LTC3370 datasheet(11/20 Pages) AD

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LTC3302

Rev. 0

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OPERATION

Synchronizing the Oscillator to an External Clock

The LTC3302’s internal oscillator can be synchronized to

an external frequency by applying a square wave clock

signal to the MODE/SYNC pin. During synchronization,

the top power switch turn-on is locked to the rising edge

of the external frequency source. While synchroniz-

ing, the switcher operates in forced continuous mode.

The synchronization frequency range is ±20% of the

nominal frequency.

After detecting an external clock on the MODE/SYNC pin,

the rising edge of the internal clock is compared with the

rising edge of the external clock. When the edges are

aligned, the LTC3302 switches over to the external clock.

If the external clock is removed for more than 1.5 clock

cycles, the clock instantly reverts back to the internal clock.

Output Power Good

When the LTC3302’s output voltage is within the

–2%/+10% window of the nominal regulation voltage the

output is considered good and the open-drain PGOOD pin

goes high impedance and is typically pulled high with an

external resistor. Otherwise, the internal pull-down device

pulls the PGOOD pin low. The PGOOD pin is also pulled

low during the following fault conditions: EN pin is low,

VIN is too low, or thermal shutdown. To filter noise and

short duration output voltage transients, the lower thresh-

old has a hysteresis of 1.2%, the upper threshold has a

hysteresis of 2%, and both have a built-in time delay to

report PGOOD, typically 120µs.

Output Overvoltage Protection

During an output overvoltage event, when the FB pin volt-

age is greater than 110% of nominal, the LTC3302 top

power switch turns off. If the output remains out of regu-

lation for more than 120µs, the PGOOD pin is pulled low.

An output overvoltage event should not happen under

normal operating conditions.

Overtemperature Protection

To prevent thermal damage to the LTC3302 and its sur-

rounding components, the device incorporates an over-

temperature (OT) function. When the die temperature

reaches 165°C (typical, not tested) the switcher is shut

down and remains in shutdown until the die temperature

falls to 160°C (typical, not tested).

Output Voltage Soft-Start

Soft-starting the output prevents current surge on the

input supply and/or output voltage overshoot. During soft-

start, the output voltage proportionally tracks the internal

node voltage ramp. An active pull-down circuit discharges

that internal node in the case of fault conditions. The ramp

restarts when the fault is cleared. Fault conditions that

initiate the soft-start 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 toward 100%. Further reduc-

tion of the supply voltage forces the main switch to remain

on for more than one cycle, eventually reaching 100%

duty cycle. The output voltage is then determined by the

input voltage minus the DC voltage drop across the inter-

nal P-channel MOSFET and the inductor.

Low Supply Operation

The LTC3302 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

LTC3302 power dissipation and die junction temperature

at the lowest input voltages.

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