LTC1735CS datasheet(16/32 Pages) LINER

16 / 32 page

LTC1735

Topside MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor CB connected to the

BOOST pin supplies the gate drive voltage for the topside

MOSFET. Capacitor CBintheFunctionalDiagramischarged

though external diode DB from INTVCC when the SW pin

is low. Note that the voltage across CB is about a diode

drop below INTVCC. When the topside MOSFET is to be

turned on, the driver places the CB voltage across the

gate-source of the MOSFET. This enhances the MOSFET

and turns on the topside switch. The switch node voltage

SW rises to VIN and the BOOST pin rises to VIN + INTVCC.

The value of the boost capacitor CB needs to be 100 times

greater than the total input capacitance of the topside

MOSFET. In most applications 0.1

µF to 0.33µF is ad-

equate. The reverse breakdown on DB must be greater

than VIN(MAX).

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 in

step-down applications is guaranteed throughout this

range allowing output voltages 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 to either be

sourced or sunk by 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. This

forces a minimum load current that can be fulfilled by the

APPLICATIO S I FOR ATIO

Figure 4. Setting the LTC1735 Output Voltage

VOSENSE

SGND

VOUT

1735 F04

LTC1735

47pF

VOUT resistive divider. The maximum 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 4 to have a maximum value to

absorb this current:

MAX

OUT

124

()

.–









Regulating an output voltage of 1.8V, the maximum value

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

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

sense pin 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.

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 into

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

be driven directly from logic as shown in Figure 5. Releas-

ing 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

begins operating with a current limit at approximately

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

LTC1735CS 数据表(PDF) 16 Page - Linear Technology

LTC1735CS 数据表(HTML) 16 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

数据表下载