Datasheet

LTC3455/LTC3455-1
13
3455fc
APPLICATIONS INFORMATION
Handling Large Transient Loads when USB Powered
Many portable devices have nominal loads that can easily
be supported by the USB supply, but they have brief tran-
sient loads that can exceed the maximum available USB
power. The LTC3455/LTC3455-1 are designed to handle
these overloads while drawing as much power as possible
from the USB port. If the USB bus is providing power but
the LTC3455/LTC3455-1 (or any other devices connected
to the V
MAX
pin) need more total power than the USB bus
can supply, the battery charger turns off completely and
the USB power controller becomes a 500mA (or 100mA)
current source and the V
MAX
voltage begins to decrease.
At this point, the capacitance connected to the V
MAX
pin
provides the additional current needed by the system. As
long as the USB pin stays above 3.9V, the USB bus will
continue to provide as much current as possible. Once the
V
MAX
pin drops just below the V
BAT
voltage, the battery
will provide the additional current needed. This operation
allows the LTC3455/LTC3455-1 to withstand load transients
that briefl y demand more power than can be provided by
the USB bus.
The oscilloscope photographs in Figure 1 show how the
LTC3455/LTC3455-1 handle load transients when USB
powered. The top photo shows a brief transient load that
turns off the charger but does not dip the V
MAX
voltage.
The bottom photo shows a prolonged transient condition
that turns off the charger and completely dips the V
MAX
voltage to the point where the battery must provide cur-
rent. For both cases, normal operation resumes as soon
as the transient passes.
Extra capacitance can be connected to the V
MAX
pin to
act as a reservoir to help support large transient cur-
rents. For most systems this is not necessary, as the
LTC3455/LTC3455-1 cleanly handle heavy transients. For
some designs, however, it may be desirable to use a larger
capacitor connected to V
MAX
to act as a larger reservoir.
Up to 50μF of ceramic capacitance may be connected to
the V
MAX
pin without diffi culty. More than 50μF requires
using a capacitor with some ESR or adding some resistance
in series with some of the ceramic capacitance. This is
necessary to ensure loop stability in the battery charger
loop when under USB power.
USB Maximum Current Condition
USB Heavy Over-Current Condition
Figure 1. Handling Load Transients when USB Powered
100μs/DIV
V
MAX
2V/DIV
I
MAX
500mA/DIV
I
USB
500mA/DIV
I
BAT
500mA/DIV
3455 F01a
100μs/DIV
V
MAX
2V/DIV
I
MAX
500mA/DIV
I
USB
500mA/DIV
I
BAT
500mA/DIV
3455 F01b