LTC3857-1_15 Datasheet, PDF (11/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3857-1
OPERATION (Refer to the Functional Diagram)
Main Control Loop
The LTC3857-1 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 I_TH 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 I_TH 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 left open or 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 LTC3857-1 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 LTC3857-1 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_CC LDOs. In this state, the LTC3857-1 draws only 8μA
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_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 LTC3857-1 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 the absolute maximum
rating of 6V), 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 Ap-
plications Information section).
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