LTC7803 datasheet(24/34 Pages) AD

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LTC7802-3.3

Rev. 0

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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 VIN LDO resulting in an efficiency

penalty of up to 10% or more at high input voltages.

2. EXTVCC connected directly to one of the regulator out-

puts. This is the normal connection for an application

with an output in the range of 5V to 30V and provides

the highest efficiency. If both outputs are in the 5V to

30V range, connect EXTVCC to the lesser of the two

outputs to maximize efficiency.

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

supply is available, it may be used to power EXTVCC

provided that it is compatible with the MOSFET gate

drive requirements. This 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 or charge

pump. For regulators where both outputs are below

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

ing EXTVCC to an output-derived voltage that has been

boosted to greater than 4.8V.

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors CB connected to the

BOOST pins supply the gate drive voltages for the top-

side MOSFETs. Capacitor CB in the Functional Diagram is

charged though external diode DB from INTVCC when the

SW pin is low.

When one of the topside MOSFETs is to be turned 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 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 MOSFET(s). For a typical application, a value of

CB = 0.1μF is generally sufficient.

The external diode DB can be a Schottky diode or sili-

con diode, but in either case it should have low leakage

and fast recovery. The reverse breakdown of the diode

must be greater than VIN(MAX). Pay close attention to the

reverse leakage at high temperatures where it generally

increases substantially.

A leaky diode not only increases the quiescent current

of the regulator, but it can create current path from the

BOOST pin to INTVCC. This will cause INTVCC to rise if

the diode leakage exceeds the current consumption on

INTVCC, which is primarily a concern in Burst Mode opera-

tion where the load on INTVCC can be very small. There

is an internal voltage clamp on INTVCC that prevents the

INTVCC voltage from running away, but this clamp should

be regarded as a failsafe only.

Minimum On-Time Considerations

Minimum on-time tON(MIN) is the smallest time duration

that the LTC7802-3.3 is capable of turning on the top

MOSFET. It is determined by internal timing delays and

the gate charge required to turn on the MOSFET. Low duty

cycle applications may approach this minimum on time

limit and care should be taken to ensure that:

tON(MIN) <

VOUT

VIN•fOSC

If the duty cycle falls below what can be accommodated

by the minimum on-time, the controller will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple voltage and current will increase. The mini-

mum on-time for the LTC7802-3.3 is approximately 40ns.

However, as the peak sense voltage decreases the mini-

mum on-time gradually increases up to about 60ns. This

is of particular concern in forced continuous applications

with low ripple current at light loads. If the duty cycle

drops below the minimum on-time limit in this situation,

a significant amount of cycle skipping can occur with cor-

respondingly larger current and voltage ripple.

APPLICATIONS INFORMATION

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