LTC3589HUJ#PBF Linear Technology, LTC3589HUJ#PBF Datasheet - Page 25

• Manufacturer Part Number: LTC3589HUJ#PBF
• Description: IC DC/DC CONV 8-OUTPUT 40QFN
• Manufacturer: Linear Technology

Specifications of LTC3589HUJ#PBF

• Applications: Handheld/Mobile Devices
• Current - Supply: 8µA
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: *
• Primary Input Voltage: 5.5V
• No. Of Outputs: 8
• Output Voltage: 5V
• Output Current: 1.6A
• No. Of Pins: 40
• Operating Temperature Range: -40°C To +150°C
• Msl: MSL 1 - Unlimited
• No. Of Ldo Regulators: 3
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

OPERATION
Operating Modes
Table 10 shows the I
the operating modes of the LTC3589/LTC3589-1 buck-boost
converter. When command register SCR1 bit 6 is LOW,
the LTC3589 buck-boost switching regulator operates in
a fi xed frequency pulse width modulation mode using
voltage mode feedback control. A proprietary switching
algorithm allows the converter to transition between buck,
buck-boost, and boost modes without discontinuity in in-
ductor current or loop characteristics. The switch topology
is shown in the application circuit in Figure 4.
When the input voltage is significantly greater than the
output voltage, the buck-boost converter operates in
buck mode. Switch D turns on continuously and switch C
remains off. Switches A and B are pulse width modulated
to produce the required duty cycle to support the output
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
I
OUT(MAX)
OUT(MAX)
=
V
0.28 • V
OUT
in Burst Mode operation is equal to:
2
C command registers used to control
+ V
IN
IN
(A)
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/LTC3589-1 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 opera-
tion regardless of the state of the SCR1 command register.
Current Limit Operation
The LTC3589/LTC3589-1 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 current 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 average 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.
LTC3589/LTC3589-1
25
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