LTC3556 Datasheet, PDF (23/36 Pages) Linear Technology – High Effi ciency USB Power Manager with Dual Buck and Buck-Boost DC/DCs

LTC3556
OPERATION
tors and deliver continuous power from their SWx pins
through their respective inductors. This mode gives the
lowest possible output noise as well as low quiescent
current at light loads.
The buck regulators allow mode transition on the fly,
providing seamless transition between modes even under
load. This allows the user to switch back and forth between
modes to reduce output ripple or increase low current
efficiency as needed.
Buck Regulator in Shutdown
The buck regulators are in shutdown when not enabled for
operation. In shutdown, all circuitry in the buck regulator
is disconnected from the buck regulator input supply
leaving only a few nanoamperes of leakage current. The
buck regulator outputs are individually pulled to ground
through a 10k resistor on the switch pins (SW1 and SW2)
when in shutdown.
Buck Regulator Dropout Operation
It is possible for a buck regulator’s input voltage, V_INX, to
approach its programmed output voltage (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
until it is turned on continuously at 100%. In this dropout
condition, the respective output voltage equals the buck
regulator’s input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
Buck Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing the
peak inductor current for each buck regulator over a
500μs period. This allows each output to rise slowly,
helping minimize the battery surge current. A soft-start
cycle occurs whenever a given buck regulator is enabled,
or after a fault condition has occurred (thermal shutdown
or UVLO). A soft-start cycle is not triggered by changing
operating modes. This allows seamless output operation
when transitioning between forced Burst Mode, Burst
Mode, pulse skip mode or LDO operation.
Buck Regulator Switching Slew Rate Control
The buck regulators contain new patent pending circuitry
to limit the slew rate of the switch node (SW1 and SW2).
This new circuitry is designed to transition the switch node
over a period of a couple of nanoseconds, significantly
reducing radiated EMI and conducted supply noise.
Low Supply Operation
The LTC3556 incorporates an undervoltage lockout circuit
on V_OUT which shuts down both buck regulators (as well
as the buck-boost) when V_OUT drops below V_OUTUVLO.
This UVLO prevents unstable operation.
Buck-Boost DC/DC Switching Regulator
The LTC3556 contains a 2.25MHz constant-frequency volt-
age mode buck-boost switching regulator. The regulator
provides up to 1A of output load current. The buck-boost
can be programmed to a minimum output voltage of 2.5V
and can be used to power a microcontroller core, micro-
controller I/O, memory, disk drive or other logic circuitry.
When controlled by I²C, the buck-boost has programmable
set-points for on-the-fly power savings. To suit a variety of
applications, a selectable mode function allows the user to
trade off noise for efficiency. Two modes are available to
control the operation of the LTC3556’s buck-boost regula-
tor. At moderate to heavy loads, the constant frequency
PWM mode provides the least noise switching solution. At
lighter loads Burst Mode operation may be selected. The
full-scale output voltage is programmed by a user-supplied
resistive divider returned to the FB3 pin. An error amplifier
compares the divided output voltage with a reference and
adjusts the compensation voltage accordingly until the FB3
has stabilized to the selected reference voltage (0.425V to
0.8V). The buck-boost regulator also includes a soft-start to
limit inrush current and voltage overshoot when powering
on, short circuit current protection, and switch node slew
limiting circuitry for reduced radiated EMI.
3556f
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