LTC3455_15 Datasheet, PDF (10/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455/LTC3455-1
OPERATION
The LTC3455/LTC3455-1 are designed to be a complete
power management solution for a wide variety of portable
systems. The device incorporates two current mode step-
down switching regulators, a full-featured battery charger,
a USB power controller, a Hot Swap output, a low-battery
comparator (which can also be configured as an LDO)
and numerous protection features into a single package.
When only battery power is available, the battery PMOS
switch connects the V_MAX pin to the V_BAT pin to provide
power to both switching regulators (and any other devices
powered from V_MAX). When external power is applied, the
LTC3455/LTC3455-1 seamlessly transition from battery
power (a single-cell Li-Ion cell) to either the USB supply or
a wall adapter. The battery PMOS switch is turned off, the
charger is activated and all internal power for the device
is drawn from the appropriate external power source.
Maximum charge current and charge time are programmed
using an external resistor and capacitor, respectively. The
USB power manager provides accurate current limiting for
the USB pin under all conditions. The Hot Swap output is
ideal for powering memory cards and other devices that
can be inserted while the system is fully powered.
APPLICATIONS INFORMATION
Undervoltage Lockout (UVLO)
If no external power is present, the LTC3455/LTC3455-1 will
start only if the battery voltage is above 3.0V. This prevents
starting up with a battery that is too close to deep discharge.
Once started, the battery must drop below 2.6V before the
LTC3455/LTC3455-1 will shut off. This hysteresis is set
intentionally large to prevent the LTC3455/LTC3455-1 from
turning off at an inappropriate time, like during the read- or
write-cycle of a hard-disk drive (which could potentially
damage the drive). The internal UVLO is meant only as a
last chance safety measure to prevent running the battery
voltage too low and damaging it. An accurate, user-settable
low-battery threshold can be implemented using the gain
block (see the “Gain Block” section for details) which gives
the microcontroller complete control over the timing of a
shutdown due to a low-battery condition.
If external power is present and the battery voltage is less
than 3.0V, the V_MAX pin voltage must be greater than 3.9V
for the LTC3455/LTC3455-1 to start, and once started, the
V_MAX pin must stay above 3.1V for the device to continue
running.
Selecting the Input Power Source
The priority for supplying power to both DC/DC convert-
ers, all internal circuitry, and the V_MAX pin is: Wall adapter,
USB, battery.
Whenever the WALLFB pin is above 1.23V, system power
is drawn from the wall adapter via the V_MAX pin, and the
battery charger is active. The 5V wall adapter output is
connected to the V_MAX pin through a Schottky diode, and
a resistor divider from the 5V wall input is connected to
the WALLFB pin to signal the LTC3455/LTC3455-1 that
wall power is present. A higher voltage adapter can also
be used, but the 6V maximum rating on the V_MAX pin
requires the use of an additional regulator to step down
the voltage.
If USB power is present and above 3.9V (and wall power
is not available), system power is drawn from the USB pin.
The battery charger is active, but charge current will be
held off until the USB pin increases above 4.0V to prevent
the battery charger from further loading down an already
low USB supply. As long as the USB pin stays above 3.9V,
the USB port supplies all other system power.
If the system needs more power than the USB bus can
supply, the charger turns off completely, the USB power
controller becomes a 500mA (or 100mA) current source
and the V_MAX voltage begins to decrease. If V_MAX continues
to decrease, eventually the battery will provide the additional
current needed. This allows the LTC3455/LTC3455-1 to
withstand load current transients that briefly require more
power than the USB power supply can provide.
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