ltc3455euf-trpbf Linear Technology Corporation, ltc3455euf-trpbf Datasheet - Page 12

• Manufacturer Part Number: ltc3455euf-trpbf
• Description: Dual Dc/dc Converter With Usb Power Manager And Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3455
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.
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 permis-
sion 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 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.
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
regulators are loaded, especially during startup. Keep in
12
MAX
pin and the output of the two switching
U
U
W
U
mind that when 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. 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
regulator outputs are loaded too heavily, the LTC3455 will
be unable to regulate the outputs (due to insufficient input
power), and an internal protection circuit will turn off the
part after 200ms. This protection feature is discussed in
detail in the section entitled “Low or Bad Battery Protec-
tion (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 must start
up in USB low power mode with no battery present.
1. Minimize the load currents on the V
2. Minimize the load current on the output of Switcher 1
3. Delay the turn-on of Switcher 2 until after USB high
For some applications, USB high power mode should be
selected during startup (at least briefly) to allow the
LTC3455 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.
the turn on of all devices that are powered from V
until after USB high power mode is available.
since Switcher 1 turns on automatically whenever USB
power is available.
power mode is available.
MAX
pin or the switching
MAX
pin by delaying
3455fb
MAX