LTC1735 Ver la hoja de datos (PDF) - Linear Technology

Número de pieza

componentes Descripción

Fabricante

LTC1735

High Efficiency Synchronous Step-Down Switching Regulator

Linear Technology 

LTC1735

APPLICATIO S I FOR ATIO

VOUT

VOSENSE

LTC1735

SGND

R2

47pF R1

1735 F04

Figure 4. Setting the LTC1735 Output Voltage

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 CB in the Functional Diagram is charged

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 0.1µF 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

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:

R1(MAX)

=



24k

0.8V

2.4V – VOUT





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

shutdown. This pin can be driven directly from logic as

shown in Figure 5. The VIN quiescent current is a function

of RUN/SS voltage (refer to Typical Performance Charac-

teristics graphs on page 6). Releasing the RUN/SS pin

allows an internal 1.2µA 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:

( ) tDELAY

=

1.5V

1.2µA

CSS

=

1.25s/µF

CSS

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.

1735fc

17

Share Link: