LTC3827 Datasheet, PDF (11/36 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3827
OPERATION (Refer to Functional Diagram)
Main Control Loop
The LTC3827 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, I_CMP, resets the RS
latch. The peak inductor current at which I_CMP 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 resis-
tor 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, an internal 5.25V low dropout linear regulator
supplies INTV_CC power from V_IN. If EXTV_CC is taken above
4.7V, the 5.25V regulator is turned off and a 7.5V low
dropout linear regulator is enabled that supplies INTV_CC
power from EXTV_CC. If EXTV_CC is less than 7.5V (but
greater than 4.7V), the 7.5V regulator is in dropout and
INTV_CC is approximately equal to EXTV_CC. When EXTV_CC
is greater than 7.5V (up to an absolute maximum rating
of 10V), INTV_CC is regulated to 7.5V. Using the EXTV_CC
pin allows the INTV_CC power to be derived from a high
efficiency external source such as one of the LTC3827
switching regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap capacitor, C_B, which normally recharges during each
off 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 LTC3827 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either
of these pins below 0.7V shuts down the main control
loop for that controller. Pulling both pins low disables
both controllers and most internal circuits, including the
INTV_CC regulator, and the LTC3827 draws only 8μA of
quiescent current.
Releasing either RUN pin allows an internal 0.5μA current
to pull up the pin and enable that controller. Alternatively,
the RUN pin may be externally pulled up or driven directly
by logic. Be careful not to exceed the Absolute Maximum
rating of 7V on this pin.
The start-up of each controller’s output voltage V_OUT is
controlled by the voltage on the TRACK/SS1 and TRACK/
SS2 pin. When the voltage on the TRACK/SS pin is less
than the 0.8V internal reference, the LTC3827 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), 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. Typi-
cally, this requires connecting to the TRACK/SS pin an
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