LTC3455EUF#TRPBF Linear Technology, LTC3455EUF#TRPBF Datasheet - Page 13

• Manufacturer Part Number: LTC3455EUF#TRPBF
• Description: IC CONV DC/DC DUAL USBBATT 24QFN
• Manufacturer: Linear Technology

Specifications of LTC3455EUF#TRPBF

• Applications: Handheld/Mobile Devices
• Voltage - Supply: 3 V ~ 6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Supply: -

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
can supply, the battery charger turns off completely and
the USB power controller becomes a 500mA (or 100mA)
current source and the V
At this point, the capacitance connected to the V
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
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.
MAX
pin drops just below the V
MAX
pin) need more total power than the USB bus
MAX
voltage begins to decrease.
BAT
Figure 1. Handling Load Transients when USB Powered
500mA/DIV
500mA/DIV
500mA/DIV
500mA/DIV
500mA/DIV
500mA/DIV
voltage, the battery
2V/DIV
2V/DIV
V
V
I
I
I
I
MAX
MAX
I
MAX
MAX
I
USB
USB
BAT
BAT
USB Heavy Over-Current Condition
USB Maximum Current Condition
MAX
pin
100μs/DIV
100μs/DIV
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
The bottom photo shows a prolonged transient condition
that turns off the charger and completely dips the V
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
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
Up to 50μF of ceramic capacitance may be connected to
the V
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.
MAX
pin without diffi culty. More than 50μF requires
3455 F01b
3455 F01a
LTC3455/LTC3455-1
MAX
to act as a larger reservoir.
MAX
MAX
voltage.
13
pin to
3455fc
MAX