LTC1735CS datasheet(15/32 Pages) LINER

15 / 32 page

LTC1735

APPLICATIO S I FOR ATIO

EXTVCC Connection

The LTC1735 contains an internal P-channel MOSFET

switch connected between the EXTVCC and INTVCC pins.

Whenever the EXTVCC pin is above 4.7V the internal 5.2V

regulator shuts off, the switch closes and INTVCC power is

supplied via EXTVCC until EXTVCC drops below 4.5V. This

allows the MOSFET gate drive and control power to be

derived from the output or other external source during

normal operation. When the output is out of regulation

(start-up, short circuit) power is supplied from the internal

regulator. Do not apply greater than 7V to the EXTVCC pin

and ensure that EXTVCC ≤ VIN.

Significant efficiency gains can be realized by powering

INTVCC from the output, since the VIN current resulting

from the driver and control currents will be scaled by a

factor of (Duty Cycle)/(Efficiency). For 5V regulators this

simply means connecting the EXTVCC pin directly to VOUT.

However, for 3.3V and other lower voltage regulators,

additional circuitry is required to derive INTVCC power

from the output.

The following list summarizes the four possible connec-

tions for EXTVCC:

1. EXTVCC left open (or grounded). This will cause INTVCC

to be powered from the internal 5.2V regulator resulting in

an efficiency penalty of up to 10% at high input voltages.

2. EXTVCC connected directly to VOUT. This is the normal

connection for a 5V output regulator and provides the

highest efficiency. For output voltages higher than 5V,

EXTVCC is required to connect to VOUT so the SENSE pins’

absolute maximum ratings are not exceeded.

3. EXTVCC connected to an output-derived boost network.

For 3.3V and other low voltage regulators, efficiency gains

can still be realized by connecting EXTVCC to an output-

derived voltage that has been boosted to greater than

4.7V. This can be done with either the inductive boost

winding as shown in Figure 3a or the capacitive charge

pump shown in Figure 3b. The charge pump has the

advantage of simple magnetics.

4. EXTVCC connected to an external supply. If an external

supply is available in the 5V to 7V range (EXTVCC ≤ VIN),

such as notebook main 5V system power, it may be used

to power EXTVCC providing it is compatible with the

MOSFET gate drive requirements. This is the typical case

as the 5V power is almost always present and is derived by

another high efficiency regulator.

Figure 3a. Secondary Output Loop and EXTVCC Connection

Figure 3b. Capacitive Charge Pump for EXTVCC

EXTVCC

FCB

SGND

VIN

PGND

LTC1735

RSENSE

VOUT

VSEC

6.8V

COUT

µF

1735 F03a

N-CH

CIN

VIN

1:N

1N4148

OPTIONAL EXTVCC

CONNECTION

≤ VSEC ≤ 7V

EXTVCC

VIN

PGND

LTC1735

RSENSE

VOUT

VN2222LL

COUT

1735 F03b

N-CH

CIN

µF

VIN

BAT85

0.22

µF

Output Voltage Programming

The output voltage is set by an external resistive divider

according to the following formula:

OUT =+









The resistive divider is connected to the output as shown

in Figure 4 allowing remote voltage sensing.

LTC1735CS 数据表(PDF) 15 Page - Linear Technology

LTC1735CS 数据表(HTML) 15 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

链接网址

LTC1735CS 数据表(PDF) 15 Page - Linear Technology

LTC1735CS 数据表(HTML) 15 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

链接网址

数据表下载