LTC3890
operaTion
(Refer to the Functional Diagram)
Main Control Loop
The LTC3890 uses a constant frequency, current mode
step-down architecture with the two controller channels
operating 180 degrees out-of-phase. During normal op-
eration, each external top MOSFET is turned on when the
clock for that channel sets the RS latch, and is turned off
when the main current comparator, ICMP resets the RS
,
latch. The peak inductor current at which ICMP trips and
resets the latch is controlled by the voltage on the ITH pin,
which is the output of the error amplifier, EA. The error
amplifier compares the output voltage feedback signal at
the V
FB
pin, (which is generated with an external resistor
divider connected across the output voltage, V
OUT
, to
ground) to the internal 0.800V reference voltage. When the
load current increases, it causes a slight decrease in V
FB
relative to the reference, which causes the EA to increase
the ITH voltage until the average inductor current matches
the new load current.
After the top MOSFET is turned off each cycle, the bottom
MOSFET is turned on until either the inductor current starts
to reverse, as indicated by the current comparator IR, or
the beginning of the next clock cycle.
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
CC
pin.
When the EXTV
CC
pin is tied to a voltage less than 4.7V,
the V
IN
LDO (low dropout linear regulator) supplies 5.1V
from V
IN
to INTV
CC
. If EXTV
CC
is taken above 4.7V, the V
IN
LDO is turned off and an EXTV
CC
LDO is turned on. Once
enabled, the EXTV
CC
LDO supplies 5.1V from EXTV
CC
to
INTV
CC
. Using the EXTV
CC
pin allows the INTV
CC
power
to be derived from a high efficiency external source such
as one of the LTC3890 switching regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap capacitor, C
B
, which normally recharges during each
cycle through an external diode when the top MOSFET
turns off. If the input voltage, 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 detects this and forces the top MOSFET off for
about one twelfth of the clock period every tenth cycle to
allow C
B
to recharge.
Shutdown and Start-Up (RUN1, RUN2 and
TRACK/ SS1, TRACK/SS2 Pins)
The two channels of the LTC3890 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either of
these pins below 1.15V shuts down the main control loop
for that controller. Pulling both pins below 0.7V disables
both controllers and most internal circuits, including the
INTV
CC
LDOs. In this state, the LTC3890 draws only 14µA
of quiescent current.
Releasing either RUN pin allows a small internal current to
pull up the pin to enable that controller. The RUN1 pin has
a 7µA pull-up current while the RUN2 pin has a smaller
0.5µA. The 7µA current on RUN1 is designed to be large
enough so that the RUN1 pin can be safely floated (to
always enable the controller) without worry of condensa-
tion or other small board leakage pulling the pin down.
This is ideal for always-on applications where one or both
controllers are enabled continuously and never shut down.
The RUN pin may be externally pulled up or driven directly
by logic. When driving the RUN pin with a low impedance
source, do not exceed the absolute maximum rating of
8V. The RUN pin has an internal 11V voltage clamp that
allows the RUN pin to be connected through a resistor to a
higher voltage (for example, V
IN
), so long as the maximum
current into the RUN pin does not exceed 100µA.
The start-up of each controller’s output voltage V
OUT
is
controlled by the voltage on the TRACK/SS pin for that
channel. When the voltage on the TRACK/SS pin is less
than the 0.8V internal reference, the LTC3890 regulates
the V
FB
voltage to the TRACK/SS pin voltage instead of the
0.8V reference. This allows the TRACK/SS pin to be used
to program a soft-start by connecting an external capacitor
from the TRACK/SS pin to SGND. An internal 1µA pull-up
current charges this capacitor creating a voltage ramp on
the TRACK/SS pin. As the TRACK/SS voltage rises linearly
from 0V to 0.8V (and beyond up to 5V), the output voltage
V
OUT
rises smoothly from zero to its final value.
Alternatively the TRACK/SS pin can be used to cause the
start-up of V
OUT
to track that of another supply. Typically,
this requires connecting to the TRACK/SS pin an external
resistor divider from the other supply to ground (see the
Applications Information section).
3890fb
11
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
