LTC3614_15 Datasheet, PDF (12/30 Pages) Linear Technology – 4A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3614
Operation
Burst Mode Operation—External Clamp
Connecting the MODE pin to a voltage in the range of 0.45V
to 0.8V enables Burst Mode operation with external clamp.
During this mode of operation the minimum voltage on
the ITH pin is externally set by the voltage on the MODE
pin. It is recommended to use Burst Mode operation with
internal burst clamp for temperatures above 85°C ambient.
Pulse-Skipping Mode Operation
Pulse-skipping mode is similar to Burst Mode operation,
but the LTC3614 does not disable power to the internal
circuitry during sleep mode. This improves output voltage
ripple but uses more quiescent current, compromising
light load efficiency.
Tying the MODE pin to SV_IN enables pulse-skipping mode.
As the load current decreases, the peak inductor current
will be determined by the voltage on the ITH pin until the
ITH voltage drops below the voltage level corresponding to
0A. At this point, the peak inductor current is determined
by the minimum on-time of the current comparator. If
the load demand is less than the average of the minimum
on-time inductor current, switching cycles will be skipped
to keep the output voltage in regulation.
Forced Continuous Mode
In forced continuous mode the inductor current is con-
stantly cycled which creates a minimum output voltage
ripple at all output current levels.
Connecting the MODE pin to a voltage in the range of
1.1V to SV_IN • 0.58 will enable forced continuous mode
operation.
At light loads, forced continuous mode operation is less
efficient than Burst Mode or pulse-skipping operation, but
may be desirable in some applications where it is neces-
sary to keep switching harmonics out of the signal band.
Forced continuous mode must be used if the output is
required to sink current.
Dropout Operation
As the input supply voltage approaches the output voltage,
the duty cycle increases toward the maximum on-time.
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 voltage drop
across the internal P-channel MOSFET and the inductor.
Low Supply Operation
The LTC3614 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 LTC3614 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 at which 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 LTC3614
clamps the maximum ITH pin voltage at approximately 0.8V
which corresponds typically to 9A peak inductor current.
When the output is shorted to ground, the inductor current
decays very slowly during a single switching cycle. The
LTC3614 uses two techniques to prevent current runaway
from occurring.
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