feedback point. No level shiſting circuits are needed for this connection. Figure 4 shows a 1A load transient response waveform with <5μs recovery time and <1mV output voltage ripple.
Pre-charge signals drive silent switcher 3 family for fast transient response In some cases, the signal processing unit is powerful with enough GPIOs, and the signal processing is well scheduled as the transient event can be known ahead of time. Tis usually happens in some FPGA power supply designs where the pre-charge signal can be generated to help power the supply transient response. Figure 5 shows a typical application circuit using the pre-charge signal generated by the FPGA to provide a bias before the real load transition happens so that the LT8625SP can have extra
Figure 5. T8625SP with a pre-charge signal fed into OUTS pin to achieve fast transient response.
time to accommodate the load disturbance without too large of a VOUT deviation and recovery time. Te tuning circuit from FPGA’s GPIO to the input of the inverter
has been omitted as the pre-charge signal is acting as a disturbance on the feedback. Te level is controlled to be 35mV. Moreover, to avoid the pre-charge signal effect on the steady state, a high-pass filter is implemented between the pre-charge signal and the OUTS. Figure 6 shows a 1.7A to 4.2A load transient response waveform. Te pre-charge signal is applied to the feedback (OUTS) ahead of the real load transient, whereas less than 5µs recovery time is achieved.
Active drooping on circuit for ultrafast recovery transient In beamformer applications, the supply voltage changes all the time to accommodate different power levels. As a result, the accuracy requirement for the supply voltage is usually 5 per cent to 10 per cent. In this application, stability is more important than the voltage accuracy as a minimised recovery time during the load transient will maximise the data processing efficiency. A drooping circuit perfectly fits into this application as the
Figure 6. LT8625SP feedback affected by both the pre-charge signal and the load transient achieving a fast recovery time.
36 February 2025
www.electronicsworld.co.uk
drooping voltage will reduce or even eliminate the recovery time. Figure 7 shows the schematic for an active drooping circuit for the LT8627SP. An extra drooping resistor has been added between the error amplifier’s negative input (OUTS) and the output (VC) to maintain a steady state error in the feedback control loop during
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54
