Datasheet

LTC3101
16
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OPERATION
MAX OUTPUT
The MAX output generates a protected output rail that tracks
the higher of the two input supplies, BAT2 or USB2. The
MAX output is current-limit protected and is guaranteed
to support a 200mA load.
The MAX output is an always-alive output, meaning it is
always enabled independent of the state of the pushbutton
interface. This allows the MAX output to power additional
LDOs or critical circuitry that must remain powered in
standby. In addition, the MAX output can be used to
effi ciently power additional application circuits that can
operate directly from a wide input voltage range without
burdening one of the switching converters. The MAX output
is also a convenient supply for forcing logic inputs (such
as PWM, ENA1, ENA2 and ENA3) high since it is powered
whenever either input supply is present.
The MAX output is disabled in undervoltage lockout and
during overtemperature shutdown. Since the MAX output
serves as the input to the LDO, it is recommended that
it be bypassed with a 1µF or greater ceramic capacitor if
the LDO is to be used in the application.
Hot Swap (HSO) OUTPUT
The HSO output is generated by a protected power switch
from the output of the buck-boost converter. It provides
a current-limited output that can be shorted to ground
without disrupting the buck-boost output voltage. This
is primarily intended to be used as a supply rail for fl ash
memory cards which can be hot-plugged in the applica-
tion. When a card is hot-plugged into the HSO output,
the supply bypass capacitors on the card are gradually
charged via the current-limited output without affecting
the buck-boost output rail. The HSO output is not enabled
until the buck-boost is enabled and the buck-boost power
good comparator indicates it is in regulation.
BUCK CONVERTER OPERATION
The LTC3101 contains two independent buck DC/DC
converters each capable of supplying a 350mA load. Each
has an adjustable output voltage that can be set as low as
0.6V. In addition, each buck converter supports low drop-
out operation to extend battery life. These converters can
be utilized in Burst Mode operation to improve light-load
effi ciency and no-load standby current or in PWM mode
to ensure low noise operation. Each buck converter has
dual P-channel power switches and a single N-channel
synchronous rectifi er. The dual P-channel power switches
allow the buck converters to operate directly from either
the battery or USB inputs (BAT1 or USB1). The buck
converters will automatically and seamlessly transition
to operate from the higher voltage supply. Both buck
converters feature short-circuit protection and frequency
foldback to prevent inductor current run-away during low
resistance output short conditions.
PWM Mode Operation
If the PWM pin is forced high, both buck converters will
operate in fi xed frequency pulse width modulation mode
using current mode control. At the start of each oscillator
cycle, the active P-channel switch is turned on and remains
on until the inductor current with superimposed slope
compensation ramp exceeds the error amplifi er output.
At this point, the synchronous rectifi er is turned on and
remains on until the inductor current falls to zero or a new
switching cycle is initiated. As a result, the buck converter
operates with discontinuous inductor current at light loads
in order to improve effi ciency. At extremely light loads, the
minimum on-time of the P-channel switch will be reached
and the buck converter will begin turning off for multiple
cycles in order to maintain regulation.
Burst Mode Operation
When the PWM pin is forced low, both buck converters
will automatically and independently transition between
Burst Mode operation at suffi ciently light loads (below
approximately 10mA) and PWM mode at heavier loads.
Burst Mode entry is determined by the peak inductor cur-
rent and therefore the load current at which Burst Mode
operation will be entered depends on the input voltage,
the output voltage and the inductor value. Typical curves
for Burst Mode entry threshold are provided in the Typical
Performance Characteristics section of this data sheet.
In dropout operation, the active P-channel switch will
remain on continuously and Burst Mode operation will
not be entered.