LTC3314AACBZ-R7 datasheet(15/26 Pages) AD

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LTC3314A

Rev. 0

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output remains out of regulation for more than 100µs, the

PGOOD pin will be pulled low.

An output overvoltage event should not happen under

normal operating conditions.

Overtemperature Protection

To prevent thermal damage to the LTC3314A, the device

incorporates an overtemperature (OT) function. If the die

temperature reaches 165°C (typical, not tested), both

buck switching regulators will be shut down and remain

in shutdown until the die temperature falls to 160°C (typi-

cal, not tested).

Output Voltage Soft-Start

Soft-starting the output limits VIN inrush current and

reduces start-up output voltage overshoot. Each buck

regulator has an internal voltage ramp that initiates when

that regulator is enabled. During soft-start, the output

voltage will proportionately track the voltage ramp until

it reaches the regulation voltage.

An active pull-down circuit is connected to the internal

soft-start node to discharge it in the case of fault con-

ditions. The soft-start ramp will restart when the fault

is cleared. Fault conditions that discharge the soft-start

voltage ramp are the EN pin transitioning low, VIN voltage

falling too low, or thermal shutdown.

Dropout Operation

As the input supply voltage approaches the output volt-

age, the duty cycle increases until SW is low for a mini-

mum time of 30ns (typical, not tested). Further reduction

of the supply voltage forces the main switch to remain

on for more than one cycle, allowing the duty cycle to

increase despite the internally set minimum SW low-time.

As the input drops further, the main switch stays on for

more cycles, until it reaches the maximum allowed 16

cycles. At least one minimum low time is forced every

16 cycles in dropout, limiting the maximum duty cycle

to 99% (typical).

Deep in dropout, the output voltage will be approximately

determined by 0.99 times the input voltage, minus the

voltage drops across the internal high-side MOSFET and

the inductor.

Low Supply Operation

The LTC3314A 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) of the inter-

nal power switches increases. Calculate the worst case

LTC3314A power dissipation and die junction temperature

at the lowest input voltages.

Output Short-Circuit Protection and Recovery

The peak inductor current level, at which the current com-

parator shuts off the top power switch, is controlled by

the internal VC voltage. If the output current increases,

the error amplifier raises VC until the average inductor

current matches the load current. The LTC3314A clamps

the maximum VC voltage, thereby limiting the peak induc-

tor current.

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

inductor current measured through the bottom power

switch is greater than IVALLEYMAX at the end of a cycle, the

top power switch will be held off. Subsequent switching

cycles will be skipped until the inductor current is reduced

below IVALLEYMAX.

Recovery from an output short-circuit may involve a soft

start cycle if VFB falls more than approximately 100mV

below regulation. During such a recovery, VFB will quickly

charge up by ~100mV and then follow the soft-start ramp

until regulation is reached.

OPERATION

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