LTC3890EGN-1#TRPBF Linear Technology, LTC3890EGN-1#TRPBF Datasheet - Page 11

• Manufacturer Part Number: LTC3890EGN-1#TRPBF
• Description: IC BUCK SYNC ADJ DUAL 28SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3890EGN-1#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 24 V
• Frequency - Switching: 350kHz ~ 535kHz
• Voltage - Input: 4 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

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 amplifier, EA. The error
amplifier 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
When the EXTV
the V
from V
LDO is turned off and an EXTV
enabled, the EXTV
INTV
to be derived from a high efficiency external source such
as one of the LTC3890-1 switching regulator outputs.
Each top MOSFET driver is biased from the floating 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
CC
FB
IN
IN
/EXTV
. Using the EXTV
B
LDO (low dropout linear regulator) supplies 5.1V
pin, (which is generated with an external resistor
to INTV
to recharge.
OUT
CC
, the loop may enter dropout and attempt
CC
Power
CC
B
, which normally recharges during each
CC
pin is tied to a voltage less than 4.7V,
. If EXTV
(Refer to the Functional Diagram)
LDO supplies 5.1V from EXTV
CC
CC
pin allows the INTV
is taken above 4.7V, the V
CC
IN
, decreases to a voltage
LDO is turned on. Once
CC
OUT
CC
power
CC
, to
pin.
FB
to
IN
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 1.15V 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
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 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
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 the
Applications Information section).
OUT
CC
FB
rises smoothly from zero to its final value.
voltage to the TRACK/SS pin voltage instead of the
LDOs. In this state, the LTC3890-1 draws only
OUT
to track that of another supply. Typically,
IN
), so long as the maximum
LTC3890-1
11
OUT
38901fa
is