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L7981 Datasheet(PDF) 11 Page - STMicroelectronics |
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L7981 Datasheet(HTML) 11 Page - STMicroelectronics |
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11 / 44 page  L7981 Functional description Doc ID 15182 Rev 3 11/44 5.2 Soft-start The soft-start is essential to assure correct and safe start up of the step-down converter. It avoids inrush current surge and makes the output voltage increases monothonically. The soft -start is performed by a staircase ramp on the non-inverting input (VREF) of the error amplifier. So the output voltage slew rate is: Equation 1 where SRVREF is the slew rate of the non-inverting input, while R1and R2 is the resistor divider to regulate the output voltage (see Figure 7). The soft-start stair case consists of 64 steps of 9.5 mV each one, from 0 V to 0.6 V. The time base of one step is of 32 clock cycles. So the soft-start time and then the output voltage slew rate depend on the switching frequency. Figure 7. Soft-start scheme Soft-start time results: Equation 2 For example with a switching frequency of 250kHz the SSTIME is 8ms. SR OUT SR VREF 1 R1 R2 -------- + ⎝⎠ ⎛⎞ ⋅ = SS TIME 32 64 ⋅ Fsw ----------------- = |
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