Datasheet

LTC3677-3
29
36773f
operaTion
Typical values for R1 are in the range of 40k to 1M. The
capacitor C
FB
cancels the pole created by feedback resis-
tors and the input capacitance of the FB pin and also helps
to improve transient response for output voltages much
greater than 0.8V. A variety of capacitor sizes can be used
for C
FB
but a value of 10pF is recommended for most ap-
plications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
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
regulators automatically switch between fixed frequency
PWM operation and hysteretic control as a function of
the load current. At light loads the step-down switch-
ing 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-skipping opera-
tion 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 I
2
C 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.