LTC3785 LINER [Linear Technology], LTC3785 Datasheet - Page 11

• Manufacturer Part Number: LTC3785
• Description: High Efficiency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

operaTion
MAIN CONTROL LOOP
The LTC3789 is a current mode controller that provides an
output voltage above, equal to or below the input voltage.
The LTC proprietary topology and control architecture
employs a current-sensing resistor. The inductor current
is controlled by the voltage on the I
output of the error amplifier EA. The V
voltage feedback signal, which is compared to the internal
reference voltage by the EA. If the input/output current
regulation loop is implemented, the sensed inductor cur-
rent is controlled by either the sensed feedback voltage
or the input/output current.
INTV
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
When the EXTV
than 4.5V, an internal 5.5V low dropout (LDO) regulator
supplies INTV
4.8V, the 5.5V regulator is turned off, and another LDO
regulates INTV
the INTV
external source such as the LTC3789 regulator output.
The absolute maximum voltage on EXTV
Internal Charge Pump
Each top MOSFET driver is biased from the floating boot-
strap capacitors C
by INTV
is turned off. When the LTC3789 operates exclusively in
the buck or boost regions, one of the top MOSFETs is
constantly on. An internal charge pump recharges the
bootstrap capacitor to compensate for the small leakage
current through the bootstrap diode so that the MOSFET
can be kept on. However, if a high leakage diode is used
such that the internal charge pump cannot provide sufficient
CC
/EXTV
CC
CC
through an external diode when the top MOSFET
power to be derived from a high efficiency
CC
CC
CC
Power
CC
power from V
from EXTV
A
is left open or tied to a voltage less
and C
B
, which are normally recharged
CC
IN
. The EXTV
. If EXTV
TH
FB
pin, which is the
CC
CC
pin receives the
CC
is taken above
is 14V.
LDO allows
CC
pin.
charges to the external bootstrap capacitor, an internal
UVLO comparator, which constantly monitors the drop
across the capacitor, will sense the (BOOST – SW) voltage
when it is below 3.6V. It will turn off the top MOSFET for
about one-twelfth of the clock period every four cycles to
allow C
Shutdown and Start-Up
The controller can be shut down by pulling the RUN
pin low. When the RUN pin voltage is below 0.5V, the
LTC3789 goes into low quiescent current mode. Releas-
ing RUN allows an internal 1.2µA current to pull up the
pin and enable the controller. When RUN is above the
accurate threshold of 1.22V, the internal LDO will power
up the INTV
will kick in to provide more RUN pin hysteresis. The RUN
pin may be externally pulled up or driven directly by logic.
Be careful not to exceed the absolute maximum rating of
6V on this pin.
The start-up of the controller’s output voltage V
controlled by the voltage on the SS pin. When the voltage
on the SS pin is less than the 0.8V internal reference, the
LTC3789 regulates the V
of the 0.8V reference. This allows the SS pin to be used
to program soft-start by connecting an external capacitor
from the SS pin to SGND. An internal 3µA pull-up current
charges this capacitor, creating a voltage ramp on the SS
pin. As the SS voltage rises linearly from 0V to 0.8V (and
beyond), the output voltage V
to its final value. Alternatively, the SS pin can be used to
cause the start-up of V
When RUN is pulled low to disable the controller, or when
INTV
3.4V, the SS pin is pulled low by an internal MOSFET. In
undervoltage lockout, the controller is disabled and the
external MOSFETs are held off.
CC
A
is below the undervoltage lockout threshold of
or C
CC
B
. At the same time, a 6µA pull-up current
to recharge.
OUT
FB
voltage to the SS voltage instead
to track that of another supply.
OUT
rises smoothly from zero
LTC3789
11
OUT
3789f
is