LTC3559/LTC3559-1
19
3559fb
APPLICATIONS INFORMATION
most applications. Experimentation with capacitor sizes
between 2pF and 22pF may yield improved transient
response if so desired by the user.
Buck Switching Regulator 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 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 step-down switching regulator requiring
only a single ceramic output capacitor for stability. At
light loads in pulse skip 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 (SW1 or SW2) 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 skip mode, the step-down switching
regulators will automatically skip pulses as needed to
maintain output regulation. At high duty cycle (V
OUT
>
PV
IN
/2) in pulse skip mode, it is possible for the inductor
current to reverse causing the buck converter to switch
continuously. Regulation and low noise operation are
maintained but the input supply current will increase to a
couple mA due to the continuous gate switching.
During Burst Mode operation, the step-down switching
regulators automatically switch between fi xed 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. During 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 operation at high loads. For
applications that can tolerate some output ripple at low
output currents, Burst Mode operation provides better
effi ciency than pulse skip at light loads.
The step-down switching regulators allow mode transition
on-the-fl y, 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 effi ciency as needed. Burst Mode operation is
set by driving the MODE pin high, while pulse skip mode
is achieved by driving the MODE pin low.
Buck Switching Regulator in Shutdown
The buck switching regulators are in shutdown when
not enabled for operation. In shutdown, all circuitry in
the buck switching regulator is disconnected from the
regulator input supply, leaving only a few nanoamps of
+
PWM
CONTROL
GND
EN
MODE
0.8V
MN
FB
MP
C
FB
V
OUT
P
VIN
C
O
R1
R2
L
SW
3559 F08
Figure 8. Buck Converter Application Circuit
