EDITOR’S CHOICE
supply voltage changes all the time to accommodate different power levels. As a result, the accuracy requirement for the supply voltage is usually 5% to 10%. 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 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 the transient. The drooping voltage can be expressed as:
Figure 6: LT8625SP feedback affected by both the pre-charge signal and the load tran- sient achieving a fast recovery time
Whereas ∆VOUT
caused by the load transient, ∆IOUT
is the initial voltage variation is the load
transient current, and g is the VC pin to switch current gain. In designing the drooping circuit shown in Figure 7, special considerations need to be taken:
The drooping current should not exceed the VC pin current limit. For LT8627SP’s error amplifier output, it is proper to limit the current to below 200µA to avoid saturation and this can be achieved by changing R7 and R8 values. The drooping voltage needs to accommodate the output capacitance so that the voltage deviation during the transient is on a similar level of the drooping voltage to achieve minimum recovery time during the transient. Figure 8 shows typical waveforms for the above-mentioned circuit during a 1A to 16A to 1A load transient. It is worth noticing that now the 16A to 1A load transient speed is no longer bottlenecked by the bandwidth but by the minimum on time of the regulator.
Conclusion
The wireless RF field is becoming increasingly calculation dependent and sensitive to the transient response time due to the time critical nature of the high speed signal processing. System design engineers face challenges to increase the power supply transient response speed so that the blanking time can be minimised.
The Silent Switcher 3 family is the next- generation of monolithic regulators optimised for noise sensitive, intensive dynamic load transient solutions in wireless, industrial, defence, and healthcare fields. Based on the load conditions, special techniques and circuits can be applied to further improve the transient response.
Figure 7: LT8627SP with an active drooping resistor placed between OUTS and VC to achieve a fast transi- ent recovery time
Figure 8: Droop transient response can be achieved to minimise the transient recovery time for LT8627SP
About the author Xinyu Liang is the applications engineering manager in the Industrial and Multimarket Group at Analog Devices focusing on power products. He received
his Ph.D. degree in electrical engineering in 2018 from North Carolina State University and he started his career at ADI in 2019 after graduation.
NOVEMBER 2023 | ELECTRONICS FOR ENGINEERS
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