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LTC1438/LTC1439

APPLICATIONS INFORMATION

the internal current limit.

Power supply sequencing can

also be accomplished using this pin.

An internal 3

µA current source charges up an external

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

reaches 1.3V the particular controller is permitted to start

operating. As the voltage on the pin continues to ramp

from 1.3V to 2.4V, the internal current limit is also ramped

at a proportional linear rate. The current limit begins at

approximately 50mV/RSENSE (at VRUN/SS = 1.3V) and ends

at 150mV/RSENSE (VRUN/SS ≥ 2.7V). 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 500ms/

µF, followed by a similar time to

reach full current on that controller.

By pulling both RUN/SS controller pins below 1.3V, the

LTC1438/LTC1439 are put into low current shutdown

(IQ < 25µA). These pins can be driven directly from logic

as shown in Figure 6. Diode D1 in Figure 6 reduces the start

delay but allows CSS to ramp up slowly providing the soft

start function; this diode and CSS can be deleted if soft start

is not needed. Each RUN/SS pin has an internal 6V Zener

clamp (See Functional Diagram).

Figure 5b. LTC1438/LTC1439 Adjustable Applications

LTC1438

LTC1439

VPROG1

SENSE – 1

SGND

GND: VOUT = 3.3V

INTVCC: VOUT = 5V

VOUT

1438 F05a

COUT

Figure 5a. LTC1438/LTC1439 Fixed Output Applications

Power-On Reset Function (POR)

The power-on reset function (not available on the

LTC1438X) monitors the output voltage of the second

controller and turns on an open drain device when it is

below its properly regulated voltage. An external pull-up

resistor is required on the POR2 pin.

When power is first applied or when coming out of

shutdown, the POR2 output is held at ground. When the

output voltage rises above a level which is 5% below the

final regulated output value, an internal counter starts.

After this counter counts 216 (65536) clock cycles, the

POR2 pull-down device turns off.

The POR2 output will go low whenever the output voltage

of the second controller drops below 7.5% of its regulated

value for longer than approximately 30

µs, signaling an

out-of-regulation condition. In shutdown, when RUN/SS1

and RUN/SS2 are both below 1.3V, the POR2 output is

pulled low even if the regulator’s output is held up by an

external source. The POR2 output is active during shut-

down if VIN is powered.

Run/ Soft Start Function

The RUN/SS1 and RUN/SS2 pins each serve two func-

tions. Each pin provides the soft start function and a

means to shut down each controller. Soft start reduces

surge currents from VIN by providing a gradual ramp-up of

CSS

3.3V

OR 5V

RUN/SS1

(RUN/SS2)

CSS

1438 F06

RUN/SS1

(RUN/SS2)

Figure 6. RUN/SS Pin Interfacing

Foldback Current Limiting

As described in Power MOSFET and D1 Selection, the

worst-case dissipation for either MOSFET occurs with a

short-circuited output, when the synchronous MOSFET

conducts the current limit value almost continuously. In

most applications this will not cause excessive heating,

even for extended fault intervals. However, when heat

sinking is at a premium or higher RDS(ON) MOSFETs are

being used, foldback current limiting should be added to

reduce the current in proportion to the severity of the fault.

Foldback current limiting is implemented by adding diode

DFB between the output and the ITH pin as shown in the

LTC1438

LTC1439

VPROG2*

VOSENSE1,2

SGND

OPEN (DC)

1.19V

≤ VOUT ≤ 9V

1438 F05b

100pF

VOUT = 1.19V 1 +

()

*LTC1439 ONLY

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