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LTC1267

Dual High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

UU W U

APPLICATIO S I FOR ATIO

the voltage comparator. This causes Burst Mode opera-

tion to be activated when the LTC1267 would normally be

in continuous operation. The effect is most pronounced

with low values of RSENSE and can be improved by oper-

ating at higher frequencies with lower values of L. The

output remains in regulation at all times.

EXT VCC Pin Connection

The LTC1267 contains an internal PNP switch connected

between the EXT VCC and VCC pins. The switch closes and

supplies the VCC power whenever the EXT VCC pin is higher

in voltage than the 4.5V internal regulator. This allows the

MOSFET driver and control power to be derived from the

output during normal operation and from the internal

regulator when the output is out of regulation (start-up,

short circuit).

Significant efficiency gain can be realized by powering VCC

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 LTC1267, LTC1267-ADJ or

LTC1267-ADJ5 this simply means connecting the EXT

VCC pin directly to VOUT of the 5V regulator.

The following list summarizes the four possible connec-

tions for EXT VCC:

1. EXT VCC left open. This will cause VCC to be powered

only from the internal 4.5V regulator, resulting in re-

duced MOSFET gate drive levels and an efficiency

penalty of up to 10% at high input voltages.

2. EXT VCC connected directly to highest VOUT of the two

regulators. This is the normal connection for LTC1267/

LTC1267-ADJ/LTC1267-ADJ5 and provides the high-

est efficiency.

3. EXT VCC connected to an output-derived boost net-

work. For 3.3V and other low voltage regulators, effi-

ciency gains can still be realized by connecting EXT VCC

to an output-derived voltage which has been boosted to

greater than 4.5V. This can be done either with the

inductive boost winding shown in Figure 5a or the

capacitive charge pump shown in Figure 5b. The charge

pump has the advantage of simple magnetics and

generally provides the highest efficiency at the expense

of a slightly higher parts count.

LTC1267

LTC1267-ADJ/LTC1267-ADJ5

EXT VCC VIN

PGATE 3

PDRIVE 3

LTC1267

NGATE 3

VIN

P-CH

N-CH

1N4148

CIN

L

1:1 •

•

RSENSE

PGND3

LTC1267 • F05A

Figure 5a. Inductive Boost Circuit for EXT VCC

1µF

VOUT

3.3V

COUT

EXT VCC VIN

PGATE 3

PDRIVE 3

LTC1267

NGATE 3

PGND3

VIN

CIN BAT 85

1µF BAT 85

0.22µF

P-CH

VN2222LL

L RSENSE

BAT 85

N-CH

LTC1267 • F05B

Figure 5b. Capacitive Charge Pump for EXT VCC

VOUT

3.3V

COUT

4. EXT VCC connected to an external supply. If an external

supply is available in the 5V to 10V range it may be used

to power EXT VCC providing it is compatible with the

MOSFET gate drive requirements. When driving stan-

dard threshold MOSFETs, the external supply must

always be present during operation to prevent MOSFET

failure due to insufficient gate drive.

Under the condition that EXT VCC is connected to VOUT1

which is greater than 5.5V, to power down the whole

regulator, both the pins MSHDN and SHDN1 have to be

pulled high. If SHDN1 is left floating or grounded the

EXT VCC may self-power from VOUT1, preventing com-

plete shutdown.

LTC1267 Adjustable Applications

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

the LTC1267-ADJ and LTC1267-ADJ5 adjustable ver-

sions are used with an external resistive divider from VOUT

to the VFB1, 2 pins. This is shown in Figure 6. The regulated

voltage is determined by:

) VOUT =

1 + R2

R1

1.25V

11

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