LTC3455 Datasheet, PDF (11/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
APPLICATIO S I FOR ATIO
Operation When No Battery Is Present
As long as USB or wall power is available, the LTC3455 will
operate with no battery present, a crucial requirement for
systems with a removable battery. Keep in mind, however,
that if the LTC3455 is USB powered and the battery is not
present, absence of the battery means that there is no
reservoir if the system needs more power than the USB
port can supply. Similarly, if external power is available,
the LTC3455 will operate even if the battery is bad or in
deep-discharge.
The LTC3455 is also a good choice for systems that are
always powered by a USB supply or wall adapter. The
charger can then be used to charge a large capacitor or
backup battery, which can briefly provide power to the
system after the external power has been removed. This
gives the microcontroller enough time to follow proper
shutdown procedures even after the main power source is
abruptly removed. If USB powered, the large capacitor or
backup battery will also be used to provide additional
current if the system briefly needs more power than the
USB bus can provide.
Concerns When Wall Adapter Powered
Always choose a wall adapter that can provide power for
all load and battery charging requirements. Choosing a
wall adapter with a power rating that is too small will result
in very long charge times and erratic system operation. If
the total current needed (load and battery charging) ex-
ceeds what the adapter can provide, the voltage on the
V_MAX pin will begin to droop. If it droops close enough to
the battery voltage (the V_BAT pin), the charge current
decreases and eventually reduces to zero. If the load
current is still too much for the wall adapter to provide, the
wall adapter will provide what it can and the battery will
provide the rest. When wall powered, this operation is
intended only for surviving fault conditions and should not
be a normal mode of operation.
Concerns When USB Powered
The popularity of USB (Universal Serial Bus) makes it an
attractive choice for transferring data in a variety of por-
table devices. Therefore, utilizing the USB port to power
these portable devices while charging their battery is very
desirable, but it is not necessarily an easy task. As the
performance of digital cameras, handheld computers, and
MP3 players increases, the power needed to operate them
also increases. The power available from a single USB port
(maximum 2.5W) is barely enough to support the peak
power needed by many full-featured portable devices,
even without the power needed to quickly charge their
batteries.
To further complicate matters, a USB port is not the ideal
power source. Each device can draw a maximum of
500mA (in high power mode), but the voltage provided to
the portable device can vary quite significantly. Although
a USB power supply has a 5V nominal rating, when you
include normal supply variations, cable losses, and tran-
sient conditions, the USB voltage showing up at the
portable device is typically much lower—often falling to
only 4V. Since the USB port has a strict current limit of
500mA, this means the amount of power available to the
portable device can be as low as 2W. The reduced USB
voltage also presents problems when trying to fully charge
a single-cell Lithium-Ion battery (that has a 4.2V final
charge voltage) when the USB voltage may itself be below
or near 4.2V.
The LTC3455 is specifically designed to alleviate these
problems and make the most of the power the USB port
has to offer. See the sections entitled ”Large Transient
Loads when USB powered” and ”Special Charger Features
when USB powered” for more detailed discussions of the
LTC3455’s special USB features.
USB High Power/Low Power/Suspend Modes
There are three basic modes for the USB power manager:
high power, low power, and suspend. High power mode
allows the LTC3455 to draw up to 500mA from the USB
port, and is selected by pulling the USBHP pin high. Low
power mode reduces the allowable current drawn to
100mA, and is selected by pulling the USBHP pin low. The
USBHP pin has a weak internal pulldown current source to
ensure that the LTC3455 always starts up in USB low
power mode. The SUSPEND pin will disable the USB
power manager completely, reducing the USB pin current
to under 20µA.
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