LTC3576-1_15 Datasheet, PDF (32/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go Triple Step-Down DC/DCs

LTC3576/LTC3576-1
OPERATION
In Burst Mode operation, the switching regulator automati-
cally switches between fixed frequency PWM operation and
hysteretic control as a function of the load current. At light
loads, the regulator operates in hysteretic mode and uses
a constant current algorithm to control the inductor cur-
rent. While in Burst Mode operation, the output capacitor
is charged to a voltage slightly higher than the regulation
point. The step-down switching regulator then goes into
sleep mode, during which the output capacitor provides
the load current. In sleep mode, most of the regulator’s
circuitry is powered down, conserving battery power.
When the output voltage drops below a pre-determined
value, the switching regulator circuitry is powered on and
another burst cycle begins. The duration for which the
regulator operates in sleep mode depends on the load
current. The sleep time decreases as the load current
increases. Burst Mode operation provides a significant
improvement in efficiency at light loads at the expense
of higher output ripple when compared to pulse-skipping
mode. At heavy loads Burst Mode operation functions in
the same manner as pulse-skipping mode.
Finally, the switching regulators have an LDO mode that
gives a DC option for regulating their output voltages. In
LDO mode, the switching regulators are converted to linear
regulators and deliver continuous power from their SWx
pins through their respective inductors. This mode gives
the lowest possible output noise as well as low quiescent
current at light loads.
The step-down switching regulators allow on-the-fly mode
transitions, providing seamless transition between modes
even under load. This allows the user to switch back and
forth between modes to reduce output ripple or increase
low current efficiency as needed.
Step-Down Switching Regulator Dropout Operation
It is possible for a switching regulator’s input voltage,
V_INx, to 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
regulator’s input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
Step-Down Switching Regulator Low Supply Operation
The LTC3576/LTC3576-1 incorporate an undervoltage
lockout circuit on V_OUT which shuts down the general
purpose switching regulators when V_OUT drops below
V_OUT(UVLO). This UVLO prevents unstable operation.
Step-Down Switching Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing the
peak inductor current for each switching regulator over a
500μs period. This allows each output to rise slowly, help-
ing minimize the battery surge current. A soft-start cycle
occurs whenever a given switching 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 Burst Mode operation, pulse-
skipping mode or LDO mode.
Step-Down Switching Regulator Switching
Slew Rate Control
The step-down switching regulators contain new patent
pending circuitry to limit the slew rate of the switch node
(SWx). 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.
Step-Down Switching Regulator in Shutdown
The step-down switching regulators are in shutdown when
not enabled for operation. In shutdown, all circuitry in
the step-down switching regulator is disconnected from
the switching regulator input supply leaving only a few
nanoamperes of leakage current. The step-down switching
regulator outputs are individually pulled to ground through
a 10k resistor on their SWx pins when in shutdown.
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