LTC3789_15 Datasheet, PDF (12/30 Pages) Linear Technology – High Efficiency, Synchronous, 4-Switch Buck-Boost Controller

LTC3789
Operation
Main Control Loop
The LTC3789 is a current mode controller that provides
an output voltage above, equal to or below the input volt-
age. The LTC proprietary topology and control architecture
employs a current-sensing resistor. The inductor current
is controlled by the voltage on the I_TH pin, which is the
output of the error amplifier EA. The V_FB pin receives the
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_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 is left open or tied to a voltage less
than 4.5V, an internal 5.5V low dropout (LDO) regulator
supplies INTV_CC power from V_IN. If EXTV_CC is taken above
4.8V, the 5.5V regulator is turned off, and another LDO
regulates INTV_CC from EXTV_CC. The EXTV_CC LDO allows
the INTV_CC power to be derived from a high efficiency
external source such as the LTC3789 regulator output
to reduce IC power dissipation. The absolute maximum
voltage on EXTV_CC is 14V.
Internal Charge Pump
Each top MOSFET driver is biased from the floating boot-
strap capacitors C_A and C_B, which are normally recharged
by INTV_CC through an external diode when the top MOSFET
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
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_A or C_B to recharge.
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_CC. At the same time, a 6µA pull-up current
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_OUT is
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_FB voltage to the SS voltage instead
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_OUT rises smoothly from zero
to its final value. Alternatively, the SS pin can be used to
cause the start-up of V_OUT to track that of another supply.
When RUN is pulled low to disable the controller, or when
INTV_CC is below the undervoltage lockout threshold of
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.
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