LTC3576 Datasheet, PDF (31/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go + Triple Step-Down DC/DCs

LTC3576/LTC3576-1
OPERATION
Step-Down Switching Regulators
The LTC3576/LTC3576-1 contain three general purpose
2.25MHz step-down constant-frequency current mode
switching regulators. Two regulators provide up to 400mA
and a third switching regulator can provide up to 1A.
All three switching regulators can be programmed for
a minimum start-up output voltage of 0.8V and can be
used to power a microcontroller core, microcontroller
I/O, memory, disk drive or other logic circuitry. All three
switching regulators have I²C programmable set points for
on-the-fly power savings. They also support 100% duty
cycle operation (low dropout mode) when their input volt-
age drops very close to their output voltage. To suit a variety
of applications, selectable mode functions can be used to
trade off noise for efficiency. Three modes are available to
control the operation of the LTC3576/LTC3576-1’s general
purpose switching regulators. At moderate to heavy loads,
the pulse skip mode provides the lowest noise switching
solution. At lighter loads, Burst Mode operation or LDO
mode may be selected. The switching regulators include
soft-start to limit inrush current when powering on, short-
circuit current protection and switch node slew limiting
circuitry to reduce radiated EMI. No external compensa-
tion components are required. The operating mode of the
regulators may be set by either I²C control or by manual
control of the SDA and SCL pins if the I²C port is not used.
Each converter may be individually enabled by either their
external control pins EN1, EN2, EN3 or by the I²C port. All
three switching regulators have individual programmable
feedback servo voltages via I²C control. The switching
regulator input supplies V_IN1, V_IN2 and V_IN3 will generally
be connected to the system load pin V_OUT.
Step-Down Switching Regulator Operating Modes
The LTC3576/LTC3576-1’s general purpose switching
regulators include three possible operating modes to meet
the noise/power needs of a variety of applications.
In pulse skip mode, an internal latch is set at the start of
every cycle which turns on the main P-channel MOSFET
switch. During each cycle, a current comparator compares
the peak inductor current to the output of an error amplifier.
The output of the current comparator resets the internal
latch which causes the main P-channel MOSFET switch to
turn off and the N-channel MOSFET synchronous rectifier
to turn on. The N-channel MOSFET synchronous rectifier
turns off at the end of the 2.25MHz cycle or if the current
through the N-channel MOSFET synchronous rectifier
drops to zero. Using this method of operation, the error
amplifier adjusts the peak inductor current to deliver the
required output power. All necessary compensation is
internal to the switching regulator requiring only a single
ceramic output capacitor for stability. At light loads in PWM
mode, the inductor current may reach zero on each pulse
which will turn off the N-channel MOSFET synchronous
rectifier. In this case, the switch node (SW) goes high
impedance and the switch node voltage will “ring”. This
is discontinuous mode operation, and is normal behavior
for a switching regulator. At very light loads in pulse skip
mode, the switching regulators will automatically skip
pulses as needed to maintain output regulation.
At high duty cycles (V_OUTx > V_INx /2) it is possible for the
inductor current to reverse, causing the regulator to operate
continuously at light loads. This is normal and regulation is
maintained, but the supply current will increase to several
mA due to continuous switching.
3576f
31