ltc3890-1 Linear Technology Corporation, ltc3890-1 Datasheet - Page 11

• Manufacturer Part Number: ltc3890-1
• Description: 60v Low Iq, Dual, 2-phase Synchronous Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

OPERATION
Main Control Loop
The LTC3890-1 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 amplifi er, EA. The error
amplifi er compares the output voltage feedback signal at
the V
divider connected across the output voltage, V
ground) to the internal 0.800V reference voltage. When
the load current increases, it causes a slight decrease
in V
increase the ITH voltage until the average inductor current
matches the new load current.
After the top MOSFET is turned off each cycle, the bot-
tom 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
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
When the EXTV
than 4.7V, the V
plies 5.1V from V
4.7V, the V
turned on. Once enabled, the EXTV
from EXTV
the INTV
external source such as one of the LTC3890-1 switching
regulator outputs.
Each top MOSFET driver is biased from the fl oating boot-
strap capacitor C
cycle through an external diode when the top MOSFET
turns off. If the input voltage, V
close to V
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
FB
CC
FB
relative to the reference, which causes the EA to
/EXTV
B
pin, (which is generated with an external resistor
CC
to recharge.
OUT
IN
CC
power to be derived from a high effi ciency
CC
LDO is turned off and an EXTV
, the loop may enter dropout and attempt
to INTV
IN
CC
Power
B
IN
LDO (low dropout linear regulator) sup-
, which normally recharges during each
pin is left open or tied to a voltage less
to INTV
(Refer to the Functional Diagram)
CC
. Using the EXTV
CC
. If EXTV
IN
, decreases to a voltage
CC
LDO supplies 5.1V
CC
is taken above
CC
pin allows
CC
OUT
CC
LDO is
, to
pin.
Shutdown and Start-Up (RUN1, RUN2 and
TRACK/ SS1, TRACK/SS2 Pins)
The two channels of the LTC3890-1 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either
of these pins below 1.2V 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
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 fl oated (to always en-
able the controller) without worry of condensation 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
current into the RUN pin does not exceed 100μA.
The start-up of each controller’s output voltage V
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 LTC3890-1 regulates
the V
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
its fi nal value.
Alternatively the TRACK/SS pin can be used to cause the
start-up of V
this requires connecting to the TRACK/SS pin an external
resistor divider from the other supply to ground (see Ap-
plications Information section).
CC
FB
voltage to the TRACK/SS pin voltage instead of
LDOs. In this state, the LTC3890-1 draws only
OUT
to track that of another supply. Typically,
OUT
rises smoothly from zero to
IN
), so long as the maximum
LTC3890-1
11
OUT
38901f
is