LTC7813 datasheet(21/32 Pages) LINER

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LTC7800

7800f

For more information www.linear.com/LTC7800

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).

Fault Conditions: Current Limit and Current Foldback

The LTC7800 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 LTC7800 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

LTC7800 (≈45ns), the input voltage and inductor value:

ΔIL(SC) = tON(MIN)

VIN













The resulting average short-circuit current is:

ISC = 45% •ILIM(MAX) –

ΔIL(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

aslongastheovervoltageconditionpersists;ifVOUTreturns

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.

Frequency Synchronization and Selection

The LTC7800 has an internal phase-locked loop (PLL)

comprised of a phase frequency detector, a lowpass filter,

and a voltage-controlled oscillator (VCO). This allows the

turn-on of the top MOSFET to be locked to the rising edge

of an external clock signal applied to the PLLIN/MODE pin.

The phase detector is an edge sensitive digital type that

provides zero degrees phase shift between the external

and internal oscillators. This type of phase detector does

not exhibit false lock to harmonics of the external clock.

If the external clock frequency is greater than the inter-

nal oscillator’s frequency, fOSC, then current is sourced

continuously from the phase detector output, pulling up

the VCO input. When the external clock frequency is less

than fOSC, current is sunk continuously, pulling down the

VCO input.

If the external and internal frequencies are the same but

exhibit a phase difference, the current sources turn on for

an amount of time corresponding to the phase difference.

The voltage at the VCO input is adjusted until the phase

and frequency of the internal and external oscillators are

identical. At the stable operating point, the phase detector

output is high impedance and the internal filter capacitor,

CLP, holds the voltage at the VCO input.

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