LTC3118_15 Datasheet, PDF (20/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

LTC3118
Operation
The RUN comparators include a built-in hysteresis of
approximately 170mV, so that the turn-off threshold will
be approximately 15% lower than the turn-on threshold.
Put another way, the internal threshold levels for the RUN
comparators to disable switching from a particular input
is 1.05V.
V_TURNOFF
=
1.05V
⎛
⎜¹
⎝
+
R_T
R_B
⎞
⎟
⎠
The RUN comparator is relatively noise insensitive, but
there may be cases due to PCB layout, very large value
resistors for R_T and R_B (Figure 4a), or proximity to noisy
components where noise pickup is unavoidable and may
cause the turn-on or turn-off levels to be intermittent. In
these cases, a small value filter capacitor can be added
across R_B to ensure proper operation.
Selecting Priority or Ideal Diode Mode Operation
Priority Mode (SEL=0)
Priority mode operation is suggested for most applications,
since powering from one of the sources is typically
preferred. In priority mode, the primary input is connected
to V_IN1 and the auxiliary input is connected to V_IN2. The
LTC3118 will maintain operation from V_IN1 until either
the RUN1 or minimum V_IN1 UVLO circuits transition the
LTC3118 to V_IN2 operation if valid. It is important that the
RUN1 turn-off threshold programs the minimum V_IN1
above 2.5V in Priority Mode unless V_CC is back-fed and
held above 2.5V. This prevents an unintended soft-start
cycle from occurring if V_CC hits its UVLO threshold when
the V_IN1 source is removed, before transitioning to V_IN2
operation.
Depending on the maximum load current of the application,
the RUN1 and RUN2 minimum V_IN turn-off thresholds
may need to set well above 2.5V to prevent V_OUT from
losing regulation, especially in step-up mode. Please
refer to Maximum Load Current vs V_IN curves found in
the Typical Performance Characteristics. Maximum load
current capability when V_IN1 or V_IN2 is less than 3.8V can
be improved if V_CC is boot-strapped to 5V as shown in
Figure 7.
Ideal Diode Mode (SEL=1)
Ideal Diode mode operation is available on the LTC3118 for
systems with low ESR sources or where the programmed
operating range of the two inputs can be separated as will
be discussed. In Ideal Diode mode, an internal comparator
monitors the voltage on both V_IN1 and V_IN2 to determine
which input is higher. The comparator has approximately
800mV of hysteresis to help prevent the part from switching
between the two inputs if the source voltages are equal. The
comparator has an approximate 250µs filter delay to prevent
rapid switching between inputs and erratic operation. When
the LTC3118 switches between inputs, current supplied
from one source is suspended before transitioning to the
other source. Depending on the impedance of each source
and the amount of input current required to support the
load on V_OUT, it is possible for the voltage ripple on one
or both inputs to exceed this comparator’s hysteresis.
As an example, if both input sources have 300mΩ of
impedance and 2A of current is being drawn from the
active source, a 600mV step will occur on the inputs
during switchover, approaching the comparator’s 800mV
of typical hysteresis. When the input voltages are equal,
the LTC3118 could toggle between V_IN1 and V_IN2 operation
at high load currents. For such systems, operation in
Priority mode is recommended, unless the RUN pins can
be programmed such that the minimum operating voltage
of one input is set above the maximum source voltage of
the other input. As with priority mode, the minimum V_IN
operating voltages should be set by their RUN pins above
V_CC UVLO and higher if needed to support maximum load
current. Low ESR 100µF to 220µF aluminum electrolytic
capacitors close to both input pins help to reduce resonant
ringing during V_IN switchover, due to cable inductances
found in some applications and bench evaluation set-ups.
3118f
20
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