LTC3892-1_15 Datasheet, PDF (13/36 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3892/LTC3892-1
Operation (Refer to the Functional Diagrams)
Main Control Loop
The LTC3892/LTC3892-1 uses a constant frequency,
current mode step-down architecture. The two controller
channels operate 180° out of phase with each other. Dur-
ing normal operation, the 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 ITH pin, which is the output of the error ampli-
fier, 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 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 I_R, or
the beginning of the next clock cycle.
DRV_CC/EXTV_CC/INTV_CC Power
Power for the top and bottom MOSFET drivers is derived
from the DRV_CC pin. The DRV_CC supply voltage can be
programmed from 5V to 10V through control of the
DRVSET pin. When the EXTV_CC pin is tied to a voltage
below its switchover voltage (4.7V or 7.7V depending on
the DRVSET voltage), the V_IN LDO (low dropout linear
regulator) supplies power from V_IN to DRV_CC. If EXTV_CC is
taken above its switchover voltage, the V_IN LDO is turned
off and an EXTV_CC LDO is turned on. Once enabled, the
EXTV_CC LDO supplies power from EXTV_CC to DRV_CC. Us-
ing the EXTV_CC pin allows the DRV_CC power to be derived
from a high efficiency external source such as one of the
LTC3892/LTC3892-1 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 internal switch whenever SW goes low.
If the input voltage decreases to a voltage close to its
output, 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, resulting in about 99% duty cycle.
The INTV_CC supply powers most of the other internal
circuits in the LTC3892/LTC3892-1. The INTV_CC LDO
regulates to a fixed value of 5V and its power is derived
from the DRV_CC supply.
Shutdown and Start-Up (RUN, TRACK/SS Pins)
The two channels of the LTC3892/LTC3892-1 can be in-
dependently shut down using the RUN1 and RUN2 pins.
Pulling a RUN pin below 1.2V shuts down the main control
loop for that channel. Pulling both pins below 0.7V dis-
ables both controllers and most internal circuits, including
the DRV_CC and INTV_CC LDOs. In this state, the LTC3892/
LTC3892-1 draws only 3.6μA of quiescent current.
Releasing a RUN pin allows a small 150nA internal current
to pull up the pin to enable that controller. Each RUN pin
may be externally pulled up or driven directly by logic. Each
RUN pin can tolerate up to 65V (absolute maximum), so it
can be conveniently tied to V_IN in always-on applications
where one or both controllers are enabled continuously
and never shut down.
The start-up of each controller’s output voltage V_OUT is
controlled by the voltage on the TRACK/SS pin (TRACK/
SS1 for channel 1, TRACK/SS2 for channel 2). When the
voltage on the TRACK/SS pin is less than the 0.8V inter-
nal reference, the LTC3892/LTC3892-1 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 GND. An internal 10μA 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 about 4V), the output
voltage V_OUT rises smoothly from zero to its final value.
Alternatively the TRACK/SS pins can be used to make 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
Applications Information section).
For more information www.linear.com/LTC3892
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