LTC3455_15 Datasheet, PDF (12/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455/LTC3455-1
APPLICATIONS INFORMATION
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/LTC3455-1 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/LTC3455-1 always start up in
USB low power mode. The SUSPEND pin will disable the
USB power manager completely, reducing the USB pin
current to under 20μA.
Operation in USB Low Power Mode
Most applications that draw power from the USB bus
should be in low power mode only for a brief amount
of time. Devices should be in low power mode (draw no
more than 100mA of current from the USB bus) upon
power-up, and can transition to high power mode (draw
up to 500mA from the USB bus) after the device has been
given permission to do so by the USB host controller. The
change to high power mode is usually very quick, so the
full 500mA of current is available shortly after connecting
to the USB bus. While the LTC3455/LTC3455-1 will operate
when in low power mode, the amount of power available
is so small that it is difficult or impossible to charge a
battery or even provide enough current to power the rest
of the system. For this reason, USB high power operation
should always be used with the LTC3455/LTC3455-1.
Startup Issues in USB Low Power Mode When No
Battery Is Present
For applications that must operate in USB low power
mode when no battery is present, careful attention must
be given to how the V_MAX pin and the output of the two
switching regulators are loaded, especially during startup.
Keep in mind that when the LTC3455/LTC3455-1 are 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. Since the USB
can only provide 100mA maximum current in low power
mode, this gives, at best, only 500mW (5V • 100mA) of
power available to get everything up and running. With
a low USB voltage and a low USB current limit, less than
300mW may be available to start up the device. For some
applications (if the outputs are too heavily loaded), this
is simply not enough power to start up the system. If the
V_MAX pin or the switching regulator outputs are loaded too
heavily, the LTC3455/LTC3455-1 will be unable to regulate
the outputs (due to insufficient input power), and an in-
ternal protection circuit will turn off the part after 200ms.
This protection feature is discussed in detail in the section
entitled “Low or Bad Battery Protection (200ms Timeout)”.
Once this protection circuit is tripped, USB power must
be removed and reapplied to restart the part.
Several steps can be taken to help lighten the total system
load which will help greatly when the LTC3455/LTC3455-1
must start up in USB low power mode with no battery
present.
1. Minimize the load currents on the V_MAX pin by delaying
the turn on of all devices that are powered from V_MAX
until after USB high power mode is available.
2. Minimize the load current on the output of Switcher 1
since Switcher 1 turns on automatically whenever USB
power is available.
3. Delay the turn-on of Switcher 2 until after USB high
power mode is available.
For some applications, USB high power mode should
be selected during startup (at least briefly) to allow the
LTC3455/LTC3455-1 to turn on properly. Startup in USB high
power mode is typically not a problem, as more than 2W
of power is available from the USB port in this mode.
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