LTC3555 Datasheet, PDF (23/32 Pages) Linear Technology – High Effi ciency USB Power Manager + Triple Step-Down DC/DC

LTC3555/LTC3555-X
OPERATION
operation provides a significant improvement in efficiency
at light loads at the expense of higher output ripple when
compared to pulse skip mode. For many noise-sensitive
systems, forced Burst Mode operation might be undesirable
at certain times (i.e., during a transmit or receive cycle
of a wireless device), but highly desirable at others (i.e.,
when the device is in low power standby mode). The I²C
port can be used to enable or disable forced Burst Mode
operation at any time, offering both low noise and low
power operation when they are needed.
In Burst Mode operation, the switching regulator automati-
cally switches between fixed frequency PWM operation and
hysteretic control as a function of the load current. At light
loads, the regulators operate in hysteretic mode in much
the same way as described for the forced Burst Mode
operation. Burst Mode operation provides slightly less
output ripple at the expense of slightly lower efficiency than
forced Burst Mode operation. At heavy loads the switch-
ing regulator operates in the same manner as pulse skip
operation at high loads. For applications that can tolerate
some output ripple at low output currents, Burst Mode
operation provides better efficiency than pulse skip at light
loads while still providing the full specified output current
of the switching regulator.
Finally, the switching regulators have an LDO mode that
gives a DC option for regulating their output voltages. In
LDO mode, the switching regulators are converted to linear
regulators 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 step-down switching 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.
Step-Down Switching Regulator in Shutdown
The step-down switching regulators are in shutdown when
not enabled for operation. In shutdown, all circuitry in
the step-down switching regulator is disconnected from
the switching regulator input supply leaving only a few
nanoamperes of leakage current. The step-down switch-
ing regulator outputs are individually pulled to ground
through a 10k resistor on the switch pins (SW1-SW3)
when in shutdown.
General Purpose Switching Regulator Dropout
Operation
It is possible for a switching 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
regulator’s input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
Step-Down Switching Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing the
peak inductor current for each switching 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 switching 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.
Step-Down Switching Regulator Switching Slew Rate
Control
The step-down switching regulators contain new patent
pending circuitry to limit the slew rate of the switch nodes
(SWx). This new circuitry is designed to transition the
switch nodes over a period of a couple of nanoseconds,
significantly reducing radiated EMI and conducted supply
noise.
Low Supply Operation
The LTC3555 family incorporates an undervoltage lockout
circuit on V_OUT which shuts down the general purpose
switching regulators when V_OUT drops below V_OUTUVLO.
This UVLO prevents unstable operation.
3555fd
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