LTC3118_15 Datasheet, PDF (18/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

LTC3118
Operation
Soft-Start
The LTC3118 soft-start circuit minimizes inrush current
and output voltage overshoot on initial power up. The
required timing components for soft-start are internal to
the LTC3118 and produce typical soft-start durations of
approximately 1ms. The internal soft-start circuit slowly
ramps the error amplifier output at VC. In doing so, the
current command of the IC is slowly increased, starting
from zero. After initial power-up, soft-start can be reset by
UVLO on V_CC, both V_IN1GOOD and V_IN2GOOD de-asserting,
thermal shutdown, or a V_OUT short circuit.
V_CC Regulator
An internal low dropout regulator (LDO) generates a
nominal 3.8V rail from the active input V_IN1 or V_IN2. The
V_CC rail powers the internal control circuitry and power
device gate drivers of the LTC3118, including the BST pin
capacitors. The V_CC regulator is disabled in shutdown to
reduce quiescent current and is enabled by forcing one
RUN pin above its logic threshold. The V_CC regulator
includes current-limit protection to safeguard against
accidental short-circuiting of the LDO rail. In 5V V_OUT ap-
plications, V_CC can be powered by V_OUT through an external
Schottky diode. This technique is commonly referred to
as bootstrapping. Bootstrapping can provide a significant
efficiency improvement, particularly when the active V_IN
is high, and also allows operation to the minimum rated
input voltage of 2V. For more information see Bootstrap-
ping the V_CC Regulator with 5V V_OUT or External Supply,
in the Applications Information section.
Undervoltage Lockouts
The LTC3118 undervoltage lockout (UVLO) circuits disable
operation of the internal power switches if both V_IN1 and
V_IN2 or the V_CC voltages are below their respective UVLO
thresholds (see Figure 2). There are three UVLO circuits,
one for each V_IN and another that monitors V_CC. The V_IN
UVLO comparators have a falling voltage threshold of 1.8V
(typical at room temperature). If both input voltages fall
below this level, switching is disabled until one V_IN rises
above 2V, as long as V_CC is above its UVLO threshold. The
V_CC UVLO has a falling voltage threshold of 2.2V (typical).
If V_CC falls below this threshold, IC operation is disabled
until V_CC rises above 2.35V as long as one V_IN is above
its UVLO threshold level.
Depending on the particular application, any of these
UVLO thresholds could be the limiting factor affecting
the minimum input voltage required for operation. The
LTC3118 V_CC regulator uses V_IN1 or V_IN2 for its power
input, whichever is active (see the Input Select Logic and
V_IN Power Good Indicators section). If V_CC is not boot-
strapped, there exists a voltage drop between the active
V_IN and V_CC. The dropout voltage is proportional to the
loading on V_CC due to the gate charge to the internal power
switches. The Typical Performance Characteristics section
of this data sheet provides information on the dropout
voltage between V_IN1 (or V_IN2) and V_CC.
In applications where V_CC is bootstrapped (powered by
V_OUT through a Schottky diode or auxiliary power rail),
the minimum input voltage for operation (after start-up)
will be limited only by the V_IN UVLO thresholds (1.8V
typical). Please note: If the bootstrap voltage is derived
from the LTC3118 V_OUT and not an independent power
rail, then the minimum input voltages required for initial
start-up are still limited by the minimum V_CC voltage
(2.35V typical).
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