LTC1876EG datasheet(21/36 Pages) LINER

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LTC1876

1876fa

Soft-Start/Run Function

The RUN/SS1 and RUN/SS2 pins are multipurpose pins

that provide a soft-start function and a means to shut

down the LTC1876 step-down controllers. Soft-start re-

duces the input power source’s surge currents by gradu-

ally increasing the controller’s current limit (proportional

to VITH). This pin can also be used for power supply

sequencing.

An internal 1.2

µA current source charges up the CSS

capacitor. When the voltage on RUN/SS1 (RUN/SS2)

reaches 1.5V, the particular controller is permitted to start

operating. As the voltage on RUN/SS increases from 1.3V

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 1.2s/

µF to reach full cur-

rent. The output current thus ramps up slowly, reducing

the starting surge current required from the input power

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

there is a delay before starting of approximately:

F C

DELAY

=µ

()

125

F C

IRAMP

−

=µ

()

31 5

125

By pulling both RUN/SS pins below 1.0V and/or pulling the

STBYMD pin below 0.2V, the controllers are 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 Dia-

gram).

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 of the controller. After the control-

ler 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 4.1V, CSS

begins discharging on the assumption 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 voltage is recycled. If the

overload occurs during start-up, the time can be approxi-

mated 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)

If an overload occurs on one channel, it will also latch off

the other channel. 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

prototype 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.

APPLICATIO S I FOR ATIO

Figure 7. RUN/SS Pin Interfacing

3.3V OR 5V

RUN/SS

VIN

INTVCC

RUN/SS

CSS

RSS*

CSS

RSS*

1876 F07

(a)

(b)

*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF

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