LTC1876
11
1876fa
Main Control Loop
The LTC1876 uses a constant frequency, current mode
scheme to provide excellent line and load regulation for all
its outputs. The step-down controllers have two of its
switch drivers operating at 180 degrees out of phase from
each other. During normal operation, each top MOSFET is
turned on when the clock for that channel sets the R
S
latch,
and turned off when the main current comparator, I1,
resets the R
S
latch. The peak inductor current at which I1
resets the R
S
latch is controlled by the voltage on the I
TH
pin, which is the output of each error amplifier EA. The
V
OSENSE
pin receives the voltage feedback signal, which is
compared to the internal reference voltage by the EA.
When the load current increases, it causes a slight de-
crease in V
OSENSE
relative to the 0.8V 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 I2, 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 opera-
tion. When both RUN/SS1 and RUN/SS2 are low, all
LTC1876 controller functions are shut down, and the
STBYMD pin determines if the standby 5V and 3.3V
regulators are kept alive.
AUX Regulator
The auxiliary boost regulator is completely independent
from other LTC1876 circuits. It can be operated even
though the LTC1876 step-down controllers are in shut-
down. The operation of the boost regulator is similar to the
controllers. The oscillator, OSC
AUX
, sets the R
S
latch and
turns on the monolithic power switch. A voltage propor-
tional to the switch current is added to a stabilizing ramp
and the resulting sum is fed into the positive terminal of the
PWM comparator, A1
AUX
. When this voltage exceeds the
level at the negative input of A1
AUX
, the SR latch is reset,
turning off the power switch. The level at the negative input
of A1
AUX
is set by the error amplifier EA
AUX
and is simply
an amplified version of the difference between the feed-
back voltage and the reference voltage. Hence the error
amplifier sets the correct peak current level to keep the
output in regulation. To protect the power switch from
excessive current, a 1A minimum limit is internally set.
When the switch reaches this limit, it will force the latch to
reset, turning it off. Applying a voltage less than 0.5V on
the shutdown pin will put the boost regulator in shutdown.
Low Current Operation
The FCB pin is a multifunction pin providing two functions:
1) to provide regulation for a secondary winding by
temporarily forcing continuous PWM operation on both
controllers; and 2) select between
two
modes of low
current operation. When the FCB pin voltage is below 0.8V,
the controller forces continuous PWM current operation.
In this mode, the top and bottom MOSFETs are alternately
turned on to maintain the output voltage independent of
direction of inductor current. When the FCB pin is below
V
INTVCC
␣ –␣ 2V but greater than 0.8V, the controller enters
Burst Mode operation. Burst Mode operation sets a mini-
mum output current level before turning off the top switch
and turns off the synchronous MOSFET(s) when the
inductor current goes negative. This combination of re-
quirements 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
slightly. There is 60mV of hysteresis in the burst compara-
tor B tied to the I
TH
pin. This hysteresis produces output
signals to the MOSFETs that turn them on for several
(Refer to Functional Diagram)
OPERATIO
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