LTC3455 Datasheet, PDF (10/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
U
OPERATIO
The LTC3455 is designed to be a complete power manage-
ment solution for a wide variety of portable systems. The
device incorporates two current mode step-down switch-
ing regulators, a full-featured battery charger, a USB
power controller, a Hot Swap output, a low-battery com-
parator (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
seamlessly transitions 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 cur-
rent and charge time are programmed using an external
resistor and capacitor, respectively. The USB power man-
ager 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.
APPLICATIO S I FOR ATIO
Undervoltage Lockout (UVLO)
If no external power is present, the LTC3455 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 will shut off. This hysteresis is set intentionally
large to prevent the LTC3455 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 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 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 contin-
ues to decrease, eventually the battery will provide the
additional current needed. This allows the LTC3455 to
withstand load current transients that briefly require more
power than the USB power supply can provide.
3455f
10