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LTC3807

3807fa

For more information www.linear.com/LTC3807

4. EXTVCCConnectedtoanOutput-DerivedBoostNetwork.

For 3.3V and other low voltage regulators, efficiency

gains can still be realized by connecting EXTVCC to an

output-derivedvoltagethathasbeenboostedtogreater

than 4.7V. This can be done with the capacitive charge

pump shown in Figure 8. Ensure that EXTVCC < VIN.

APPLICATIONS INFORMATION

Topside MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor, CB, connected to the

BOOST pin supplies the gate drive voltage for the topside

MOSFET.CapacitorCBintheFunctionalDiagramischarged

though 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 top-

side MOSFET(s). The reverse breakdown of the external

Schottky diode must be greater than VIN(MAX).

When adjusting the gate drive level, the final arbiter is the

total input current for the regulator. If a change is made

and the input current decreases, then the efficiency has

improved. If there is no change in input current, then there

is no change in efficiency.

EXTVCC

VIN

PGND

LTC3807

RSENSE

VOUT

NDS7002

COUT

3807 F08

MBOT

MTOP

CIN

BAT85

Figure 8. Capacitive Charge Pump for EXTVCC

Fault Conditions: Current Limit and Current Foldback

The LTC3807 includes current foldback to help limit

load current when the output is shorted to ground. If

the output voltage falls below 70% of its nominal output

level, then the maximum sense voltage is progressively

lowered from 100% to 45% of its maximum selected

value. Under short-circuit conditions with very low duty

cycles, the LTC3807 will begin cycle skipping in order to

limit the short-circuit current. In this situation the bottom

MOSFET will be dissipating most of the power but less

than in normal operation. The short-circuit ripple current

is determined by the minimum on-time, tON(MIN), of the

LTC3807 (≈95ns), the input voltage and inductor value:

DIL(SC) = tON(MIN)

VIN









The resulting average short-circuit current is:

ISC = 45%•ILIM(MAX)–

DIL(SC)

Fault Conditions: Overvoltage Protection (Crowbar)

The overvoltage crowbar is designed to blow a system

input fuse when the output voltage of the regulator rises

muchhigherthannominallevels.Thecrowbarcauseshuge

currents to flow, that blow the fuse to protect against a

shorted top MOSFET if the short occurs while the control-

ler is operating.

A comparator monitors the output for overvoltage condi-

tions. The comparator detects faults greater than 10%

above the nominal output voltage. When this condition

is sensed, the top MOSFET is turned off and the bottom

MOSFET is turned on until the overvoltage condition is

cleared. The bottom MOSFET remains on continuously

for as long as the overvoltage condition persists; if VOUT

returns to a safe level, normal operation automatically

resumes.

AshortedtopMOSFETwillresultinahighcurrentcondition

which will open the system fuse. The switching regulator

will regulate properly with a leaky top MOSFET by altering

the duty cycle to accommodate the leakage.

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