ltc3891 Linear Technology Corporation, ltc3891 Datasheet - Page 11

• Manufacturer Part Number: ltc3891
• Description: Ltc3891 - Low Iq, 60v Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

OPERATION
Main Control Loop
The LTC3891 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
pin (which is generated with an external resistor divider
connected across the output voltage, V
the internal 0.800V reference voltage. When the load cur-
rent increases, it causes a slight decrease in V
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
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
When the EXTV
the V
from V
LDO is turned off and an EXTV
enabled, the EXTV
INTV
to be derived from a high efficiency external source such
as one of the LTC3891 switching regulator outputs.
The top MOSFET driver is biased from the floating bootstrap
capacitor, C
through an external diode when the top MOSFET turns
off. If the input voltage, V
to V
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
to recharge.
OUT
CC
CC
IN
IN
/EXTV
. Using the EXTV
LDO (low dropout linear regulator) supplies 5.1V
, the loop may enter dropout and attempt to turn
to INTV
B
, which normally recharges during each cycle
CC
CC
Power
CC
CC
pin is tied to a voltage less than 4.7V,
. If EXTV
LDO supplies 5.1V from EXTV
CC
IN
CC
, decreases to a voltage close
pin allows the INTV
is taken above 4.7V, the V
CC
LDO is turned on. Once
OUT
, to ground) to
FB
CC
relative
CC
power
CC
pin.
FB
to
IN
B
Shutdown and Start-Up (RUN, TRACK/SS Pins)
The LTC3891 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
In this state, the LTC3891 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
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
The start-up of the controller’s output voltage V
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 LTC3891 regulates the V
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 5V), the output voltage V
smoothly from zero to its final value. Alternatively the
TRACK/SS pin can be used to cause the start-up of V
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).
IN
(see Applications Information section).
IN
), so long as the maximum
LTC3891
FB
voltage to the
CC
OUT
11
OUT
LDOs.
rises
OUT
3891f
is