LTC3555 Linear Technology, LTC3555 Datasheet - Page 22

• Manufacturer Part Number: LTC3555
• Description: High Efficiency USB Power Manager + Triple Step-Down DC/DC
• Manufacturer: Linear Technology

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Step-Down Switching Regulator Operating Modes
The LTC3555’s general purpose switching regulators
include four 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 amplifi er.
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 rectifi er
to turn on. The N-channel MOSFET synchronous rectifi er
turns off at the end of the 2.25MHz cycle or if the current
through the N-channel MOSFET synchronous rectifi er
drops to zero. Using this method of operation, the error
amplifi er adjusts the peak inductor current to deliver the
required output power. All necessary compensation is
internal to the switching regulator requiring only a single
ceramic output capacitor for stability. At light loads in PWM
mode, the inductor current may reach zero on each pulse
which will turn off the N-channel MOSFET synchronous
rectifi er. In this case, the switch node (SW) goes high
impedance and the switch node voltage will “ring”. This
is discontinuous mode operation, and is normal behavior
for a switching regulator. At very light loads in Pulse Skip
mode, the switching regulators will automatically skip
pulses as needed to maintain output regulation.
At high duty cycles (V
inductor current to reverse, causing the regulator to operate
continuously at light loads. The is normal and regulation is
maintained, but the supply current will increase to several
milliamperes due to continuous switching.
In Forced Burst Mode operation, the switching regulators
use a constant current algorithm to control the inductor
current. By controlling the inductor current directly and
using a hysteretic control loop, both noise and switching
losses are minimized. In this mode output power is limited.
While in Forced Burst Mode operation, the output capacitor
is charged to a voltage slightly higher than the regulation
point. The step-down converter then goes into sleep mode,
during which the output capacitor provides the load cur-
rent. In sleep mode, most of the regulator’s circuitry is
LTC3555
OPERATION
22
OUTx
> V
INx
/2) it is possible for the
powered down, helping conserve battery power. When the
output voltage drops below a pre-determined value, the
switching regulator circuitry is powered on and another
burst cycle begins. The duration for which the regulator
operates in sleep mode depends on the load current. The
sleep time decreases as the load current increases. The
maximum output current in Forced Burst Mode operation is
about 100mA for switching regulators 1 and 2, and about
250mA for switching regulator 3. The step-down switching
regulators will not enter Sleep mode if the maximum output
current is exceeded in Forced Burst Mode operation and
the output will drop out of regulation. Forced Burst Mode
operation provides a signifi cant improvement in effi ciency
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 undesir-
able 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 I
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 fi xed 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 operation. Burst
Mode operation provides slightly less output ripple at the
expense of slightly lower effi ciency than Forced Burst mode
operation. At heavy loads the switching 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 ef-
fi ciency than Pulse Skip at light loads while still providing the
full specifi ed 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.
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