LTC3617_15 Datasheet, PDF (9/20 Pages) Linear Technology – 6A Monolithic Synchronous Step-Down Regulator for DDR Termination

LTC3617
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 V_FB 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 SV_IN, 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
reference voltage of the error amplifier which controls the
output voltage. Therefore, large transients on this pin will
impact the behavior of the output.
V_IN Overvoltage Protection
In order to protect the internal power MOSFET devices
against transient voltage spikes, the LTC3617 constantly
monitors the V_IN pin for an overvoltage condition. When
V_IN 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 R_DS(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
clamps the maximum ITH pin voltage at approximately 1.05V
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.
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