A7986ATR datasheet(11/42 Pages) STMICROELECTRONICS

Nome de Peças	A7986ATR

Descrição Electrónicos	3 A step-down switching regulator for automotive applications

PDF	42 Pages

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Fabricante Electrônico	STMICROELECTRONICS [STMicroelectronics]

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11 / 42 page

DocID022801 Rev 5

11/42

A7986A

Functional description

5.2

Soft-start

Soft-start is essential to assure a correct and safe startup of the step-down converter. It

avoids inrush current surge and makes the output voltage increase monothonically.

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 staircase consists of 64

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 scheme

Soft-start time results:

Equation 2

For example, with a switching frequency of 250 kHz, the SSTIME is 8 ms.

OUT

VREF

--------





⋅

TIME

32 64

⋅

Fsw

-----------------

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