ltc3455euf-trpbf Linear Technology Corporation, ltc3455euf-trpbf Datasheet - Page 13

• 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
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
transient loads that can exceed the maximum available
USB power. The LTC3455 is 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 (or any other devices connected to the V
needs 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
capacitance connected to the V
tional 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
below the V
tional current needed. This operation allows the LTC3455
to withstand load transients that briefly demand more
power than can be provided by the USB bus.
MAX
BAT
voltage begins to decrease. At this point, the
voltage, the battery will provide the addi-
U
U
MAX
Figure 1. Handling Load Transients when USB Powered
pin provides the addi-
W
500mA/DIV
500mA/DIV
500mA/DIV
500mA/DIV
500mA/DIV
500mA/DIV
MAX
2V/DIV
2V/DIV
V
V
I
I
I
I
MAX
MAX
I
USB
I
BAT
MAX
MAX
USB
BAT
pin drops just
USB Heavy Over-Current Condition
USB Maximum Current Condition
U
MAX
pin)
100µs/DIV
100µs/DIV
The oscilloscope photographs in Figure 1 show how the
LTC3455 handles load transients when USB powered. The
top photo shows a brief transient load that turns off the
charger but does not dip the V
photo shows a prolonged transient condition that turns off
the charger and completely dips the V
point where the battery must provide current. For both
cases, normal operation resumes as soon as the transient
passes.
Extra capacitance can be connected to the V
as a reservoir to help support large transient currents. For
most systems this is not necessary, as the LTC3455
cleanly handles heavy transients. For some designs, how-
ever, it may be desirable to use a larger capacitor con-
nected to V
ceramic capacitance may be connected to the V
without difficulty. More than 50µF requires using a capaci-
tor 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.
MAX
to act as a larger reservoir. Up to 50µF of
3455 F01a
3455 F01b
MAX
voltage. The bottom
MAX
LTC3455
voltage to the
MAX
pin to act
MAX
13
3455fb
pin