Datasheet

LTC4085
21
4085fd
Consider the above example with an ambient temperature of
55°C. The charge current will be reduced to approximately:
I
BAT
=
105°C–55°C
5V – 3V
()
•37°C/W
=
50°C
74°C/A
= 0.675A
Board Layout Considerations
In order to be able to deliver maximum charge current
under all conditions, it is critical that the Exposed Pad on
the backside of the LTC4085 package is soldered to the
board. Correctly soldered to a 2500mm
2
double-sided
1oz. copper board the LTC4085 has a thermal resistance
of approximately 37°C/W. Failure to make thermal contact
between the Exposed Pad on the backside of the package
and the copper board will result in thermal resistances far
greater than 37°C/W. As an example, a correctly soldered
LTC4085 can deliver over 1A to a battery from a 5V supply
at room temperature. Without a backside thermal connec-
tion, this number could drop to less than 500mA.
V
IN
and Wall Adapter Bypass Capacitor
Many types of capacitors can be used for input bypassing.
However, caution must be exercised when using multilayer
ceramic capacitors. Because of the self resonant and high
Q characteristics of some types of ceramic capacitors, high
voltage transients can be generated under some start-up
conditions, such as connecting the charger input to a hot
power source. For more information, refer to Application
Note 88.
Stability
The constant-voltage mode feedback loop is stable without
any compensation when a battery is connected. However,
a 4.7μF capacitor with a 1Ω series resistor to GND is
recommended at the BAT pin to keep ripple voltage low
when the battery is disconnected.
APPLICATIONS INFORMATION
USB Power Control Application with Wall Adapter Input
IN
SUSP
HPWR
SUSPEND USB POWER
500mA/100mA SELECT
OUT
PROG
LTC4085
CLPROG
GND
BAT
GATE
V
NTC
NTC
Li-Ion
CELL
TO LDOs
REGs, ETC
CHRG
ACPR
WALL
TIMER
0.15μF
4085 TA02
R
NTCBIAS
10k
R
NTC
10k
510Ω
R
PROG
71.5k
*SERIES 1W RESISTOR ONLY
NEEDED FOR INDUCTIVE
INPUT SUPPLIES
R
CLPROG
2.1k
+
4.7μF
4.7μF
510Ω1k
5V WALL
ADAPTER
INPUT
5V (NOM)
FROM USB
CABLE V
BUS
1Ω*
4.7μF
1Ω*
TYPICAL APPLICATION