LTC3567 Datasheet, PDF (19/28 Pages) Linear Technology – High Effi ciency USB Power Manager Plus 1A Buck-Boost Converter with I2C Control

LTC3567
OPERATION
Buck-Boost DC/DC Switching Regulator
The LTC3567 contains a 2.25MHz constant-frequency volt-
age mode buck-boost switching regulator. The regulator
provides up to 1A of output load current. The buck-boost
can be programmed to a minimum output voltage of 2.75V
and can be used to power a microcontroller core, micro-
controller I/O, memory, disk drive, or other logic circuitry.
When controlled by I²C, the buck-boost has programmable
set-points for on-the-fly power savings. To suit a variety of
applications, a selectable mode function allows the user to
trade off noise for efficiency. Two modes are available to
control the operation of the LTC3567’s buck-boost regula-
tor. At moderate to heavy loads, the constant frequency
PWM mode provides the least noise switching solution. At
lighter loads Burst Mode operation may be selected. The
full-scale output voltage is programmed by a user-supplied
resistive divider returned to the FB1 pin. An error amplifier
compares the divided output voltage with a reference and
adjusts the compensation voltage accordingly until the FB1
has stabilized to the selected reference voltage (0.425V to
0.8V). The buck-boost regulator also includes a soft-start to
limit inrush current and voltage overshoot when powering
on, short circuit current protection, and switch node slew
limiting circuitry for reduced radiated EMI.
Input Current Limit
The input current limit comparator will shut the input
PMOS switch off once current exceeds 2.5A (typical). The
2.5A input current limit also protects against a grounded
V_OUT1 node.
Output Overvoltage Protection
If the FB1 node were inadvertently shorted to ground, then
the output would increase indefinitely with the maximum
current that could be sourced from V_IN1. The LTC3567
protects against this by shutting off the input PMOS if
the output voltage exceeds a 5.6V (typical).
Low Output Voltage Operation
When the output voltage is below 2.65V (typical) during
start-up, Burst Mode operation is disabled and switch D
is turned off (allowing forward current through the well
diode and limiting reverse current to 0mA).
Buck-Boost Regulator PWM Operating Mode
In PWM mode the voltage seen at FB1 is compared to the
selected reference voltage (0.425V to 0.8V). From the FB1
voltage an error amplifier generates an error signal seen
at V_C1. This error signal commands PWM waveforms
that modulate switches A, B, C, and D. Switches A and B
operate synchronously as do switches C and D. If V_IN1 is
significantly greater than the programmed V_OUT1, then the
converter will operate in buck mode. In this mode switches
A and B will be modulated, with switch D always on (and
switch C always off), to step down the input voltage to the
programmed output. If V_IN1 is significantly less than the
programmed V_OUT1, then the converter will operate in boost
mode. In this mode switches C and D are modulated, with
switch A always on (and switch B always off), to step up
the input voltage to the programmed output. If V_IN1 is close
to the programmed V_OUT1, then the converter will operate
in 4-switch mode. In this mode the switches sequence
through the pattern of AD, AC, BD to either step the input
voltage up or down to the programmed output.
Buck-Boost Regulator Burst-Mode Operation
In Burst Mode operation, the buck-boost regulator uses
a hysteretic FB1 voltage algorithm to control the output
voltage. By limiting FET switching and using a hysteretic
control loop, switching losses are greatly reduced. In this
mode output current is limited to 50mA typical. While
operating in Burst Mode operation, the output capacitor
is charged to a voltage slightly higher than the regulation
point. The buck-boost converter then goes into a sleep
state, during which the output capacitor provides the
load current. The output capacitor is charged by charg-
ing the inductor until the input current reaches 275mA
typical and then discharging the inductor until the reverse
current reaches 0mA typical. This process is repeated
until the feedback voltage has charged to 6mV above the
regulation point. In the sleep state, most of the regulator’s
circuitry is powered down, helping to conserve battery
power. When the feedback voltage drops 6mV below the
regulation point, the switching regulator circuitry is pow-
ered on and another burst cycle begins. The duration for
which the regulator sleeps depends on the load current
and output capacitor value. The sleep time decreases as
the load current increases. The maximum load current in
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