LTC3557_15 Datasheet, PDF (18/28 Pages) Linear Technology – USB Power Manager with Li-Ion Charger and Three Step-Down Regulators

LTC3557/LTC3557-1
OPERATION
Step-Down Switching Regulator RST2 Operation
The RST2 pin is an open-drain output used to indicate that
step-down switching regulator 2 has been enabled and
has reached its final voltage. A 230ms delay is included
from the time switching regulator 2 reaches 92% of its
regulation value to allow a system controller ample time to
reset itself. RST2 may be used as a power-on reset to the
microprocessor powered by regulator 2 or may be used to
enable regulators 1 and/or 3 for supply sequencing. RST2
is an open-drain output and requires a pull-up resistor to
the output voltage of regulator 2 or another appropriate
power source.
Step-Down Switching Regulator Operating Modes
The step-down switching regulators include two possible
operating modes to meet the noise/power needs of a
variety of applications.
In pulse-skip mode, an internal latch is set at the start of
every cycle, which turns on the main P-channel MOSFET
switch. During each cycle, a current comparator compares
the peak inductor current to the output of an error amplifier.
The output of the current comparator resets the internal
latch, which causes the main P-channel MOSFET switch to
turn off and the N-channel MOSFET synchronous rectifier
to turn on. The N-channel MOSFET synchronous rectifier
turns off at the end of the 2.25MHz cycle or if the current
through the N-channel MOSFET synchronous rectifier
drops to zero. Using this method of operation, the error
amplifier adjusts the peak inductor current to deliver the
required output power. All necessary compensation is
internal to the step-down switching regulator requiring only
a single ceramic output capacitor for stability. At light loads
in pulse-skip mode, the inductor current may reach zero
on each pulse which will turn off the N-channel MOSFET
synchronous rectifier. In this case, the switch node (SW1,
SW2 or SW3) goes high impedance and the switch node
voltage will “ring”. This is discontinuous operation, and is
normal behavior for a switching regulator. At very light loads
in pulse-skip mode, the step-down switching regulators
will automatically skip pulses as needed to maintain
output regulation. At high duty cycle (V_OUTX > V_INX/2) it is
possible for the inductor current to reverse at light loads
causing the stepped down switching regulator to operate
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continuously. When operating continuously, regulation
and low noise output voltage are maintained, but input
operating current will increase to a few milliamps.
In Burst Mode operation, the step-down switching regula-
tors automatically switch between fixed frequency PWM
operation and hysteretic control as a function of the load
current. At light loads the step-down switching regulators
control the inductor current directly and use a hysteretic
control loop to minimize both noise and switching losses.
While operating in Burst Mode operation, the output capaci-
tor is charged to a voltage slightly higher than the regulation
point. The step-down switching regulator then goes into
sleep mode, during which the output capacitor provides
the load current. In sleep mode, most of the switching
regulator’s circuitry is powered down, helping conserve
battery power. When the output voltage drops below a
pre-determined value, the step-down switching regulator
circuitry is powered on and another burst cycle begins. The
sleep time decreases as the load current increases. Beyond
a certain load current point (about 1/4 rated output load
current) the step-down switching regulators will switch to
a low noise constant frequency PWM mode of operation,
much the same 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.
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. Burst Mode operation is
set by driving the MODE pin high, while pulse-skip mode
is achieved by driving the MODE pin low.
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
nanoamps of leakage current. The step-down switching
regulator outputs are individually pulled to ground through
a 10k resistor on the switch pin (SW1, SW2 or SW3) when
in shutdown.
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