ltc3586 Linear Technology Corporation, ltc3586 Datasheet - Page 22

• Manufacturer Part Number: ltc3586
• Description: High Ef? Ciency Usb Power Manager With Boost, Buck-boost And Dual Bucks
• Manufacturer: Linear Technology Corporation

OPERATION
LTC3586
Buck Regulator Dropout Operation
It is possible for a buck regulator’s input voltage, V
approach its programmed output voltage (e.g., a battery
voltage of 3.4V with a programmed output voltage of 3.3V).
When this happens, the PMOS switch duty cycle increases
until it is turned on continuously at 100%. In this dropout
condition, the respective output voltage equals the buck
regulator’s input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
Buck Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing the
peak inductor current for each buck regulator over a 500μs
period. This allows each output to rise slowly, helping
minimize the battery in-rush current. A soft-start cycle
occurs whenever a given buck regulator is enabled, or
after a fault condition has occurred (thermal shutdown
or UVLO). A soft-start cycle is not triggered by changing
operating modes. This allows seamless output operation
when transitioning between modes.
Buck Regulator Switching Slew Rate Control
The buck regulators contain new patent pending circuitry
to limit the slew rate of the switch node (SW1 and SW2).
This new circuitry is designed to transition the switch node
over a period of a couple of nanoseconds, significantly
reducing radiated EMI and conducted supply noise.
BUCK-BOOST DC/DC SWITCHING REGULATOR
The LTC3586 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 2.5V
and can be used to power a microcontroller core, micro-
controller I/O, memory, disk drive, or other logic circuitry.
The converter is enabled by pulling EN3 high. To suit a
variety of applications, a selectable mode function allows
the user to trade-off noise for effi ciency. Two modes are
available to control the operation of the LTC3586’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-sup-
22
INx
, to
plied resistive divider returned to FB3. An error amplifi er
compares the divided output voltage with a reference and
adjusts the compensation voltage accordingly until the
FB3 pin has stabilized to the reference voltage (0.8V). The
buck-boost regulator 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
Output Overvoltage Protection
If the FB3 node were inadvertently shorted to ground, then
the output would increase indefi nitely with the maximum
current that could be sourced from V
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
reference voltage (0.8V). From the FB3 voltage an error
amplifi er generates an error signal seen at V
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
than the programmed V
erate 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
V
this case switches C and D are modulated, with switch A
always on (and switch B always off), to step-up the input
OUT3
OUT3
, then the converter will operate in boost mode. In
node.
IN3
is signifi cantly less than the programmed
OUT3
, then the converter will op-
IN3
is signifi cantly greater
IN3
. The LTC3586
C3
. This error
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