LTC3115-2_15 Datasheet, PDF (15/42 Pages) Linear Technology – 40V, 2A Synchronous Buck-Boost DC/DC Converter

LTC3115-2
Operation
of this current limit circuit is limited by the dynamics of
the error amplifier. On a hard output short, it is possible
for the inductor current to increase substantially beyond
the current limit threshold before the average current limit
has time to react and reduce the inductor current. For this
reason, there is a second current limit circuit which turns
off power switch A if the current through switch A exceeds
approximately 160% of the primary inductor current limit
threshold. This provides additional protection in the case of
an instantaneous hard output short and provides time for
the primary current limit to react. In addition, if V_OUT falls
below 1.85V, the inductor current limit is folded back to half
its nominal value in order to minimize power dissipation.
Reverse Current Limit
In PWM mode operation, the LTC3115-2 synchronously
switches all four power devices. As a result, in addition to
being able to supply current to the output, the converter
has the ability to actively conduct current away from the
output if that is necessary to maintain regulation. If the
output is held above regulation, this could result in large
reverse currents. This situation can occur if the output of
the LTC3115-2 is held up momentarily by another supply
as may occur during a power-up or power-down sequence.
To prevent damage to the part under such conditions, the
LTC3115-2 has a reverse current comparator that monitors
the current entering power switch D from the load. If this
current exceeds 1.5A (typical) switch D is turned off for
the remainder of the switching cycle in order to prevent
the reverse inductor current from reaching unsafe levels.
Output Current Capability
The maximum output current that can be delivered by the
LTC3115-2 is dependent upon many factors, the most
significant being the input and output voltages. For V_OUT
= 5V and V_IN ≥ 3.6V, the LTC3115-2 is able to support
up to a 0.8A load continuously. For V_OUT = 12V and V_IN
≥ 12V, the LTC3115-2 is able to support up to a 2A load
continuously. Typically, the output current capability is
greatest when the input voltage is approximately equal to
the output voltage. At larger step-up voltage ratios, the
output current capability is reduced because the lower duty
cycle of switch D results in a larger inductor current being
needed to support a given load. Additionally, the output
current capability generally decreases at large step-down
voltage ratios due to higher inductor current ripple which
reduces the maximum attainable inductor current.
The output current capability can also be affected by induc-
tor characteristics. An inductor with large DC resistance
will degrade output current capability, particularly in
boost mode operation. Larger value inductors generally
maximize output current capability by reducing inductor
current ripple. In addition, higher switching frequencies
(especially above 750kHz) will reduce the maximum output
current that can be supplied (see the Typical Performance
Characteristics for details).
Burst Mode OPERATION
When the PWM/SYNC pin is held low, the buck-boost
converter employs Burst Mode operation using a vari-
able frequency switching algorithm that minimizes the
no-load input quiescent current and improves efficiency
at light load by reducing the amount of switching to the
minimum level required to support the load. The output
current capability in Burst Mode operation is substantially
lower than in PWM mode and is intended to support light
standby loads (typically under 50mA). Curves showing
the maximum Burst Mode load current as a function of
the input and output voltage can be found in the Typical
Characteristics section of this data sheet. If the converter
load in Burst Mode operation exceeds the maximum Burst
Mode current capability, the output will lose regulation.
Each Burst Mode cycle is initiated when switches A and
C turn on producing a linearly increasing current through
the inductor. When the inductor current reaches the Burst
Mode current limit (1A typically) switches B and D are
turned on, discharging the energy stored in the inductor
into the output capacitor and load. Once the inductor
current reaches zero, all switches are turned off and the
cycle is complete. Current pulses generated in this manner
are repeated as often as necessary to maintain regulation
of the output voltage. In Burst Mode operation, the error
amplifier is not used but is instead placed in a low current
standby mode to reduce supply current and improve light
load efficiency.
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