LTC1735CS datasheet(17/32 Pages) LINER

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LTC1735

Diode D1 in Figure 5 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).

capacitor during a severe overcurrent and/or short-circuit

condition. When deriving the 5

µA current from VIN as in

Figure 6a, current latchoff is always defeated. Diode con-

necting this pull-up resistor to INTVCC , as in Figure 6b,

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 does not

exceed 4.1V the overcurrent latch is not armed and the

function is disabled.

Why should you defeat overcurrent latchoff? During the

prototyping stage of a design, there may be a problem with

noise pickup or poor layout causing the protection circuit

to latch off. Defeating this feature will easily allow trouble-

shooting of the circuit and PC layout. The internal short-

circuit and foldback current limiting still remains active,

thereby protecting the power supply system from failure.

After the design is complete, a decision can be made

whether to enable the latchoff feature.

The value of the soft-start capacitor CSS will need to be

scaled with output current, output capacitance and load

current characteristics. The minimum soft-start capaci-

tance is given by:

CSS > (COUT )(VOUT)(10–4)(RSENSE)

The minimum recommended soft-start capacitor of

CSS = 0.1µF will be sufficient for most applications.

Fault Conditions: Current Limit and Current Foldback

The LTC1735 current comparator has a maximum sense

voltage of 75mV resulting in a maximum MOSFET current

of 75mV/RSENSE.

The LTC1735 includes current foldback to help further

limit load current when the output is shorted to ground.

The foldback circuit is active even when the overload

shutdown latch described above is defeated. If the output

falls by more than half, then the maximum sense voltage

is progressively lowered from 75mV to 30mV. Under

short-circuit conditions with very low duty cycle, the

LTC1735 will begin cycle skipping in order to limit the

short-circuit current. In this situation the bottom MOSFET

will be conducting the peak current. The short-circuit

ripple current is determined by the minimum on-time

APPLICATIO S I FOR ATIO

Figure 6. RUN/SS Pin Interfacing with Latchoff Defeated

Figure 5. RUN/SS Pin Interfacing

3.3V OR 5V

RUN/SS

CSS

1735 F05

3.3V OR 5V

RUN/SS

VIN

INTVCC

RUN/SS

CSS

RSS

CSS

RSS

1735 F06

(a)

(b)

Fault Conditions: Overcurrent Latchoff

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

controller and latch off 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 it’s 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 6. This current shortens the soft-start

period but also prevents net discharge of the RUN/SS

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