LTC3875_15 Datasheet, PDF (11/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

LTC3875
Operation
Main Control Loop
The LTC3875 is a constant frequency, current mode step-
down controller with two channels operating 180° or 240°
out of phase. During normal operation, each top MOSFET
is turned on when the clock for that channel sets the R_S
latch, and turned off when the main current comparator,
I_CMP, resets the R_S latch. The peak inductor current at
which I_CMP resets the R_S latch is controlled by the voltage
on the I_TH pin, which is the output of each error amplifier
EA. The remote sense amplifier (DIFFAMP) converts the
sensed differential voltage across the output feedback
resistor divider to an internal voltage (V_FB) referred to
SGND. The V_FB signal is then compared to the internal
0.6V reference voltage by the EA. When the load current
increases, it causes a slight decrease in V_FB relative to
the 0.6V reference, which in turn causes the I_TH voltage
to increase until the average inductor current matches the
new load current. After the top MOSFET has turned off,
the bottom MOSFET is turned on until either the inductor
current starts to reverse, as indicated by the reverse cur-
rent comparator, I_REV, or the beginning of the next 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 4.5V, an internal 5.5V linear regulator supplies INTV_CC
power from V_IN. If EXTV_CC is taken above 4.7V, the 5.5V
regulator is turned off and an internal switch is turned on
connecting EXTV_CC to INTV_CC. When using EXTV_CC, the V_IN
voltage has to be higher than EXTV_CC voltage at all time and
has to come before EXTV_CC is applied. Otherwise, EXTV_CC
current will flow back to V_IN through the internal switch’s
body diode and potentially damage the device. Using the
EXTV_CC pin allows the INTV_CC power to be derived from
a high efficiency external source.
Each top MOSFET driver is biased from the floating
bootstrap capacitor, C_B, which normally recharges dur-
ing 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 plus 100ns
every third cycle to allow C_B to recharge. However, it is
recommended that a load be present or the IC operates
at low frequency during the drop-out transition to ensure
that C_B is recharged.
Shutdown and Start-Up
(RUN1, RUN2 and TK/SS1, TK/SS2 Pins)
The two channels of the LTC3875 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either
of these pins below 1.14V shuts down the main control
loop for that channel. Pulling both pins low disables both
channels and most internal circuits, including the INTV_CC
regulator. Releasing either RUN pin allows an internal
1µA current to pull up the pin and enable the controller.
Alternatively, the RUN pins may be externally pulled up
or driven directly by logic. Be careful not to exceed the
absolute maximum rating of 6V on these pins.
The start-up of each channel’s output voltage, V_OUT, is
controlled by the voltage on its TK/SS pin. When the
voltage on the TK/SS pin is less than the 0.6V internal
reference, the LTC3875 regulates the V_FB voltage to the
TK/SS pin voltage instead of the 0.6V reference. This al-
lows the TK/SS pin to be used to program the soft-start
period by connecting an external capacitor from the TK/SS
pin to SGND. An internal 1.25µA pull-up current charges
this capacitor, creating a voltage ramp on the TK/SS pin.
As the TK/SS voltage rises linearly from 0V to 0.6V (and
beyond), the output voltage V_OUT rises smoothly from zero
to its final value. Alternatively the TK/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 TK/SS
pin an external resistor divider from the other supply to
ground (see the Applications Information section). When
the corresponding RUN pin is pulled low to disable a
controller, or when INTV_CC drops below its undervoltage
lockout threshold of 3.7V, the TK/SS pin is pulled low
by an internal MOSFET. When in undervoltage lockout,
both controllers are disabled and the external MOSFETs
are held off.
Internal Soft-Start
By default, the start-up of the output voltage is normally
controlled by an internal soft-start ramp. The internal
soft-start ramp represents one of the noninverting inputs
3875fa
For more information www.linear.com/LTC3875
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