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LTC3563

3563f

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.

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