11
LTC3819
3819f
OPERATIO
U
(Refer to Functional Diagram)
Main Control Loop
The LTC3819 uses a constant frequency, current mode
step-down architecture with the two output stages oper-
ating 180 degrees out of phase. During normal operation,
each top MOSFET is turned on when the clock for that
channel sets the RS latch, and turned off when the main
current comparator, I
1
, resets the RS latch. The peak
inductor current at which I
1
resets the RS latch is con-
trolled by the voltage on the I
TH
pin, which is the output of
error amplifier EA. The EAIN pin receives the voltage
feedback signal, which is compared to the internal refer-
ence voltage by the EA. When the load current increases,
it causes a slight decrease in V
EAIN
relative to the 0.6V
reference, which in turn causes the I
TH
voltage to increase
until the average inductor current matches the new load
current. After the top MOSFET has turned off, the bottom
MOSFET is turned on until either the inductor current
starts to reverse, as indicated by current comparator I
2
, or
the beginning of the next cycle.
The top MOSFET drivers are biased from floating boot-
strap capacitor C
B
, which normally is recharged during
each off cycle through an external diode when the top
MOSFET turns off. As V
IN
decreases to a voltage close to
V
OUT
, the loop may enter dropout and attempt to turn on
the top MOSFET continuously. The dropout detector de-
tects this and forces the top MOSFET off for about 500ns
every tenth cycle to allow C
B
to recharge.
The main control loop is shut down by pulling the RUN/
SS pin low. Releasing RUN/SS allows an internal 1.2µA
current source to charge soft-start capacitor C
SS
. When
C
SS
reaches 1.5V, the main control loop is enabled with
the I
TH
voltage clamped at approximately 30% of its
maximum value. As C
SS
continues to charge, the I
TH
pin
voltage is gradually released allowing normal, full-current
operation.
Low Current Operation
T
he FCB pin selects between
two
modes of low current
operation. When the FCB pin voltage is below 0.6V, the
controller forces continuous PWM current mode opera-
tion. In this mode, the top and bottom MOSFETs are
alternately turned on to maintain the output voltage inde-
pendent of direction of inductor current. When the FCB pin
is below V
INTVCC
– 2V but greater than 0.6V, the controller
enters Burst Mode operation. Burst Mode operation sets
a minimum output current level before inhibiting the top
switch and turns off the synchronous MOSFET(s) when
the inductor current goes negative. This combination of
requirements will, at low currents, force the I
TH
pin below
a voltage threshold that will temporarily inhibit turn-on of
both output MOSFETs until the output voltage drops.
There is 60mV of hysteresis in the burst comparator B tied
to the I
TH
pin. This hysteresis produces output signals to
the MOSFETs that turn them on for several cycles, fol-
lowed by a variable “sleep” interval depending upon the
load current. The resultant output voltage ripple is held to
a very small value by having the hysteretic comparator
after the error amplifier gain block.
Constant Frequency Operation
When the FCB pin is tied to INTV
CC
, Burst Mode operation
is disabled and a forced minimum peak output current
requirement is removed. This provides constant frequency,
discontinuous (preventing reverse inductor current) cur-
rent operation over the widest possible output current
range. This constant frequency operation is not as efficient
as Burst Mode operation, but does provide a lower noise,
constant frequency operating mode down to approxi-
mately 1% of designed maximum output current.
Continuous Current (PWM) Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least efficient operating mode, but
may be desirable in certain applications. The output can
source or sink current in this mode. When sinking current
while in forced continuous operation, current will be
forced back into the main power supply potentially boost-
ing the input supply to dangerous voltage levels—
BEWARE!
