ltc3890 Linear Technology Corporation, ltc3890 Datasheet - Page 11

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

OPERATION
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 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
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
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
the EXTV
4.7V, the V
5.1V from V
the V
Once enabled, the EXTV
to INTV
to be derived from a high effi ciency external source such
as one of the LTC3890 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
CC
IN
FB
/EXTV
LDO is turned off and an EXTV
CC
B
pin, (which is generated with an external resistor
CC
to recharge.
. Using the EXTV
OUT
IN
pin is left open or tied to a voltage less than
IN
LDO (low dropout linear regulator) supplies
CC
, the loop may enter dropout and attempt
to INTV
Power
B
, which normally recharges during each
(Refer to the Functional Diagram)
CC
CC
. If EXTV
LDO supplies 5.1V from EXTV
CC
pin allows the INTV
IN
, decreases to a voltage
CC
is taken above 4.7V,
CC
LDO is turned on.
CC
pin. When
CC
OUT
power
, to
CC
FB
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 1.2V 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
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 enable 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 regulates
the V
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
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).
OUT
CC
FB
rises smoothly from zero to its fi nal value.
voltage to the TRACK/SS pin voltage instead of the
LDOs. In this state, the LTC3890 draws only 14μA
OUT
to track that of another supply. Typically,
IN
), so long as the maximum
LTC3890
11
OUT
3890f
is