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LTC1775

EXTVCC Connection

The LTC1775 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 supplying

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 cir-

cuitry 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 a low current 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.

6.8V

1775 F06

EXTVCC

VIN

Figure 6. EXTVCC Power Supplied from VIN

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 which has been boosted to

greater than 4.7V. This can be done with either an

inductive boost winding as shown in Figure 5a or a

capacitive charge pump as shown in Figure 5b.

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

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

it may be used to power EXTVCC.

Figure 6 shows how one can easily generate a suitable

EXTVCC voltage from VIN. This circuit still derives the gate

drive current from VIN, but it removes the power dissipa-

tion from the LTC1775 internal regulator and increases the

gate drive voltage.

Figure 5a: Secondary Output Loop and EXTVCC Connection

Figure 5b: Capacitive Charge Pump for EXTVCC

VIN

LTC1775

SGND

FCB

EXTVCC

OPTIONAL

EXTVCC

CONNECTION

5V < VSEC < 7V

1775 F05a

1:N

PGND

CSEC

µF

VOUT

VSEC

VIN

CIN

1N4148

• +

COUT

VIN

LTC1775

EXTVCC

VPUMP ≈ 2(VOUT – VD) < 7V

1775 F05b

PGND

COUT

VOUT

BAT85

VN2222LL

VIN

CIN

µF

0.22

µF

Note that RDS(ON) also varies with the gate drive level. If

gate drives other than the 5.2V INTVCC are used, this must

be accounted for when selecting the MOSFET RDS(ON).

Particular care should be taken with applications where

EXTVCC is connected to the output. When the output

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