LTC3621/LTC3621-2
8
3621fc
For more information www.linear.com/LTC3621
operaTion
The LTC3621 uses a constant-frequency, peak current
mode architecture. It operates through a wide V
IN
range
and regulates with ultralow quiescent current. The opera-
tion frequency is set at either 2.25MHz or 1MHz and can
be synchronized to an external oscillator ±40% of the
inherent frequency
. T
o suit a variety of applications, the
selectable MODE/SYNC pin allows the user to trade off
output ripple for efficiency.
The output voltage is set by an external divider returned to
the FB pin. An error amplifier compares the divided output
voltage with a reference voltage of 0.6V and adjusts the
peak inductor current accordingly. In the MS8E package,
overvoltage and undervoltage comparators will pull the
PGOOD output low if the output voltage is not within 7.5%
of the programmed value. The PGOOD output will go high
immediately after achieving regulation and will go low 32
clock cycles after falling out of regulation.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle.
The inductor current is allowed to ramp up to a peak level.
Once that level is reached, the top power switch is turned
off and the bottom switch (N-channel MOSFET) is turned
on until the next clock cycle. The peak current level is con
-
trolled by the internally compensated ITH voltage, which is
the output of the error amplifier. This amplifier compares
the FB voltage to the 0.6V internal reference. When the
load current increases, the FB voltage decreases slightly
below the reference, which causes the error amplifier to
increase the ITH voltage until the average inductor current
matches the new load current.
The main control loop is shut down by pulling the RUN
pin to ground.
Low Current Operation
Two discontinuous-conduction modes (DCMs) are available
to control the operation of the LTC3621 at low currents.
Both modes, Burst Mode operation and pulse-skipping,
automatically switch from continuous operation to the
selected mode when the load current is low.
To optimize efficiency, Burst Mode operation can be selected
by tying the MODE/SYNC pin to INTV
CC
. In Burst Mode
operation, the peak inductor current is set to be at least
400mA, even if the output of the error amplifier demands
less. Thus, when the switcher is on at relatively light output
loads, FB voltage will rise and cause the ITH voltage to
drop. Once the ITH voltage goes below 0.2V, the switcher
goes into its sleep mode with both power switches off.
The switcher remains in this sleep state until the external
load pulls the output voltage below its regulation point.
During sleep mode, the part draws an ultralow 3.5µA of
quiescent current from V
IN
.
To minimize V
OUT
ripple, pulse-skipping mode can be se-
lected by grounding the MODE/SYNC pin. In the LTC3621,
pulse-skipping mode is implemented similarly to Burst
Mode operation with the peak inductor current set to be
at about 66mA. This results in lower output voltage ripple
than in Burst Mode operation with the trade-off
being
slightly lower efficiency.
Forced Continuous Mode Operation
Aside from the two discontinuous-conduction modes,
the LTC3621 also has the ability to operate in the forced
continuous mode by setting the MODE/SYNC voltage
between 1V and V
INTVCC
– 1V. In forced continuous mode,
the switcher will switch cycle by cycle regardless of what
the output load current is. If forced continuous mode is
selected, the minimum peak current is set to be –133mA
in order to ensure that the part can operate continuously
at zero output load.
High Duty Cycle/Dropout Operation
When the input supply voltage decreases towards the output
voltage, the duty cycle increases and slope compensation
is required to maintain the fixed switching frequency. The
LTC3621 has internal circuitry to accurately maintain the
peak current limit (I
LIM
) of 1.6A even at high duty cycles.
As the duty cycle approaches 100%, the LTC3621 enters
dropout operation. During dropout, if force continuous
mode is selected, the top PMOS switch is turned on
continuously, and all active circuitry is kept alive. How
-
ever, if Burst Mode operation or pulse-skipping mode is
