LTC3542-1_15 Datasheet, PDF (8/16 Pages) Linear Technology – 500mA, 2.25MHz 2.8V Output Synchronous Step-Down DC/DC Converter

LTC3542-1
OPERATION
The LTC3542-1 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 internal resistor divider. 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 divided output 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 divided output volt-
age decreases slightly below the reference. This decrease
causes the error amplifier to increase its output 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 Load Current Operation
By selecting MODE/SYNC pin, two modes are available to
control the operation of the LTC3542-1 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
125mA 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
8
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 LTC3542-1 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 LTC3542-1 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 2.8V with a 500mA
load and V_IN = 3.6V (refer to Figure 3a).
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