LTC3554-3_15 Datasheet, PDF (21/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators

LTC3554/LTC3554-1/
LTC3554-2/LTC3554-3
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-impedance
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.
In hard reset, the PGOOD pin is placed in high impedance
state to minimize current draw from the battery in this ul-
tralow power state. This will cause the PGOOD pin to signal
the wrong state (high level) if it is pulled up to a supply
that is not shut down in hard reset (e.g. BAT). If PGOOD
is pulled up to one of the step-down switching regulator
outputs then the PGOOD pin will indicate the correct state
(low level) in hard reset because the switching regulator
output will be low.
Normal Operating Mode (STBY Pin Low)
In normal mode (STBY pin low), the regulators perform
as traditional constant-frequency current mode switching
regulators. Switching frequency is determined by an inter-
nal 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 necessary 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 capacitor.
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 opera-
tion 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.
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