LTC4055 Linear Technology, LTC4055 Datasheet - Page 20

• Manufacturer Part Number: LTC4055
• Description: USB Power Controller and Li-Ion Linear Charger
• Manufacturer: Linear Technology

OPERATIO
The LTC4055 can be used above 46 C, but the charge
current will be reduced below 0.8A. The approximate
charge current at a given ambient temperature can be
approximated by:
Consider the above example with an ambient temperature
of 55 C. The charge current will be reduced to approxi-
mately:
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 LTC4055 package is soldered to the
board. Correctly soldered to a 2500mm
1oz. copper board, the LTC4055 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
LTC4055
20
I
I
BAT
BAT
5
V
OUT
V
105
105
–
3
–
U
V
C
V
C T
BAT
–
•
–
37
55
A
•
C W
C
/
JA
Figure 4. LTC4055 vs Schottky Diode Forward Voltage Drop
I
I
MAX
FWD
74
50
0
V
FWD
C A
C
/
SLOPE: 1/R
2
double-sided
0 675
SLOPE: 1/R
LTC4055
.
FWD
FORWARD VOLTAGE (V)
A
DIO,ON
SCHOTTKY
DIODE
LTC4055 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.
STABILITY
The constant-voltage mode feedback loop is stable with-
out any compensation when a battery is connected. How-
ever, a 1 F capacitor with a 1 series resistor to GND is
recommended at the BAT pin to keep ripple voltage low
when the battery is disconnected.
Ideal Diode from BAT to OUT
Forward regulation for the LTC4055 from BAT to OUT has
three operational ranges, depending on the magnitude of
the load current. For small load currents, the LTC4055 will
provide a constant voltage drop; this operating mode is
referred to as “constant V
exceeds I
the current with a slope of 1/R
is referred to as “constant R
increases further, exceeding I
drop will increase rapidly; this operating mode is referred
to as “constant I
following parameters: R
specified with the aid of Figure 4.
CONSTANT
I
CONSTANT
R
CONSTANT
V
FWD
ON
ON
ON
, the voltage drop will increase linearly with
ON
” regulation. The characteristics for the
4055 F04
FWD
ON
ON
, R
” regulation. As the current
DIO,ON
” regulation. As the current
MAX
ON
, V
, the forward voltage
; this operating mode
FWD
, and I
FWD
4055p
are