LTC3554 Datasheet, PDF (21/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators

LTC3554
OPERATION
PGOOD Operation
The PGOOD pin is an open-drain output which indicates
that all enabled step-down switching regulators have
reached their final regulation voltage. It goes high-imped-
ance 230ms after all enabled switching regulators reach
92% of their regulation value. The delay allows ample time
for an external processor to reset itself. PGOOD may be
used as a power-on reset to a microprocessor powered
by the step-down switching regulators. Since PGOOD is
an open-drain output, a pull-up resistor to an appropriate
power source is needed. A suggested approach is to con-
nect the pull-up resistor to one of the step-down switching
regulator output voltages so that power is not dissipated
while the regulators are disabled.
Normal Operating Mode (STBY Pin Low)
In normal mode (STBY pin low), the regulators perform
as traditional constant-frequency current mode switch-
ing regulators. Switching frequency is determined by an
internal oscillator whose frequency is selectable via the
FSEL pin. An internal latch is set at the start of every
oscillator cycle, turning on the main P-channel MOSFET
switch. During each cycle, a current comparator compares
the 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 clock cycle, or when
the current through the N-channel MOSFET synchronous
rectifier drops to zero, whichever happens first. Via this
mechanism, the error amplifier adjusts the peak inductor
current to deliver the required output power. All neces-
sary compensation is internal to the step-down switching
regulator requiring only a single ceramic output capacitor
for stability.
At light load and no-load conditions, the buck automatically
switches to a power-saving hysteretic control algorithm
that operates the switches intermittently to minimize
switching losses. Known as Burst Mode operation, the
buck cycles the power switches enough times to charge
the output capacitor to a voltage slightly higher than
the regulation point. The buck then goes into a reduced
quiescent current sleep mode. In this state, power loss
is minimized while the load current is supplied by the
output capacitor. Whenever the output voltage drops
below a predetermined value, the buck wakes from sleep
and cycles the switches again until the output capacitor
voltage is once again slightly above the regulation point.
Sleep time thus depends on load current, since the load
current determines the discharge rate of the output capaci-
tor. Should load current increase above roughly 50mA, the
buck resumes constant-frequency operation.
Standby Mode (STBY Pin High)
There are situations where even the low quiescent current
of Burst Mode operation is not low enough. For instance,
in a static memory keep alive situation, load current may
fall well below 1μA. In this case, the 25μA typical BVIN
quiescent current per active regulator in Burst Mode
operation becomes the main factor determining battery
run time.
Standby mode cuts BVIN quiescent current down to just
1.5μA per active regulator, greatly extending battery run
time in this essentially no-load region of operation. The
application circuit commands the LTC3554 into and out
of standby mode via the STBY pin logic input. Bringing
the STBY pin high places both regulators into standby
mode, while bringing it low returns them to Burst Mode
operation. In standby mode, load capability drops to 5mA
per regulator.
In standby mode, each regulator operates hysteretically.
When the FB pin voltage falls below the internal 0.8V
reference, a current source from BVIN to SW turns on,
delivering current through the inductor to the switching
regulator output capacitor and load. When the FB pin
voltage rises above the reference plus a small hysteresis
voltage, that current is shut off. In this way, output regula-
tion is maintained.
3554p
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