LTC3624/LTC3624-2
9
36242fd
For more information www.linear.com/LTC3624
operaTion
The LTC3624/LTC3624-2 uses a constant-frequency, peak
current mode architecture. It operates through a wide V
IN
range and regulates with ultralow quiescent current. The
operation frequency is set at either 1MHz or 2.25MHz and
can be synchronized to an external oscillator ±40% of the
inherent frequency. To 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. Overvoltage and un
-
dervoltage 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 low 32 clock cycles after
falling out of regulation and will go high immediately after
achieving 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 LTC3624/LTC3624-2 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
800mA, 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 LTC3624/
LTC3624
-2, pulse-skipping mode is implemented similarly
to Burst Mode operation with the peak inductor current
set to be at least 132mA. This results in lower 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
LTC3624/LTC3624-2 also has the ability to operate in the
forced continuous mode by setting the MODE/SYNC volt
-
age between 1V and V
INTVCC
– 1.2V. 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 –266mA 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 com
-
pensation is required to maintain the fixed switching
frequency. The LTC3624/LTC3624-2 has internal circuitry
to accurately maintain the peak current limit (I
LIM
) of 3A
even at high duty cycles.
As the duty cycle approaches 100%, the LTC3624/
LTC3624-2 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
