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LTC3728L-1

3728l1fc

APPLICATIONS INFORMATION

stage is biased with internal resistors from an internal 2.4V

source as shown in the Functional Diagram. This requires

that current either be sourced or sunk from the SENSE

pins depending on the output voltage. If the output voltage

is below 2.4V current will ﬂow out of both SENSE pins to

the main output. The output can be easily preloaded by

the VOUT resistive divider to compensate for the current

comparator’s negative input bias current. The maximum

current ﬂowing out of each pair of SENSE pins is:

ISENSE+ + ISENSE– = (2.4V – VOUT)/24k

Since VOSENSE is servoed to the 0.8V reference voltage,

we can choose R1 in Figure 2 to have a maximum value

to absorb this current.

R1(MAX) = 24k

0.8V

2.4V – VOUT

for VOUT < 2.4V

Regulating an output voltage of 1.8V, the maximum value

of R1 should be 32k. Note that for an output voltage above

2.4V, R1 has no maximum value necessary to absorb the

sense currents; however, R1 is still bounded by the VOSENSE

feedback current.

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

LTC3728L-1. Soft-start reduces the input power source’s

surge currents by gradually increasing the controller’s

current limit (proportional to VITH). This pin can also be

used for power supply sequencing.

An internal 1.2μA current source charges up the CSScapaci-

tor.WhenthevoltageonRUN/SS1(RUN/SS2)reaches1.5V,

the particular 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 start-

ing 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:

tDELAY =

1.5V

1.2

μA

CSS = 1.25s / μF

()C

tIRAMP =

3V 1.5V

1.2

μA

CSS = 1.25s / μF

()C

By pulling both RUN/SS pins below 1V, the IC 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). Because the LTC3728L-1 is designed

for applications not requiring over current latchoff, no

pull-up resistor is required on the RUN/SS pin to defeat

latchoff. Refer to the LTC3728L/LTC3728LX datasheet if

this feature is required.

Fault Conditions: Current Limit and Current Foldback

The current comparators have a maximum sense volt-

age of 75mV resulting in a maximum MOSFET current

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

generally occurs with the largest VIN at the highest ambi-

Figure 7. RUN/SS Pin Interfacing

3.3V OR 5V

RUN/SS

(a)

(b)

CSS

RUN/SS

CSS

3728L1 F07

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