LTC4366 datasheet(23/32 Pages) AD

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LTC7862

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

Fault Conditions: Output Overvoltage Protection

(Crowbar)

The overvoltage crowbar is designed to blow a system

input fuse when the output voltage of the regulator rises

much higher than nominal levels. The crowbar causes

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

a shorted top MOSFET if the short occurs while the con-

troller 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.

A shorted top MOSFET will result in a high current con-

dition 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.

Fault Conditions: Overtemperature Protection

At higher temperatures, or in cases where the internal

power dissipation causes excessive self heating on chip,

the overtemperature shutdown circuitry will shut down

the LTC7862. When the junction temperature exceeds

approximately 175°C, the overtemperature circuitry

disables the DRVCC LDO, causing the DRVCC supply to

collapse and effectively shutting down the entire LTC7862

chip. Once the junction temperature drops back to the

approximately 155°C, the DRVCC LDO turns back on. Long

term overstress (TJ > 150°C) should be avoided as it can

degrade the performance or shorten the life of the part.

Minimum On-Time Considerations

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

that the LTC7862 is capable of turning on the top MOSFET.

It is determined by internal timing delays and the gate

charge required to turn on the top MOSFET. Low duty

cycle applications may approach this minimum on-time

limit and care should be taken to ensure that:

tON(MIN) <

VOUT

VIN(f)

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 minimum on-time for the LTC7862 is approximately

80ns. However, as the peak sense voltage decreases the

minimum on-time gradually increases up to about 130ns.

If the duty cycle drops below the minimum on-time limit

in this situation, a significant amount of cycle skipping

can occur with correspondingly larger current and volt-

age ripple.

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