LTC3622-2_15 Datasheet, PDF (10/24 Pages) Linear Technology – 17V, Dual 1A Synchronous Step-Down Regulator with Ultralow Quiescent Current

LTC3622/
LTC3622-2/LTC3622-23/5
Operation
The LTC3622 is a dual high efficiency monolithic step-
down regulator, which 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 2.25MHz or 1MHz and
can be synchronized to an external oscillator ±50% 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.
For each channel, 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 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 the 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 I_TH 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 I_TH 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 (DCM) are available
to control the operation of the LTC3622 at low currents.
Both modes, Burst Mode operation and pulse-skipping
mode, automatically switch from continuous operation to
the selected mode when the load current is low.
To optimize efficiency, Burst Mode operation can be se-
lected 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 de-
mands less. Thus, when the switcher is on at relatively
light output loads, FB voltage will rise and cause the I_TH
voltage to drop. Once the I_TH voltage drops low enough,the
switcher goes into sleep mode with both power switches off.
The switchers remain in this sleep state until the external
load pulls the output voltage below its regulation point.
When both channels are in sleep mode, the part draws an
ultralow 5µA of quiescent current from V_IN.
To minimize V_OUT ripple, pulse-skipping mode can be
selected by grounding the MODE/SYNC pin. In LTC3622,
pulse-skipping mode is implemented similarly to Burst
Mode operation with the peak inductor current set to be
at above 66mA. This results in lower ripple than in Burst
Mode operation with the trade-off being slightly lower
efficiency.
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
LTC3622 has internal circuitry to accurately maintain the
peak current limit (I_LIM) of 1.8A even at high duty cycles.
As the duty cycle approaches 100%, the LTC3622 enters
dropout operation. During dropout, the part will transition
in and out of sleep mode depending on the output load
current. This significantly reduces the quiescent current,
thus prolonging the use of the input supply.
3622fa
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For more information www.linear.com/LTC3622