LTC3536_15 Datasheet, PDF (11/28 Pages) Linear Technology – 1A Low Noise, Buck-Boost DC/DC Converter

LTC3536
Operation
Reverse Current Limit
In PWM mode operation the LTC3536 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 LTC3536 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 LTC3536 has a reverse
current comparator that monitors the current entering
power switch D from the load. If this current exceeds 0.55A
(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.
For no-load current application, the inductor current ripple
must be lower than double the minimum reverse current
limit (0.3A • 2 = 0.6A maximum inductor current ripple).
See the Inductor Selection section for information about
how to calculate the inductor current ripple.
Output Current Capability
The maximum output current that can be delivered by
the LTC3536 is dependent upon many factors, the most
significant being the input and output voltages. For V_OUT
= 3.3V and V_IN ≥ 3V, the LTC3536 is able to support a
1A load continuously. For V_OUT = 3.3V and V_IN =1.8V, the
LTC3536 is able to support a 300mA 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 cur-
rent 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. In addition, larger value inductors gener-
ally maximize output current capability by reducing inductor
current ripple. See the Typical Performance Characteristics
and the Inductor Selection section for information.
Burst Mode OPERATION
When MODE/SYNC is held high, the buck-boost converter
operates in Burst Mode operation using a variable 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 capabil-
ity in Burst Mode operation is substantially lower than in
PWM mode and is intended to support light stand-by loads.
Curves showing the maximum Burst Mode load current
as a function of the input and output voltage can be found
in the Typical Performance 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 peak current limit (0.6A typically), switches B and D
are turned on, discharging the energy stored in the induc-
tor 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 used as burst comparator. If the MODE pin
is driven externally to a level higher than V_IN, a current
limiting resistor should be used. 1M for 6V on the MODE
pin limits the current to 6µA. Also, a Schottky diode from
the MODE pin to V_IN can be used in addition to current
limiting resistor.
Soft-Start
To minimize input current transients on power-up, the
LTC3536 incorporates an internal soft-start circuit with a
nominal duration of 0.9ms. The soft-start is implemented
by a linearly increasing ramp of the error amplifier refer-
ence voltage during the soft-start duration. As a result,
the duration of the soft-start period is largely unaffected
by the size of the output capacitor or the output regula-
tion voltage. Given the closed-loop nature of the soft-start
implementation, the converter is able to respond to load
transients that occur during the soft-start interval. The
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