LTC3312SAAVPBF datasheet(15/28 Pages) AD

15 / 28 page

LTC3312SA

Rev. 0

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An output overvoltage event should not happen under

normal operating conditions.

Overtemperature Protection

To prevent thermal damage to the LTC3312SA, 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).

Soft-Start/Tracking/Temperature Monitor

The soft-start tracking function facilitates supply sequenc-

ing, limits VIN inrush current, and reduces start-up output

overshoot. Each buck regulator has an SSTT pin, which

sources a 4µA current when that buck is enabled. When the

SSTT pin voltage is low, it replaces the 500mV reference

normally used for regulating the FB voltage. A capacitor

on the SSTT pin to ground can be used to set the soft-start

ramp time for each buck regulator. When soft-starting is

completed, each SSTT pin parks itself at a voltage rep-

resentative of the LTC3312SA die junction temperature.

An alternative is to connect an SSTT pin to AVIN, which

selects an internally-set 1ms default soft-start time for

that buck regulator.

An active pull-down circuit is connected to either the SSTT

pin, or to the internal soft-start node if SSTT is connected

high. This is used to discharge it in the case of fault con-

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

cleared. Fault conditions that clear 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 voltage,

the duty-cycle increases until SW is low for a minimum

time of 30ns (typical). 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 LTC3312SA 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

LTC3312SA power dissipation and die junction tempera-

ture 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 induc-

tor current matches the load current. The LTC3312SA

clamps the maximum VC voltage, thereby limiting the

peak inductor 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 ILIMN 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 ILIMN.

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 that ~100mV and then follow the soft-start

ramp until regulation is reached.

OPERATION

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