LTC3586-2 Datasheet, PDF (22/36 Pages) –

LTC3586-2/LTC3586-3
OPERATION
V_INx
L
SWx
LTC3586-2/
LTC3586-3
C_FB
FBx
GND
V_OUTx
R1
C_OUT
R2
X = 1, 2
358623 F05
Figure 5. Buck Converter Application Circuit
Buck Regulator Operating Modes
The LTC3586-2/LTC3586-3’s buck 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 compara-
tor 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 neces-
sary compensation is internal to the switching regulator
requiring only a single ceramic output capacitor for sta-
bility. At light loads, 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) 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, the buck regulators will automatically skip pulses
as needed to maintain output regulation.
At high duty cycles (V_OUTx > V_INx /2) it is possible for the
inductor current to reverse, causing the buck regulator
to operate continuously at light loads. This is normal and
regulation is maintained, but the supply current will increase
to several milliamperes due to continuous switching.
In Burst Mode operation, the buck regulator automati-
cally switches between fixed frequency PWM operation
and hysteretic control as a function of the load current.
At light loads, the buck regulators operate in hysteretic
mode in which the output capacitor is charged to a volt-
age slightly higher than the regulation point. The buck
converter then goes into sleep mode, during which the
output capacitor provides the load current. In sleep mode,
most of the regulator’s circuitry is powered down, helping
conserve battery power. When the output voltage drops
below a predetermined value, the buck regulator circuitry
is powered on and the normal PWM operation resumes.
The duration for which the buck regulator operates in
sleep mode depends on the load current. 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 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 while still provid-
ing the full specified output current of the buck regulator.
The buck 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.
Buck Regulator in Shutdown
The buck regulators are in shutdown when not enabled for
operation. In shutdown, all circuitry in the buck regulator
is disconnected from the buck regulator input supply
leaving only a few nanoamps of leakage current. The
buck regulator outputs are individually pulled to ground
through a 10k resistor on the switch pins (SW1 and SW2)
when in shutdown.
Buck Regulator Dropout Operation
It is possible for a buck regulator’s input voltage, V_INx, to
approach its programmed output voltage (e.g., a battery
voltage of 3.4V with a programmed output voltage of 3.3V).
358623f
22