LTC3616_15 Datasheet, PDF (11/28 Pages) Linear Technology – 6A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3616
Operation
Main Control Loop
The LTC3616 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 cur-
rent at which 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 an internal 0.6V
reference. 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 0.1V to 1.05V with
0.45V corresponding to zero current.
When the top power switch shuts off, the synchronous
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 –8A for
forced continuous mode and 0A for Burst Mode operation
and pulse-skipping mode.
The operating frequency defaults to 2.25MHz when
RT/SYNC is connected to SV_IN, or can be set by an ex-
ternal resistor connected between the RT/SYNC pin and
ground, or by a clock signal applied to the RT/SYNC 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
±7.5% (typical) from the set point.
Mode Selection
The MODE pin is used to select one of four different
operating modes:
Mode Selection Voltage
SV_IN
SV_IN – 0.3V
SV_IN • 0.58
PS PULSE-SKIPPING MODE ENABLE
FC FORCED CONTINUOUS MODE ENABLE
1.1V
0.8V
0.45V
0.3V
SGND
BM
EXT
Burst Mode ENABLE—EXTERNAL CLAMP,
CONTROLLED BY VOLTAGE APPLIED AT
MODE PIN
BM Burst Mode ENABLE—INTERNAL CLAMP
3616 OP01
Burst Mode Operation—Internal Clamp
Connecting the MODE pin to SGND enables Burst Mode
operation with an internal clamp. In Burst Mode operation
the internal power switches operate intermittently at light
loads. This increases efficiency by minimizing switching
losses. During the intervals when the switches are idle,
the LTC3616 enters sleep state where many of the internal
circuits are disabled to save power. During Burst Mode
operation, the minimum peak inductor current is internally
clamped and the voltage on the ITH pin is monitored by
the burst comparator to determine when sleep mode is
enabled and disabled. When the average inductor current
is greater than the load current, the voltage on the ITH pin
drops. As the ITH voltage falls below the internal clamp,
the burst comparator trips and enables sleep mode. Dur-
ing sleep mode, the power MOSFETs are held off and the
load current is solely supplied by the output capacitor.
When the output voltage drops, the top power switch is
turned back on and the internal circuits are re-enabled.
This process repeats at a rate that is dependent on the
load current.
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