LTC3807_15 Datasheet, PDF (12/32 Pages) Linear Technology – Low IQ, Synchronous Step-Down Controller with 24V Output Voltage Capability

LTC3807
OPERATION
Main Control Loop
The LTC3807 uses a constant frequency, current mode
step-down architecture. During 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, ICMP, resets the RS latch. The
peak inductor current at which ICMP trips and resets the
latch is controlled by the voltage on the ITH 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 rela-
tive 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 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 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 the LTC3807 switching regulator output.
The top MOSFET driver is biased from the floating bootstrap
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 (RUN, TRACK/SS Pins)
The LTC3807 can be shut down using the RUN pin. Pulling
this pin below 1.16V shuts down the main control loop.
Pulling the RUN pin below 0.7V disables the controller and
most internal circuits, including the INTV_CC LDOs. In this
state, the LTC3807 draws only 14μA of quiescent current.
Releasing the RUN pin allows a small internal current to
pull up the pin to enable the controller. The RUN pin has
a 7μA pull-up which is designed to be large enough so
that the RUN pin can be safely floated (to always enable
the controller) without worry of condensation or other
small board leakage pulling the pin down. This is ideal
for always-on applications where the controller is enabled
continuously and never shut down.
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 RUN pin can also be implemented as a UVLO by
connecting it to the output of an external resistor divider
network off V_IN (see Applications Information section).
The start-up of the controller’s output voltage V_OUT is
controlled by the voltage on the TRACK/SS pin. When the
voltage on the TRACK/SS pin is less than the 0.8V internal
reference, the LTC3807 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 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 5V), 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 Applications
Information section).
3807f
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