LTC3577-3 Datasheet, PDF (31/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
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
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 efficiency 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 efficiency 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 (Buck1, Buck2 and
Buck3) are shut down when the pushbutton circuitry is in
the power-down or power-off state. Step-down switching
regulator 3 (Buck3) can also be shut down by bringing the
EN3 input low. 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 out-
puts 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
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.
Soft-Start Operation
Soft-start is accomplished by gradually increasing the peak
inductor current for each step-down switching regulator
over a 500μs period. This allows each output to rise slowly,
helping minimize inrush current required to charge up the
switching regulator output capacitor. A soft-start cycle
occurs whenever a given switching regulator is enabled.
A soft-start cycle is not triggered by changing operating
modes. This allows seamless output transition when
actively changing between operating modes.
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