LTC3589_1 Datasheet, PDF (25/46 Pages) Linear Technology – 8-Output Regulator with Sequencing and I2C

LTC3589
OPERATION
regulation voltage. As the input voltage decreases, switch A
remains on for a larger portion of the switching cycle.
When the duty cycle reaches approximately 85%, the
switch pair AC begins turning on for a small fraction of the
switching period. As the input voltage decreases further,
the AC switch pair remains on for longer durations and
the duration of the BD phase decreases proportionately.
As the input voltage drops below the output voltage, the
AC phase will eventually increase to the point that there is
no longer any BD phase. At this point, switch A remains
on continuously while switches CD operate as a boost
converter to regulate the desired output voltage.
The buck-boost is set to Burst Mode operation by writing
a 1 to command register SCR1 bit 6. Using Burst Mode
operation at light loads improves efficiency and reduces
standby current at zero loads. In Burst Mode operation,
the inductor is charged with bursts of fixed peak amplitude
current pulses. The current pulses are repeated as often
as necessary to maintain the target output voltage. The
maximum output current that can be supplied in Burst Mode
operation is dependent upon the input and output voltage.
Typically I_OUT(MAX) in Burst Mode operation is equal to:
I _{OUT(MAX )}
=
0.28
V_OUT
• V_{IN (A)}
+ V_IN
If the buck-boost load exceeds the maximum Burst Mode
current capability then the output rail will lose regula-
tion and the power good comparator will indicate a fault
condition.
When the LTC3589 buck-boost is not enabled, a 2.5k pull-
down resistor is connected between BB_OUT and ground.
Soft-Start
The buck-boost converter has an internal voltage mode
soft-start circuit that ramps the buck-boosts error amp
reference from 0V to 800mV at a rate of 2V/ms. During
soft-start, the converter is regulating to the ramping
reference and will respond to output load transients.
During soft-start the buck-boost converter is forced into
continuous mode operation regardless of the state of the
SCR1 command register.
Current Limit Operation
The LTC3589 buck-boost regulator has current limit circuits
to limit forward current through the A switch and reverse
current through the D switch. The primary forward cur-
rent limit circuit injects a small fraction of the inductor
current into the feedback node whenever the inductor
current exceeds 2.7A (typical). Forcing the current into
the feedback node in the high gain feedback circuit has
the effect of lowering the output voltage until the aver-
age current in switch A is equal to the current limit. The
average limit uses the error amplifier in its active linear
state so once the fault condition is removed the recovery
is smooth with little overshoot.
A hard short on the output of the buck-boost will cause the
inductor current to exceed the 2.7A average current limit.
A second current limit turns off switch A in the event peak
inductor current reaches 3A (typical). The instantaneous
forward current limit provides extra protection in the event
of a sudden hard short.
The reverse current comparator on the D switch moni-
tors the current entering the BB_OUT pin. When this
current exceeds 1A (typical) switch D will turn off for the
remainder of the switching cycle. This feature protects
the buck-boost converter from excessive reverse current
if the buck-boost output is held above the regulation point
by an external source.
Inductor Selection
Inductor selection criteria for the buck-boost are similar to
those given for the step-down switching regulators. The
buck-boost converter is designed to work with inductors
in the range of 1µH to 3.3µH. For most applications use a
1.5µH inductor. Choose an inductor with a DC current rating
at least two times larger than the maximum load current to
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