LTC3577_15 Datasheet, PDF (32/54 Pages) Linear Technology – Highly Integrated 6-Channel Portable PMIC

LTC3577/LTC3577-1
OPERATION
open-drain output and requires a pull-up resistor to an
appropriate power source. Optimally the pull-up resistor
is connected to one of the step-down switching regulator
output voltages so that power is not dissipated while the
regulators are disabled.
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-skipping 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-skipping 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-skipping
mode, the step-down switching regulators will automati-
cally skip pulses as needed to maintain output regulation.
At high duty cycle (V_OUTX approaching V_INX) it is possible
for the inductor current to reverse at light loads causing
the stepped down switching regulator to operate continu-
ously. 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
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control loop to minimize both noise and switching losses.
While operating in Burst Mode operation, the output
capacitor 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-
ping operation at high loads.
For applications that can tolerate some output ripple at low
output currents, Burst Mode operation provides better ef-
ficiency than pulse-skipping 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 ef-
ficiency as needed. Burst Mode operation is individually
selectable for each step-down switching regulator through
the I²C register bits BK1BRST, BK2BRST and BK3BRST.
Shutdown
The step-down switching regulators are shut down when
the pushbutton circuitry is in the power-down, power
off or hard reset states. 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
internal 10k resistors on the switch pin (SW1, SW2 or
SW3) when in shutdown.
Dropout Operation
It is possible for a step-down switching regulator’s input
voltage 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
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