LTC3553-2_15 Datasheet, PDF (21/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger, Always-On LDO and Buck Regulator

LTC3553-2
OPERATION
the power switches enough times to charge the output
capacitor to a voltage slightly higher than the regulation
point. The buck then goes into a reduced quiescent current
sleep mode. In this state, power loss is minimized while the
load current is supplied by the output capacitor. Whenever
the output voltage drops below a predetermined value, the
buck wakes from sleep and cycles the switches again until
the output capacitor voltage is once again slightly above
the regulation point. Sleep time thus depends on load
current, since the load current determines the discharge
rate of the output capacitor.
Standby Mode Buck Operation (STBY Pin High)
There are situations where even the low quiescent current
of Burst Mode operation is not low enough. For instance,
in a static memory keep alive situation, load current may
fall well below 1μA. In this case, the 22μA typical BVIN
quiescent current in Burst Mode operation becomes the
main factor determining battery run time.
Standby mode cuts BVIN quiescent current down to just
1.5μA, greatly extending battery run time in this essen-
tially no-load region of operation. The application circuit
commands the LTC3553-2 into and out of standby mode
via the STBY pin logic input. Bringing the STBY pin high
places the regulator into standby mode, while bringing it
low returns it to Burst Mode operation. In standby mode,
buck load capability drops to 10mA.
In standby mode, the buck regulator operates hyster-
etically. When the BUCK_FB pin voltage falls below the
internal 0.8V reference, a current source from BVIN to
SW turns on, delivering current through the inductor to
the switching regulator output capacitor and load. When
the FB pin voltage rises above the reference plus a small
hysteresis voltage, that current is shut off. In this way,
output regulation is maintained.
Since the power transfer from BVIN to SW is through a
high impedance current source rather than through a low
impedance MOSFET switch, power loss scales with load
current as in a linear low dropout (LDO) regulator, rather
than as in a switching regulator. For near-zero load condi-
tions where regulator quiescent current is the dominant
power loss, standby mode is ideal. But at any appreciable
load current, Burst Mode operation yields the best overall
conversion efficiency.
Shutdown
The buck regulator is shut down and enabled via the
pushbutton interface. In shutdown, it draws only a few
nanoamps of leakage current from the BVIN pin. It also
pulls down on its output with a 10k resistor from its switch
pin to ground.
Dropout Operation
It is possible for the buck regulator’s input voltage to fall
near or below its programmed output voltage (e.g., a bat-
tery voltage of 3.4V with a programmed output voltage of
3.3V). When this happens, the PMOS switch duty cycle
increases to 100%, keeping the switch on continuously.
Known as dropout operation, the output voltage equals the
regulator’s input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
Soft-Start Operation
In normal operating mode, soft-start works by gradually
increasing the maximum allowed peak inductor current
for the buck regulator over a 500μs period. This allows the
output to rise slowly, helping minimize the inrush current
needed to charge up the output capacitor. A soft-start
cycle occurs whenever the buck is enabled.
Soft-start occurs only in normal operation, but not in
standby mode. Standby mode operation is already in-
herently current-limited, since the regulator works by
intermittently turning on a current source from BVIN to
SW. Changing the state of the STBY pin while the regula-
tors are operating doesn’t trigger a new soft-start cycle,
to avoid glitching the outputs.
Inductor Selection
Many different sizes and shapes of inductors are avail-
able from numerous manufacturers. Choosing the right
inductor from such a large selection of devices can be
overwhelming, but following a few basic guidelines will
make the selection process much simpler.
35532f
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