LTC3306CACBZ-R7 datasheet(9/16 Pages) AD

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LTC3306

Rev. 0

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OPERATION

Output Voltage Soft-Start

Soft-starting the output is needed to prevent current

surge from the input supply and output voltage overshoot

during initial start-up or after a fault condition. During

soft-start, the output voltage will proportionally track an

internal voltage ramp. This ramp voltage is reset if the

EN pin transitions low (into shutdown), or during a fault

condition, allowing for graceful fault recovery. Fault condi-

tions that reset the soft-start ramp are VIN UVLO, output

short-circuit, and thermal shutdown.

Dropout Operation

As the input supply voltage approaches the output volt-

age, the duty cycle increases toward 100%. Further

reduction of the supply voltage forces the main switch to

remain on for more than one cycle, eventually reaching

100% duty cycle. The output voltage during dropout will

then be determined by the input voltage minus the DC

voltage drop across the internal P-channel MOSFET and

the inductor.

Low Supply Operation

The LTC3306 is designed to operate down to an input

supply voltage of 2.25V. One important consideration at

low input supply voltages is that the RDS(ON)s of the inter-

nal power switches increase. Calculate the worst-case

LTC3306 power dissipation and die junction temperature

at the lowest applicable input voltages.

Output Short-Circuit Protection and Recovery

When the output is shorted to ground, the inductor cur-

rent decays very slowly when the bottom power switch

is on, because the voltage across the inductor is low. To

keep the inductor current in control, a secondary limit is

imposed on the valley of the inductor current. If the induc-

tor current measured through the bottom power switch is

greater than IVALLEYMAX, the top power switch will be held

off, preventing a further increase in current. Subsequent

switching cycles will be skipped until the inductor current

is reduced below IVALLEYMAX.

Recovery from an output short-circuit initiates a soft-start

cycle. When VOUT falls below regulation, as defined by the

PGOOD threshold, the VOUT voltage will not be allowed to

rise faster than the internal soft-start ramp.

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