LTC3857IUH#TRPBF Linear Technology, LTC3857IUH#TRPBF Datasheet - Page 11

• Manufacturer Part Number: LTC3857IUH#TRPBF
• Description: IC CTRLR STP-DN SYNC DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3857IUH#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 24 V
• Frequency - Switching: 50kHz ~ 900kHz
• Voltage - Input: 4 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

OPERATION
Main Control Loop
The LTC3857 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 I
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 I
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 efficiency external source such
as one of the LTC3857 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
TH
CC
IN
FB
voltage until the average inductor current matches
/EXTV
LDO is turned off and an EXTV
CC
pin, (which is generated with an external resistor
CC
. 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
TH
power
pin,
, to
CC
FB
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
Shutdown and Start-Up (RUN1, RUN2 and
TRACK/ SS1, TRACK/SS2 Pins)
The two channels of the LTC3857 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either of
these pins below 1.26V 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.
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 LTC3857 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 the absolute maximum
rating of 6V), the output voltage V
zero to its final 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
B
voltage to the TRACK/SS pin voltage instead of the
LDOs. In this state, the LTC3857 draws only 8μA
to recharge.
OUT
to track that of another supply. Typically,
IN
), so long as the maximum
OUT
rises smoothly from
LTC3857
11
OUT
3857fc
is