LTC3542_15 Datasheet, PDF (7/16 Pages) Linear Technology – 500mA, 2.25MHz Synchronous Step-Down DC/DC Converter

LTC3542
OPERATION
The LTC3542 uses a constant frequency, current mode,
step-down architecture. The operating frequency is set at
2.25MHz and can be synchronized to an external oscillator.
To suit a variety of applications, the selectable MODE/SYNC
pin allows the user to trade-off noise for efficiency.
The output voltage is set by an external divider returned
to the V_FB pin. An error amplifier compares the divided
output voltage with a reference voltage of 0.6V and adjusts
the peak inductor current accordingly.
the sleep threshold and turns the top MOSFET on. This
process repeats at a rate that is dependent on the load
demand. By running cycles periodically, the switching
losses which are dominated by the gate charge losses of
the power MOSFETs are minimized.
For lower ripple noise at low load currents, the pulse skip
mode can be used. In this mode, the regulator continues
to switch at a constant frequency down to very low load
currents, where it will begin skipping pulses.
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_FB voltage is below the reference voltage. The
current flows into the inductor and the load increases until
the 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 output of the error amplifier. When the load
current increases, the V_FB voltage decreases slightly below
the reference. This decrease causes the error amplifier to
increase its output voltage until the average inductor cur-
rent matches the new load current. The main control loop
is shut down by pulling the RUN pin to ground.
Low Load Current Operation
By selecting MODE/SYNC pin, two modes are available to
control the operation of the LTC3542 at low load currents.
Both modes automatically switch from continuous opera-
tion to the selected mode when the load current is low.
To optimize efficiency, the Burst Mode operation can be
selected. When the converter is in Burst Mode operation,
the peak current of the inductor is set to approximately
60mA regardless of the output load. Each burst event can
last from a few cycles at light loads to almost continuously
cycling with short sleep intervals at moderate loads. In
between these burst events, the power MOSFETs and any
unneeded circuitry are turned off, reducing the quiescent
current to 26μA. In this sleep state, the load current is
being supplied solely from the output capacitor. As the
output voltage drops, the EA amplifier’s output rises above
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 LTC3542 is used
at 100% duty cycle with low input voltage (See Thermal
Considerations in the Applications Information Section).
Low Supply Operation
To prevent unstable operation, the LTC3542 incorporates
an undervoltage lockout circuit which shuts down the part
when the input voltage drops below about 2V.
Internal Soft-Start
At start-up when the RUN pin is brought high, the internal
reference is linearly ramped from 0V to 0.6V in about 1ms.
The regulated feedback voltage follows this ramp resulting
in the output voltage ramping from 0% to 100% in 1ms.
The current in the inductor during soft-start is defined
by the combination of the current needed to charge the
output capacitance and the current provided to the load
as the output voltage ramps up. The start-up waveform,
shown in the Typical Performance Characteristics, shows
the output voltage start-up from 0V to 1.8V with a 500mA
load and V_IN = 3.6V (refer to Figure 3a).
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