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LTC3707

3707fb

APPLICATIONS INFORMATION

By pulling both RUN/SS pins below 1V and/or pulling

the STBYMD pin below 0.2V, the LTC3707 is put into

low current shutdown (IQ = 20μA). The RUN/SS pins

can be driven directly from logic as shown in Figure 7.

Diode D1 in Figure 7 reduces the start delay but allows

CSS to ramp up slowly providing the soft-start function.

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

Functional Diagram).

Fault Conditions: Overcurrent Latchoff

The RUN/SS pins also provide the ability to latch off the

controller(s) when an overcurrent condition is detected.

The RUN/SS capacitor, CSS, is used initially to turn on

and limit the inrush current. After the controller has been

started and been given adequate time to charge up the

output capacitor and provide full load current, the RUN/SS

capacitor is used for a short-circuit timer. If the regulator’s

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

after CSS reaches 4.2V, CSS begins discharging on the as-

sumption that the output is in an overcurrent condition. If

the condition lasts for a long enough period as determined

by the size of the CSS and the speciﬁed discharge current,

the controller will be shut down until the RUN/SS pin volt-

age is recycled. If the overload occurs during start-up, the

time can be approximated by:

tLO1 ≈ [CSS(4.1 – 1.5 + 4.1 – 3.5)]/(1.2μA)

= 2.7 • 106 (CSS)

If the overload occurs after start-up the voltage on CSS will

begin discharging from the zener clamp voltage:

tLO2 ≈ [CSS (6 – 3.5)]/(1.2μA) = 2.1 • 106 (CSS)

This built-in overcurrent latchoff can be overridden by

providing a pull-up resistor to the RUN/SS pin as shown

in Figure 7. This resistance shortens the soft-start period

and prevents the discharge of the RUN/SS capacitor during

an over current condition. Tying this pull-up resistor to

VIN as in Figure 7a, defeats overcurrent latchoff. Diode-

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

7b, eliminates any extra supply current during controller

shutdown while eliminating the INTVCC loading from

preventing controller start-up.

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

troubleshooting 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 may need to be

scaled with output voltage, 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 LTC3707 current comparator has a maximum sense

voltage of 75mV resulting in a maximum MOSFET cur-

rent of 75mV/RSENSE. The maximum value of current

limit generally occurs with the largest VIN at the highest

ambient temperature, conditions that cause the highest

power dissipation in the top MOSFET.

The LTC3707 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 overridden. If the

output falls below 70% of its nominal output level, then

the maximum sense voltage is progressively lowered from

75mV to 25mV. Under short-circuit conditions with very

low duty cycles, the LTC3707 will begin cycle skipping in

order to limit the short-circuit current. In this situation

the bottom MOSFET will be dissipating most of the power

3.3V OR 5V

RUN/SS

VIN

INTVCC

RUN/SS

CSS

RSS*

CSS

RSS*

3707 F07

*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF

Figure 7. RUN/SS Pin Interfacing

(a)

(b)

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