LTC3313JVPBF datasheet(12/28 Pages) AD

12 / 28 page

LTC3313

Rev. 0

For more information www.analog.com

OPERATION

Soft-Start/Tracking/Temperature Monitor

The soft-start tracking function facilitates supply sequenc-

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

overshoot. When soft-start is completed, the SSTT pin

parks itself at a voltage representative of the LTC3313

die junction temperature. The SSTT capacitor is reset

during shutdown, VIN UVLO, and thermal shutdown. See

Applications Information section.

Dropout Operation

As the input supply voltage approaches the output volt-

age, the duty cycle increases. 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 will then be determined by the

input voltage minus the DC voltage drop across the inter-

nal main P-channel MOSFET and the inductor.

In many designs when the input voltage approaches the

output voltage, the amplitude of the output ripple volt-

age increases from its normally low value. To avoid any

increase in output ripple voltage under these conditions,

it is recommended to utilize a resistor divider on the EN

input and limit the VIN turn-on and turn-off thresholds to

where the output ripple voltage is acceptable for the given

application (typically 500mV above VOUT).

Low Supply Operation

The LTC3313 is designed to operate down to an input

supply voltage of 2.25V. An important thermal design

consideration is that the RDS(ON)s of the power switches

increase at low VIN. Calculate the worst case LTC3313

power dissipation and die junction temperature at the low-

est 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

voltage on the ITH pin. If the output current increases, the

error amplifier raises the ITH pin voltage until the average

inductor current matches the load current. The LTC3313

clamps the maximum ITH pin voltage, thereby limiting the

peak inductor current.

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

rent decays very slowly during a single switching cycle

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 the IVALLEY(MAX), the top power switch will

be held off. Subsequent switching cycles will be skipped

until the inductor current is reduced below IVALLEY(MAX).

Recovery from an output short circuit goes through a

soft-start cycle. When VOUT goes below regulation, as

defined by the PGOOD threshold, the SSTT voltage is

pulled to a voltage just above the FB voltage. Because

the SSTT pin is pulled low, a soft-start cycle is initiated

once the output short is removed.

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