Shutdown/Soft Start

A soft start circuit is used in switching power supplies to limit

the input inrush current upon start-up. Without a soft-start

circuit, the inrush current can be several times the

steady-state load current, and thus apply unnecessary

stress to the input source. The LM2611 does not have

soft-start circuitry, but implementing the circuit in

Figure 11

will lower the peak inrush current. The SHDN pin is coupled

to the output through C

shutdown and run states while the output slowly decreases

to its steady-state value. The energy required to reach

steady-state is spread over a longer time and the input

current spikes decrease (see

Figure 12 and Figure 13).

20018125

FIGURE 11. LM2611 Soft Start Circuit

20018142

FIGURE 12. Start-Up Waveforms with Soft Start Circuit

20018141

FIGURE 13. Start-Up Waveforms without Soft Start

Circuit

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