LTC3562 Datasheet, PDF (11/20 Pages) Linear Technology – I2C Quad Synchronous Step-Down DC/DC Regulator 2 × 600mA, 2 × 400mA

LTC3562
OPERATION
the output capacitor is charged to a voltage slightly higher
than the regulation point. The step-down converter 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, helping conserve
battery power and increase efficiency. When the output
voltage drops below a predetermined 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. Forced Burst Mode
operation has a maximum deliverable output current of
about 140mA for the 600mA regulators and 100mA for
the 400mA regulators. Beyond the maximum deliverable
output current, the step-down switching regulator will not
enter sleep mode and the output will drop out of regula-
tion. Forced Burst Mode operation provides a significant
improvement in efficiency at light loads at the expense of
higher output ripple when compared to pulse skip mode.
For many noise-sensitive systems, forced 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 I²C port can be used to
enable or disable forced Burst Mode operation at any time,
offering both low noise and low power operation when
they are needed.
In Burst Mode operation, the switching regulator automati-
cally switches between fixed frequency pulse skip operation
and hysteretic control as a function of the load current. At
light loads the regulators operate in hysteretic mode and
at heavy loads they operate in constant-frequency mode.
The constant-frequency mode provides the same output
ripple and efficiency as pulse skip mode while hysteretic
mode provides slightly lower output ripple than forced
Burst Mode operation at the expense of slightly lower
efficiency.
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.
Dropout Operation
It is possible for V_IN to approach a switching regulator’s
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 condi-
tion, the respective output voltage equals the regulator’s
input voltage minus the voltage drops across the internal
P-channel MOSFET and the inductor.
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,
helping minimize the battery in-rush current. A soft-
start cycle occurs whenever a given switching regula-
tor is enabled, or after a fault condition has occurred
(thermal shutdown). A soft-start cycle is not triggered
by changing operating modes. This allows seamless
output operation when transitioning between Burst
Mode operation, forced Burst Mode operation, pulse
skip mode or LDO mode.
Switching Slew Rate Control
The step-down switching regulators contain new pat-
ent 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 nanoseconds,
significantly reducing radiated EMI and conducted supply
noise, while keeping efficiency high.
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
nano-amps of leakage current. The step-down switch-
ing regulator outputs are individually pulled to ground
through a 2k resistor on the switch pin (SWx) when in
shutdown.
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