LTC3548-1 Datasheet, PDF (7/16 Pages) Linear Technology – Dual Synchronous, Fixed Output 2.25MHz Step-Down DC/DC Regulator

LTC3548-1
OPERATION
The LTC3548-1 uses a constant frequency, current mode
architecture. The operating frequency is set at 2.25MHz.
Both channels share the same clock and run in-phase.
The output voltage is set by an internal divider. An error
amplfier compares the divided output voltage with a
reference voltage of 0.6V and adjusts the peak inductor
current accordingly.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
when the V_OUT voltage is below the regulated voltage. The
current flows into the inductor and the load increases until
current limit is reached. The switch turns off and energy
stored in the inductor flows through the bottom switch
(N-channel MOSFET) into the load until the next
clock cycle.
The peak inductor current is controlled by the internally
compensated I_TH voltage, which is the output of the error
amplifier. This amplifier compares the feedback voltage V_FB
to the 0.6V reference (see Block Diagram). When the load
current increases, the V_FB voltage decreases slightly below
the reference. This decrease 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
When the load is relatively light, the LTC3548-1 auto-
matically switches into Burst Mode operation, in which
the PMOS switch operates intermittently based on load
demand with a fixed peak inductor current. By running
cycles periodically, the switching losses which are domi-
nated by the gate charge losses of the power MOSFETs
are minimized. The main control loop is interrupted when
the output voltage reaches the desired regulated value. A
voltage comparator trips when I_TH is below 0.35V, shutting
off the switch and reducing the power. The output capaci-
tor and the inductor supply the power to the load until I_TH
exceeds 0.65V, turning on the switch and the main control
loop which starts another cycle.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100% which
is the dropout condition. In dropout, the PMOS switch is
turned on continuously with the output voltage being equal
to the input voltage minus the voltage drops across the
internal p-channel MOSFET and the inductor.
An important design consideration is that the R_DS(ON)
of the P-channel switch increases with decreasing input
supply voltage (see Typical Performance Characteristics).
Therefore, the user should calculate the power dissipation
when the LTC3548-1 is used at 100% duty cycle with low
input voltage (see Thermal Considerations in the Applica-
tions Information section).
Low Supply Operation
To prevent unstable operation, the LTC3548-1 incorporates
an undervoltage lockout circuit which shuts down the part
when the input voltage drops below about 1.65V.
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