LTC3707IGN datasheet(18/32 Pages) LINER

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LTC3707

3707fb

APPLICATIONS INFORMATION

EXTVCC Connection

The LTC3707 contains an internal P-channel MOSFET

switch connected between the EXTVCC and INTVCC pins.

When the voltage applied to EXTVCC rises above 4.7V,

the internal regulator is turned off and the switch closes,

connecting the EXTVCC pin to the INTVCC pin thereby sup-

plying internal power. The switch remains closed as long

as the voltage applied to EXTVCC remains above 4.5V. This

allows the MOSFET driver and control power to be derived

from the output during normal operation (4.7V < VOUT <

7V) and from the internal regulator when the output is

out of regulation (start-up, short-circuit). If more current

is required through the EXTVCC switch than is speciﬁed,

an external Schottky diode can be added between the

EXTVCC and INTVCC pins. 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

supply 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 5V 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 regulator and provides the highest

efﬁciency.

3. EXTVCC Connected to an External supply. If an external

supply is available in the 5V to 7V range, it may be used to

power EXTVCC providing it is compatible with the MOSFET

gate drive requirements.

4. EXTVCCConnectedtoanOutput-DerivedBoostNetwork.

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 6a or the capacitive charge pump

shown in Figure 6b. The charge pump has the advantage

of simple magnetics.

EXTVCC

FCB

SGND

VIN

TG1

BG1

PGND

LTC3707

RSENSE

VOUT

VSEC

COUT

1μF

3707 F06a

N-CH

CIN

VIN

1:N

OPTIONAL EXTVCC

CONNECTION

5V < VSEC < 7V

EXTVCC

VIN

TG1

BG1

PGND

LTC3707

VOUT

VN2222LL

COUT

3707 F06b

N-CH

CIN

1μF

VIN

BAT85

0.22μF

RSENSE

Figure 6a. Secondary Output Loop & EXTVCC Connection

Figure 6b. Capacitive Charge Pump for EXTVCC

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