LTC1709 datasheet(16/28 Pages) LINER

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LTC1709

Figure 5a. Secondary Output Loop with EXTVCC Connection

Figure 5b. Capacitive Charge Pump for EXTVCC

external voltage source is applied to the EXTVCC pin when

the VIN supply is not present, a diode can be placed in

series with the LTC1709’s VIN pin and a Schottky diode

between the EXTVCC and the VIN pin, to prevent current

from backfeeding 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 the

ratio: (Duty Factor)/(Efficiency). For 5V regulators this

means connecting the EXTVCC pin directly to VOUT. How-

ever, for 3.3V and other lower voltage regulators, addi-

tional 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

a significant efficiency penalty 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. 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 but less than 7V. This can be done with either the

inductive boost winding as shown in Figure 5a or the

capacitive charge pump shown in Figure 5b. The charge

pump has the advantage of simple magnetics.

Topside MOSFET Driver Supply (CB,DB) (Refer to

Functional Diagram)

External bootstrap capacitors CB1 and CB2 connected to

the BOOST 1 and BOOST 2 pins supply the gate drive

voltages for the topside MOSFETs. Capacitor CB in the

Functional Diagram is charged though diode DB from

INTVCC when the SW pin is low. When the topside MOSFET

turns on, the driver places the CB voltage across the gate-

source of the desired MOSFET. This enhances the MOSFET

and turns on the topside switch. The switch node voltage,

SW, rises to VIN and the BOOST pin rises to VIN + VINTVCC.

The value of the boost capacitor CB needs to be 30 to 100

times that of the total input capacitance of the topside

MOSFET(s). The reverse breakdown of DB must be greater

than VIN(MAX).

The final arbiter when defining the best gate drive ampli-

tude level will be the input supply current. If a change is

made that decreases input current, the efficiency has

improved. If the input current does not change then the

efficiency has not changed either.

Output Voltage

The LTC1709 has a true remote voltage sense capablity.

The sensing connections should be returned from the load

back to the differential amplifier’s inputs through a com-

APPLICATIO S I FOR ATIO

1709 F05a

VIN

TG1

N-CH

1N4148

N-CH

BG1

PGND

LTC1709

SW1

EXTVCC

OPTIONAL EXTVCC CONNECTION

5V < VSEC < 7V

RSENSE

VSEC

VOUT

VIN

CIN

µF

COUT

1709 F05b

VIN

TG1

N-CH

BG1

PGND

LTC1709

SW1

EXTVCC

RSENSE

BAT85

0.22

µF

VOUT

VIN

CIN

COUT

VN2222LL

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