LTC3563 Ver la hoja de datos (PDF) - Linear Technology
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LTC3563 Datasheet PDF : 16 Pages
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LTC3563
U
OPERATIO
The LTC3563 uses a constant frequency, current mode,
step-down architecture. The operating frequency is set
at 2.25MHz.
The output voltage is set by an internal divider. An error
amplifier 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 VOUT voltage is below the regulated 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 feedback voltage
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.
Burst Mode Operation
During light load currents, the LTC3563 operates in Burst
Mode operation in which the internal power MOSFETs
operate intermittently based on load demand.
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 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.
Low Supply Operation
To prevent unstable operation, the LTC3563 incorporates
an undervoltage lockout circuit which shuts down the part
when the input voltage drops below 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 output voltage follows this ramp 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 1.87V with a 500mA load and VIN = 3.6V.
3563f
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