LTC7803 datasheet(21/36 Pages) AD

21 / 36 page

LTC7803

Rev 0

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APPLICATIONS INFORMATION

When the voltage applied to EXTVCC rises above 4.7V, the

VIN LDO is turned off and the EXTVCC LDO is enabled. The

EXTVCC LDO remains on as long as the voltage applied to

EXTVCC remains above 4.5V. The EXTVCC LDO attempts

to regulate the INTVCC voltage to 5.15V, so while EXTVCC

is less than 5.15V, the LDO is in dropout and the INTVCC

voltage is approximately equal to EXTVCC. When EXTVCC

is greater than 5.15V, up to an absolute maximum of 30V,

INTVCC is regulated to 5.15V.

Using the EXTVCC LDO allows the MOSFET driver and

control power to be derived from one of the LTC7803’s

switching regulator outputs (4.7V ≤ VOUT ≤ 30V) during

normal operation and from the VIN LDO when the output

is out of regulation (e.g., start-up, short-circuit). If more

current is required through the EXTVCC LDO than is speci-

fied, an external Schottky diode can be added between the

EXTVCC and INTVCC pins. In this case, do not apply more

than 6V to the EXTVCC pin.

Significant efficiency and thermal gains can be realized

by powering INTVCC from the output, since the VIN cur-

rent resulting from the driver and control currents will be

scaled by a factor of (Duty Cycle)/(Switcher Efficiency).

This is accomplished by tying the EXTVCC pin directly to

an output voltage that is greater than the INTVCC regula-

tion point. EXTVCC may also be connected to any other

supply in the system that is higher than the INTVCC regu-

lation point and capable of providing the MOSFET gate

drive current.

For the previous example, Tying the EXTVCC pin to an 8.5V

supply reduces the junction temperature in the previous

example from 125°C to:

TJ = 70°C + (20mA)(8.5V)(68°C/W) = 82°C

and from 125°C to 82°C in an MSOP package

TJ = 70°C + (34mA)(8.5V)(40°C/W) = 82°C

However, for 3.3V and other low voltage outputs, where

the output is less than the INTVCC regulation point and no

other supply available in the system, additional circuitry is

required to derive INTVCC power from the output.

Using the EXTVCC LDO allows the MOSFET driver and

control power to be derived from the LTC7803’s switch-

ing regulator output during normal operation and from

the VIN LDO when the output is out of regulation (e.g.,

start-up, short-circuit).

The following list summarizes the four possible connec-

tions for EXTVCC:

1. EXTVCC Grounded. This will cause INTVCC to be pow-

ered from the internal 5.15V 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 to 30V regulator and provides the

highest efficiency.

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

supply is available in the 5V to 30V range, it may be

used to power EXTVCC providing it is compatible with

the MOSFET gate drive requirements. The supply may

be higher or lower than VIN, however, a lower EXTVCC

voltage results in higher efficiency.

4. EXTVCC Connected to an Output-Derived Boost

Network. For regulators where the output is below

5V, efficiency gains can still be realized by connect-

ing EXTVCC to an output-derived voltage that has been

boosted to greater than 5V.

Topside MOSFET Driver Supply CB

An external bootstrap capacitor, CB, connected to the

BOOST pin supplies the gate drive voltage for the topside

MOSFET. Capacitor CB in the Block Diagram is charged

through external diode DB from INTVCC when the SW pin

is low. When the topside MOSFET is to be turned on, the

driver places the CB voltage across the gate-source of

the MOSFET. This enhances the top MOSFET switch and

turns it on. The switch node voltage, SW, rises to VIN and

the BOOST pin follows. With the topside MOSFET on, the

boost voltage is above the input supply: VBOOST = VIN +

VINTVCC. The value of the boost capacitor, CB, needs to be

100 times that of the total input capacitance of the topside

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