LTC1709_15 datasheet(18/28 Pages) LINER

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LTC1709

APPLICATIO S I FOR ATIO

Figure 6. RUN/SS Pin Interfacing

Diode D1 in Figure 6 reduces the start delay but allows CSS

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

RUN/SS pin has an internal 6V zener clamp (see Func-

tional Diagram).

3.3V OR 5V

RUN/SS

VIN

INTVCC

RUN/SS

D1*

CSS

RSS*

CSS

RSS*

1709 F06

*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF

floated or connected to VBIAS to get a digital high input. The

series diode is used to prevent the digital inputs from

being damaged or clamped if they are driven higher than

VBIAS. The digital inputs accept CMOS voltage levels.

VBIAS is the supply voltage for the VID section. It is

normally connected to INTVCC but can be driven from

other sources. If it is driven from another source, that

source MUST be in the range of 2.7V to 5.5V and MUST be

alive prior to enabling the LTC1709.

Soft-Start/Run Function

The RUN/SS pin provides three functions: 1) Run/Shut-

down, 2) soft-start and 3) a defeatable short-circuit latchoff

timer. Soft-start reduces the input power sources’ surge

currents by gradually increasing the controller’s current

limit ITH(MAX). The latchoff timer prevents very short,

extreme load transients from tripping the overcurrent

latch. A small pull-up current (>5

µA) supplied to the RUN/

SS pin will prevent the overcurrent latch from operating.

The following explanation describes how the functions

operate.

An internal 1.2

µA current source charges up the soft-start

capacitor, CSS. When the voltage on RUN/SS reaches

1.5V, the controller is permitted to start operating. 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. If

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

delay before starting of approximately:

F C

DELAY

=µ

()

125

The time for the output current to ramp up is then:

F C

RAMP

−

=µ

()

31 5

125

By pulling the RUN/SS pin below 0.8V the LTC1709 is put

into low current shutdown (IQ < 40µA). The RUN/SS pins

can be driven directly from logic as shown in Figure 6.

Fault Conditions: Overcurrent Latchoff

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

controllers when an overcurrent condition is detected. The

RUN/SS capacitor, CSS, is used initially to limit the inrush

current of both controllers. After the controllers have been

started and been given adequate time to charge up the

output capacitors and provide full load current, the RUN/

SS capacitor is used for a short-circuit timer. If the 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 CSS, 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 approximated by:

tLO1 ≈ (CSS • 0.6V)/(1.2µA) = 5 • 105 (CSS)

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

continue charging and will provide additional time before

latching off:

tLO2 ≈ (CSS • 3V)/(1.2µA) = 2.5 • 106 (CSS)

This built-in overcurrent latchoff can be overridden by

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

shown in Figure 6. This resistance shortens the soft-start

period and prevents the discharge of the RUN/SS capaci-

tor during a severe overcurrent and/or short-circuit con-

dition. When deriving the 5

µA current from VIN as in the

figure, current latchoff is always defeated. Diode connect-

ing this pull-up resistor to INTVCC, as in

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