LTC3714_15 Datasheet, PDF(19/28 Page) Linear Technology – Intel Compatible, Wide Operating Range, Step-Down Controller with Internal Op Amp

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LTC3714_15 Datasheet, PDF (19/28 Pages) Linear Technology – Intel Compatible, Wide Operating Range, Step-Down Controller with Internal Op Amp

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LTC3714

Applications Information

Soft-Start and Latchoff with the RUN/SS Pin

The RUN/SS pin provides a means to shut down the

LTC3714 as well as a timer for soft-start and overcurrent

latchoff. Pulling the RUN/SS pin below 1.5V puts the

LTC3714 into a low quiescent current shutdown (IQ â¤

30ÂµA). Releasing the pin allows an internal 1.2ÂµA internal

current source to charge up the external timing capacitor

CSS. If RUN/SS has been pulled all the way to ground,

there is a delay before starting of about:

tDELAY

1.5V

1.2ÂµA

CSS

(1.3s /ÂµF) CSS

When the voltage on RUN/SS reaches 1.5V, the LTC3714

begins operating with a clamp on ITH of approximately

0.9V. As the RUN/SS voltage rises to 3V, the clamp on ITH

is raised until its full 2.4V range is available. This takes an

additional 1.3s/ÂµF, during which the load current is folded

back until the output reaches 50% of its final value. The

pin can be driven from logic as shown in Figure 7. Diode

D1 reduces the start delay while allowing CSS to charge

up slowly for the soft-start function.

After the controller has been started and given adequate

time to charge up the output capacitor, CSS is used as a

short-circuit timer. After the RUN/SS pin charges above 4V,

if the output voltage falls below 75% of its regulated value,

then a short-circuit fault is assumed. A 1.7ÂµA current then

begins discharging CSS. If the fault condition persists until

the RUN/SS pin drops to 3.5V, then the controller turns

off both power MOSFETs, shutting down the converter

permanently. The RUN/SS pin must be actively pulled

down to ground in order to restart operation.

The overcurrent protection timer requires that the soft-start

timing capacitor CSS be made large enough to guarantee

that the output is in regulation by the time CSS has reached

the 4V threshold. In general, this will depend upon the

size of the output capacitance, output voltage and load

current characteristic. A minimum soft-start capacitor

can be estimated from:

CSS > COUT VOUT RSENSE (10â4 [F/Vs])

Generally 0.1ÂµF is more than sufficient.

Overcurrent latchoff operation is not always needed or

desired. Load current is already limited during a short-

circuit by the current foldback circuitry and latchoff op-

eration can prove annoying during troubleshooting. The

feature can be overridden by adding a pull-up current of

>5ÂµA to the RUN/SS pin. The additional current prevents

the discharge of CSS during a fault and also shortens

the soft-start period. Using a resistor to VIN as shown in

Figure 7a is simple, but slightly increases shutdown cur-

rent. Connecting a resistor to INTVCC as shown in Figure

7b eliminates the additional shutdown current, but requires

a diode to isolate CSS. Any pull-up network must be able

to pull RUN/SS above the 4.5V maximum threshold that

arms the latchoff circuit and overcome the 4ÂµA maximum

discharge current.

INTVCC

3.3V OR 5V

VIN

RUN/SS

RSS*

D2* RUN/SS

CSS

(7a)

*OPTIONAL TO OVERRIDE

OVERCURRENT LATCHOFF

(7b)

3714 F07

Figure 7. RUN/SS Pin Interfacing with Latchoff Defeated

3714f

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