LTC3554-1_15 Datasheet, PDF (22/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators

LTC3554/LTC3554-1/
LTC3554-2/LTC3554-3
OPERATION
In standby mode, each regulator operates hysteretically.
When the 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 regula-
tion 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
Each step-down switching regulator is shut down and
enabled via the pushbutton interface. In shutdown, each
switching regulator draws only a few nanoamps of leak-
age current from the BVIN pin. Each disabled regulator
also pulls down on its output with a 10k resistor from its
switch pin to ground.
Dropout Operation
It is possible for a step-down switching regulator’s input
voltage to fall near or below its programmed output volt-
age (e.g., a battery 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 respective
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 peak inductor current for each step-down
switching regulator over a 500μs period. This allows each
output to rise slowly, helping minimize the inrush current
needed to charge up the output capacitor. A soft-start cycle
occurs whenever a given switching regulator is enabled.
Soft-start occurs only in normal operation, but not in
standby mode. Standby mode operation is already inher-
ently current-limited, since the regulator works by inter-
mittently turning on a current source from BVIN to SW.
Changing the state of the STBY pin while the regulators
are operating doesn’t trigger a new soft-start cycle, to
avoid glitching the outputs.
Frequency/Slew Rate Select
The FSEL pin allows an application to dynamically trade off
between highest efficiency and reduced electromagnetic
interference (EMI) emission.
When FSEL is high, the switching regulator frequency is
set to 2.25MHz to stay out of the AM radio band. Also,
new patented circuitry is enabled which limits the slew rate
of the switch nodes (SW1 and SW2). This new circuitry
is designed to transition the switch node over a period of
a few nanoseconds, significantly reducing radiated EMI
and conducted supply noise.
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