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LTC1736

APPLICATIO S I FOR ATIO

Soft-Start/Run Function

The RUN/SS pin is a multipurpose pin that provides a soft-

start function and a means to shut down the LTC1736.

Soft-start reduces surge currents from VIN by gradually

increasing the controller’s current limit ITH(MAX). This pin

can also be used for power supply sequencing.

Pulling the RUN/SS pin below 1.5V puts the LTC1736 into

a low quiescent current shutdown (IQ< 25µA). This pin can

be driven directly from logic as shown in Figure 3. Releas-

ing the RUN/SS pin allows an internal 1.2

µA current

source to charge up the external soft-start capacitor CSS.

If RUN/SS has been pulled all the way to ground there is

a delay before starting of approximately:

F C

DELAY

=µ

()

125

When the voltage on RUN/SS reaches 1.5V the LTC1736

begins operating with a current limit at approximately

25mV/RSENSE. As the voltage on RUN/SS increases from

1.5V to 3.0V, the internal current limit is increased from

25mV/RSENSE to 75mV/RSENSE. The output current limit

ramps up slowly, taking an additional 1.25s/

µF to reach

full current. The output current thus ramps up slowly

reducing the starting surge current required from the input

power supply.

Diode D1 in Figure 3 reduces the start delay while allowing

CSS to charge up slowly for the soft-start function. This

diode and CSS can be deleted if soft-start is not needed.

The RUN/SS pin has an internal 6V zener clamp (See

Functional Diagram).

Fault Conditions: Overcurrent Latchoff

The RUN/SS pin also provides the ability to shut off the

controller and latchoff when an overcurrent condition is

detected. The RUN/SS capacitor CSS is used initially to

turn on and limit the inrush current of the controller. After

the controller has been started and given adequate time to

charge up the output capacitor and provide full load

current, CSS is used as a short-circuit timer. If the output

voltage falls to less than 70% of its nominal output voltage

after CSS reaches 4.1V, the assumption is made that the

output is in a severe overcurrent and/or short-circuit

condition and CSS begins discharging. If the condition

lasts for a long enough period as determined by the size of

CSS, the controller will be shut down until the RUN/SS pin

voltage is recycled.

This built-in latchoff can be overridden by providing a

current > 5

µA at a compliance of 5V to the RUN/SS pin as

shown in Figure 4. This current shortens the soft-start

period but also prevents net discharge of the RUN/SS

capacitor during a severe overcurrent and/or short-circuit

condition. When deriving the 5

µA current from VIN as in

Figure 4a, current latchoff is always defeated. A diode

connecting this pull-up resistor to INTVCC , as in Figure 4b,

eliminates any extra supply current during controller shut-

down while eliminating the INTVCC loading from prevent-

ing controller start-up. If the voltage on CSS doesnotexceed

4.1V, the overcurrent latch is not armed and the function

is disabled.

Figure 3. RUN/SS Pin Interfacing

3.3V OR 5V

RUN/SS

(a)

(b)

CSS

1736 F03

3.3V OR 5V

RUN/SS

VIN

INTVCC

RUN/SS

(a)

(b)

CSS

RSS

CSS

RSS

1736 F04

Figure 4. RUN/SS Pin Interfacing with Latchoff Defeated

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