5
LTC3801/LTC3801B
sn3801 3801fs
OPERATIO
U
(Refer to the Functional Diagram)
Main Control Loop (Normal Operation)
The LTC3801/LTC3801B are constant frequency current
mode step-down switching regulator controllers. During
normal operation, an external P-channel MOSFET is turned
on each cycle when the oscillator sets the RS latch and
turned off when the current comparator resets the latch.
The peak inductor current at which the current comparator
trips is controlled by the voltage on the I
TH
/RUN pin, which
is the output of the error amplifier. The negative input to
the error amplifier is the output feedback voltage V
FB
which is generated by an external resistor divider con-
nected between V
OUT
and ground. When the load current
increases, it causes a slight decrease in V
FB
relative to the
0.8V reference, which in turn causes the I
TH
/RUN voltage
to increase until the average inductor current matches the
new load current.
The main control loop is shut down by pulling the I
TH
/RUN
pin to ground. Releasing the I
TH
/RUN pin allows an
internal 1µA current source (2µA on LTC3801B) to charge
up the external compensation network. When the I
TH
/
RUN pin voltage reaches approximately 0.6V, the main
control loop is enabled and the I
TH
/RUN voltage is pulled
up by a clamp to its zero current level of approximately
one diode voltage drop (0.7V). As the external compensa-
tion network continues to charge up, the corresponding
peak inductor current level follows, allowing normal op-
eration. The maximum peak inductor current attainable is
set by a clamp on the I
TH
/RUN pin at 1.2V above the zero
current level (approximately 1.9V).
Burst Mode Operation (LTC3801 Only)
The LTC3801 incorporates Burst Mode operation at low
load currents (<25% of I
MAX
). In this mode, an internal
clamp sets the peak current of the inductor at a level cor-
responding to an I
TH
/RUN voltage 0.3V above its zero
current level (approximately 1V), even though the actual
I
TH
/RUN voltage is lower. When the inductor’s average
current is greater than the load requirement, the voltage at
the I
TH
/RUN pin will drop. When the I
TH
/RUN voltage falls
to 0.15V above its zero current level (approximately 0.85V),
the sleep comparator will trip, turning off the external
MOSFET. In sleep, the input DC supply current to the IC is
reduced to 16µA from 195µA in normal operation. With the
switch held off, average inductor current will decay to zero
and the load will eventually cause the error amplifier out-
put to start drifting higher. When the error amplifier output
rises to 0.225V above its zero current level (approximately
0.925V), the sleep comparator will untrip and normal op-
eration will resume. The next oscillator cycle will turn the
external MOSFET on and the switching cycle will repeat.
Low Load Current Operation (LTC3801B Only)
Under very light load current conditions, the I
TH
/RUN pin
voltage will be very close to the zero current level of 0.85V.
As the load current decreases further, an internal offset at
the current comparator input will ensure that the current
comparator remains tripped (even at zero load current)
and the regulator will start to skip cycles, as it must, in
order to maintain regulation. This behavior allows the
regulator to maintain constant frequency down to very
light loads, resulting in less low frequency noise genera-
tion over a wide load current range.
Figure 1 illustrates this result for the circuit on the front
page of this data sheet using both an LTC3801 (in Burst
Mode operation) and an LTC3801B (with Burst Mode
operation disabled). At an output current of 100mA, the
LTC3801 exhibits an output ripple of 81.6mV
P-P
, whereas
the LTC3801B has an output ripple of only 17.6mV
P-P
. At
lower output current levels, the improvement is even
greater. This comes at a tradeoff of lower efficiency for the
non Burst Mode part at light load currents (see Figure 2).
Also notice the constant frequency operation of the
LTC3801B, even at 5% of maximum output current.
Dropout Operation
When the input supply voltage decreases towards the
output voltage, the rate of change of inductor current
during the on cycle decreases. This reduction means that
at some input-output differential, the external P-channel
MOSFET will remain on for more than one oscillator cycle
(start dropping off-cycles) since the inductor current has
not ramped up to the threshold set by the error amplifier.
Further reduction in input supply voltage will eventually
cause the external P-channel MOSFET to be turned on
100%, i.e., DC. The output voltage will then be determined
by the input voltage minus the voltage drop across the
sense resistor, the MOSFET and the inductor.
