A7985ATR datasheet(13/44 Pages) STMICROELECTRONICS

13 / 44 page

DocID023128 Rev 5

13/44

A7985A

Functional description

where:

Equation 1

FSW is desired switching frequency.

5.2

Soft-start

Soft-start is essential to assure the 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 (VREF) of the error

amplifier. So the output voltage slew rate is:

Equation 2

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 3

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

RFSW

28.5 10



FSW 250 10



–

----------------------------------------- 3.23 10



–

SROUT

SRVREF 1

--------







SSTIME

32 64



Fsw

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

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