L7986 STMicroelectronics, L7986 Datasheet - Page 11

• Manufacturer Part Number: L7986
• Description: 3A step-down switching regulator
• Manufacturer: STMicroelectronics

Specifications of L7986

• Low Dropout Operation: 100% duty cycle

L7986
5.2
Soft-start
The soft-start is essential to assure correct and safe startup of the step-down converter. It
avoids inrush current surge and makes the output voltage increase monothonically.
The soft-start is performed by a staircase ramp on the non-inverting input (V
amplifier. So the output voltage slew rate is:
Equation 1
where SR
divider to regulate the output voltage (see
steps of 9.5 mV each, 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 time results:
Equation 2
For example, with a switching frequency of 250 kHz, the SS
VREF
Soft-start scheme
is the slew rate of the non-inverting input, while R1and R2 is the resistor
Doc ID 022450 Rev 1
SR
OUT
SS
=
TIME
SR
Figure
VREF
=
32 64
-----------------
Fsw
7). The soft-start staircase consists of 64
⋅
⋅
⎛
⎝
1
+
R1
------- -
R2
⎞
⎠
TIME
is 8 ms.
Functional description
REF
) of the error
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