LTC1735CS-1 datasheet(16/28 Pages) LINER

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LTC1735-1

APPLICATIO S I FOR ATIO

When adjusting the gate drive level, the final arbiter is the

total input current for the regulator. If you make a change

and the input current decreases, then you improved the

efficiency. If there is no change in input current, then there

is no change in efficiency.

SENSE +/ SENSE – Pins

The common mode input range of the current comparator

is from 0V to 1.1(INTVCC). Continuous linear operation is

guaranteed throughout this range allowing output volt-

ages anywhere from 0.8V to 7V. A differential NPN input

stage is used and is biased with internal resistors from an

internal 2.4V source as shown in the Functional Diagram.

This causes current either to be sourced or sunk by these

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. This forces a minimum load current that

can be fulfilled by the VOUT resistive divider. The maxi-

mum current flowing out of the 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 3 to have a maximum value to

absorb this current:

R Max

VVOUT

124

()

.–









Regulating an output voltage of 1.8V, the maximum value

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

2.4V no maximum value of R1 is necessary to absorb the

sense currents; however, R1 is still bounded by the

VOSENSE feedback current.

Soft-Start/Run Function

The RUN/SS pin is a multipurpose pin that provides a soft-

start function and a means to shut down the LTC1735-1.

Soft-start reduces surge currents from VIN by gradually

increasing the controller’s current limit ITH(MAX). This pin

can also be used for power supply sequencing.

Pulling the RUN/SS pin below 1.5V puts the LTC1735-1

into a low quiescent current shutdown (IQ < 25µA). This

pin can be driven directly from logic as shown in Figures

4 and 5. Releasing the RUN/SS pin allows an internal

1.2

µA current source to charge up the external soft-start

capacitor CSS. If RUN/SS has been pulled all the way to

ground there is a delay before starting of approximately:

Cs F C

DELAY

=µ

()

125

When the voltage on RUN/SS reaches 1.5V the

LTC1735-1 begins operating with a current limit at ap-

proximately 25mV/RSENSE. As the voltage on RUN/SS

increases from 1.5V to 3V, the internal current limit is

increased from 25mV/RSENSE to 75mV/RSENSE. The out-

put current limit ramps up slowly, taking an additional

1.25s/

µF to reach full current. Ramping the output cur-

rent slowly reduces the starting surge current

required from the input supply.

Diode D1 in Figure 4 and Figure 5 reduces the start delay

while allowing CSS to charge up slowly for the soft-start

function. This diode and CSS can be deleted if soft-start is

not needed. The RUN/SS pin has an internal 6V zener

clamp (see Functional Diagram).

Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated

Figure 4. RUN/SS Pin Interfacing

3.3V OR 5V

RUN/SS

CSS

1735-1 F04

3.3V OR 5V

RUN/SS

VIN

INTVCC

RUN/SS

CSS

RSS

CSS

RSS

1735-1 F05

(a)

(b)

Fault Conditions: Overcurrent Latchoff

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

controller and latchoff 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 controller has been started and given adequate time to

charge up the output capacitor and provide full load

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