L6986H datasheet(15/68 Pages) STMICROELECTRONICS

15 / 68 page

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

CSS<

−1ms

RSS_EQ⋅In(1−VSS_FINAL−0.9V

600μA−RSS_EQ)

(2)

where:

RSS_EQ= RUP⋅RDWN

RUP+RDWN VSS_FINAL=(VSTEP−VDIODE)⋅ RDWN

RUP+RDWN

(3)

The optional diode prevents the device from disabling 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 µA typ. current capability, see Table 1), so:

VSTEP−VDIODE−VSS END

RUP

≫ISS CHARGE≡4μA

(4)

where

VSS END=VSS START+ VFBSSGAIN

(5)

represents the SS/INH voltage correspondent to the end of the ramp on the error amplifier (see Figure 5. Soft-

start phase); refer to Table 1 for VSS START, VFB and SSGAIN parameters.

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

VSS (t)=VSS FINAL⋅ 1

1−e− t

CSS⋅RSS_EQ

(6)

RSS_DOWN is selected to guarantee the device stays in inhibit mode when the internal generator sources 1 µA typ.

out of the SS/INH pin and VSTEP is not present:

RDWN⋅ISSINHIBIT≡RDWN⋅1μA≪VINH≡200mV

(7)

RDWN<100kΩ

(8)

RUP and RDWN are selected to guarantee:

VSS_FINAL≅2V>VSS_END

(9)

The time to ramp the internal voltage reference can be calculated as follows

TSS=CSS⋅RSS_EQ⋅In(VSS_FINAL−VSS START

VSS_FINAL−VSS END )

(10)

that is the equivalent soft-start time to ramp the output voltage.

Figure 8. External soft-start network VSTEP driven shows the soft-start phase with the following component

selection: RUP = 180 kΩ, RDWN = 33 kΩ, CSS = 200 nF, the 1N4148 is a small signal diode and VSTEP = 13 V.

L6986H

Soft-start and inhibit

DS12905 - Rev 2

page 15/68

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