LTC3556_15 Datasheet, PDF (24/36 Pages) Linear Technology – High Effi ciency USB Power Manager with Dual Buck and Buck-Boost DC/DCs

LTC3556
OPERATION
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_OUT3 node.
Output Overvoltage Protection
If the FB3 node were inadvertently shorted to ground, then
the output would increase indefinitely with the maximum
current that could be sourced from V_IN3. The LTC3556
protects against this by shutting off the input PMOS if
the output voltage exceeds 5.6V (typical).
Low Output Voltage Operation
When the output voltage is below 2.65V (typical) during
startup, 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 FB3 is compared to the
selected reference voltage (0.425V to 0.8V). From the FB3
voltage an error amplifier generates an error signal seen
at V_C3. 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_IN3 is
significantly greater than the programmed V_OUT3, then the
converter will operate in buck mode. In this case 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_IN3 is significantly less than
the programmed V_OUT3, then the converter will operate in
boost mode. In this case 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_IN3 is
close to the programmed V_OUT3, then the converter will
operate in 4-switch mode. In this mode the switches se-
quence 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 FB3 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 charging the
inductor until the input current reaches 250mA 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 powered 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 cur-
rent increases. The maximum load current in Burst Mode
operation is 50mA typical. The buck-boost regulator will
not go to sleep if the current is greater than 50mA, and
if the load current increases beyond this point while in
Burst Mode operation the output will lose regulation. Burst
Mode operation provides a significant improvement in ef-
ficiency at light loads at the expense of higher output ripple
when compared to PWM mode. For many noise-sensitive
systems, Burst Mode operation might be undesirable at
certain times (i.e., during a transmit or receive cycle of a
wireless device), but highly desirable at others (i.e., when
the device is in low power standby mode). The B6 and
B5 bits of the I²C port are used to enable or disable Burst
Mode operation at any time, offering both low noise and
low power operation when they are needed.
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