LTC3566-2_15 Datasheet, PDF (17/28 Pages) Linear Technology – High Efficiency USB Power Manager Plus 1A Buck-Boost Converter

LTC3566/LTC3566-2
OPERATION
of the NTC thermistor rises. The LTC3566 family is also
designed to pause charging when the value of the NTC
thermistor increases to 3.25 times the value of R25. For
Vishay curve 1 this resistance, 325k, corresponds to
approximately 0°C. The hot and cold comparators each
have approximately 3°C of hysteresis to prevent oscillation
about the trip point. Grounding the NTC pin disables the
NTC charge pausing function.
Thermal Regulation
To optimize charging time, an internal thermal feedback
loop may automatically decrease the programmed charge
current. This will occur if the die temperature rises to
approximately 110°C. Thermal regulation protects the
LTC3566 family from excessive temperature due to high
power operation or high ambient thermal conditions and
allows the user to push the limits of the power handling
capability with a given circuit board design without risk of
damaging the part or external components. The benefit of
the LTC3566 family thermal regulation loop is that charge
current can be set according to actual conditions rather
than worst-case conditions with the assurance that the
battery charger will automatically reduce the current in
worst-case conditions.
Buck-Boost DC/DC Switching Regulator
The LTC3566 family contains a 2.25MHz constant-fre-
quency voltage 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 volt-
age of 2.75V and can be used to power a microcontroller
core, microcontroller I/O, memory, disk drive, or other logic
circuitry. 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
LTC3566 family’s buck-boost regulator. 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 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 compen-
sation voltage accordingly until the FB1 has stabilized at
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 LTC3566
family 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 a
0.8V reference. 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.
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