A6986 datasheet(19/67 Pages) STMICROELECTRONICS

19 / 67 page

DocID026211 Rev 6

19/67

A6986

Functional description

The maximum capacitor value has to be limited to guarantee the device can discharge it in

case of thermal shutdown and UVLO events (see Figure 9), so restart the switching activity

ramping the error amplifier reference voltage.

Equation 2

where:

Equation 3

The optional diode prevents to disable the device if the external source drops to ground.

RUP value is selected in order to make the capacitor charge at first approximation

independent from the internal current generator (4

A typ. current capability, see Table 5 on

page 8), so:

Equation 4

where:

Equation 5

represents the SS/INH voltage correspondent to the end of the ramp on the error amplifier

(see Figure 5); refer to Table 5 for VSS START, VFB and SSGAIN parameters.

As a consequence the voltage across the soft-start capacitor can be written as:

Equation 6

RSS_DOWN is selected to guarantee the device stays in inhibit mode when the internal

generator sources 1

A typ. out of the SS/INH pin and VSTEP is not present:

Equation 7

so:

Equation 8

CSS

1 msec

–

RSS_EQ

VSS_FINAL 0.9 V

–

600

AR

SS_EQ

–

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

–







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



RSS_EQ

RUP RDWN



RUP RDWN

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

VSS_FINAL

VSTEP VDIODE

–



RDWN

RUP RDWN

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



VSTEP VDIODE

–

VSS END

–

RUP

----------------------------------------------------------------------- ISS CHARGE 4 A



VSS END

VSS START

VFB

SSGAIN

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

vSS t

VSS_FINAL

CSS RSS_EQ



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

–

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



RDWN ISS INHIBIT RDWN 1 A



VINH 200 mV





RDWN 100 k



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