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LTC3617

3617fa

reference voltage of the error amplifier which controls the

output voltage. Therefore, large transients on this pin will

impact the behavior of the output.

VIN Overvoltage Protection

In order to protect the internal power MOSFET devices

against transient voltage spikes, the LTC3617 constantly

monitors the VIN pin for an overvoltage condition. When

VIN rises above 6.5V, the regulator suspends operation by

shutting off both MOSFETS. The regulator executes its

soft-start function when exiting an overvoltage condition.

Low Supply Operation

The LTC3617 is designed to operate down to an input

supply voltage of 2.25V. An important consideration at low

input supply voltages is that the RDS(ON) of the P-channel

and N-channel power switches increases. The user should

calculate the power dissipation when the LTC3617 is used

at 100% duty cycle with low input voltages to ensure that

thermal limits are not exceeded. See the Typical Perfor-

mance Characteristics graphs.

Short-Circuit Protection

The peak inductor current when the current comparator

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 new load current. In normal operation the LTC3617

clampsthemaximumITHpinvoltageatapproximately1.05V

which corresponds typically to 10A peak inductor current.

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

rent decays very slowly during a single switching cycle.

To prevent current runaway from occurring, a secondary

current limit is imposed on the inductor current. If the

inductor current measured through the bottom MOSFET

increases beyond 12A typical, the top power MOSFET will

be held off and switching cycles will be skipped until the

inductor current decreases below this limit.

operation

Main Control Loop

The LTC3617 is a monolithic, constant frequency, current

mode step-down DC/DC converter. During normal opera-

tion, the internal top power switch (P-channel MOSFET) is

turned on at the beginning of each clock cycle. Current in

the inductor increases until the current comparator trips

and turns off the top power switch. The peak inductor

current when the current comparator trips is controlled

by the voltage on the ITH pin. The error amplifier adjusts

the voltage on the ITH pin by comparing the feedback

signal from a resistor divider on the VFB pin with a refer-

ence voltage on the VTTR pin. VTTR is the output of an

op amp buffer that expresses one-half the voltage on the

VDDQIN pin. When the load current increases, it causes a

reduction in the feedback voltage relative to the reference.

The error amplifier raises the ITH voltage until the average

inductor current matches the new load current. Typical

voltage range for the ITH pin is from 0.2V to 1.05V with

0.575V corresponding to zero current.

When the top power switch shuts off, the synchronous

bottom power switch (N-channel MOSFET) turns on until

either the bottom current limit is reached or the next clock

cycle begins. The bottom current limit is typically set at

–10A.

The operating frequency defaults to 2.25MHz when RT is

connected to SVIN, or can be set by an external resistor

connected between the RT pin and ground, or by a clock

signal applied to the RT pin. The switching frequency can

be set from 300kHz to 4MHz.

Overvoltage and undervoltage comparators pull the

PGOOD output low if the output voltage varies more than

±8% (typical) from the set point.

VTTR Voltage Buffer Output

An internal high accuracy op amp buffer generates a VTTR

pin voltage that is equal to VDDQIN • 0.5. VTTR can source

and sink up to 10mA and is stable with a maximum bypass

capacitor of 0.1µF. Short-circuit current limit is set around

20mA to prevent damage to the op amp. VTTR is also the

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