L5980 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L5980' PDF : 37 Pages View PDF
Functional description
L5980
4.2
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
SROUT
=
SRVREF
⋅
⎛
⎝
1
+
RR-----12--⎠⎞
where SRVREF is the slew rate of the non-inverting input while R1 and R2 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
SSTIME
=
3----2-----⋅---6---4--
Fsw
For example with a switching frequency of 250 kHz the SSTIME is 8 ms.
10/37
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