LTC1159CS datasheet(12/20 Pages) LINER

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LTC1159

LTC1159-3.3/LTC1159-5

4. EXTVCC Connected to an External Supply. If an external

supply is available in the 5V to 12V range, it may be used

to power EXTVCC providing it is compatible with the

MOSFET gate drive requirements. There are no restrictions

on the EXTVCC voltage relative to VIN. EXTVCC may be

higher than VIN providing EXTVCC does not exceed the 15V

absolute maximum rating.

When driving standard threshold MOSFETs, the exter-

nal supply must always be present during operation to

prevent MOSFET failure due to insufficient gate drive. The

LTC1149 family should also be considered for applications

which require the use of standard threshold MOSFETs.

Important Information About LTC1159 Adjustable

Applications

When an output voltage other than 3.3V or 5V is required,

the LTC1159 adjustable version is used with an external

resistive divider from VOUT to the VFB pin (Figure 6). The

regulated voltage is determined by:

VOUT =

1.25V

)

)1+ R2

The VFB pin is extremely sensitive to pickup from the

inductor switching node. Care should be taken to isolate

the feedback network from the inductor, and the 100pF

capacitor should be connected between the VFB and SGND

pins next to the package.

In LTC1159N and LTC1159S applications with VOUT >

5.5V, the VCC pin may self-power through the SENSE pins

when SHDN2 is taken high, preventing shutdown. In these

applications, a pull-down must be added to the SENSE– pin

as shown in Figure 6. This pull-down effectively takes the

place of the SHDN1 pin, ensuring complete shutdown.

Note: For versions in which both the SHDN1 and SHDN2

pins are available (LTC1159G and all fixed output ver-

sions), the two pins are simply connected to each other and

driven together to guarantee complete shutdown.

The Figure 6 circuit cannot be used to regulate a VOUTwhich

is greater than the maximum voltage allowed on the

LTC1159 SENSE pins (13V). In applications with VOUT >

13V, RSENSE must be moved to the ground side of the

output capacitor and load. This operates the current sense

comparator at 0V common mode, increasing the off-time

approximately 40% and requiring the use of a smaller

timing capacitor CT.

Inverting Regular Applications

The LTC1159 can also be used to obtain negative output

voltages from positive inputs. In these inverting applica-

tions, the current sense resistor connects to ground while

the LTC1159 and N-channel MOSFET connections, which

would normally go to ground, instead ride on the negative

output. This allows the negative output voltage to be set by

Figure 6. High Efficiency Adjustable Regulator with 5.5V < VOUT < 13V

0.15

µF

VIN

CAP

P-DRIVE

PGND

EXTVCC

VFB

LTC1159

ITH

P-GATE

VCC

SHDN2

SENSE–

SENSE+

Si4840DY

Si4401DY

VN2222LL

0.1

µF

100pF

1N4148

VIN

1N5819

0.01

µF

100

Ω

100

Ω

100

µF

50V

150

µF

16V

OS-CON

µF

0V = NORMAL

>3V = SHUTDOWN

3300pF

390pF

N-GATE

SGND

VOUT

24.9k

LTC1159 • F06

215k

VOUT =

()

1 + R2

VALUES SHOWN FOR VOUT = 12V/2.5A

100

µH

RSENSE

0.039

Ω

1.25

APPLICATIO S I FOR ATIO

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