L5985 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L5985' PDF : 36 Pages View PDF
L5985
4.2
Functional description
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
SROUT
=
SRVREF
⋅
⎛
⎝
1
+
RR-----12--⎠⎞
where SRVREF is the slew rate of the non-inverting input and R1, R2 is the resistor divider to
regulate the output voltage (see Figure 7). The soft-start stair case consists of 64 steps of
9.5mV each one, from 0V to 0.6V. 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
SSTIME
=
3----2-----⋅---6---4--
Fsw
For example with a switching frequency of 250kHz the SSTIME is 8ms.
10/36
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