Datasheet

LTC4055/LTC4055-1
20
4055fb
OPERATION
The LTC4055/LTC4055-1 can be used above 46°C, but the
charge current will be reduced below 0.8A. The charge
current at a given ambient temperature can be approxi-
mated by:
I
CT
VV
BAT
A
OUT BAT JA
=
°
()
105 –
–•θ
Consider the above example with an ambient temperature
of 55°C. The charge current will be reduced to approxi-
mately:
I
CC
VV CW
C
CA
BAT
=
°°
()
°
=
°
°
=
105 55
53 37
50
74
0
–
–• /
/
.6675A
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/LTC4055-1 package is
soldered to the board. Correctly soldered to a 2500mm
2
double-sided 1oz. copper board, the LTC4055/LTC4055-1
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/LTC4055-1 can
deliver over 1A to a battery from a 5V supply at room
temperature. Without a backside thermal connection, this
number could drop to less than 500mA.
STABILITY
The constant-voltage mode feedback loop is stable without
any compensation when a battery is connected. However,
a 1μF capacitor with a 1Ω series resistor to GND is recom-
mended 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/LTC4055-1 from BAT
to OUT has three operational ranges, depending on the
magnitude of the load current. For small load currents,
the LTC4055/LTC4055-1 will provide a constant-voltage
drop; this operating mode is referred to as “constant
V
ON
” regulation. As the current exceeds I
FWD
, the voltage
drop will increase linearly with the current with a slope
of 1/R
DIO,ON
; this operating mode is referred to as
“constant R
ON
” regulation. As the current increases
further, exceeding I
MAX
, the forward voltage drop will
increase rapidly; this operating mode is referred to as
“constant I
ON
” regulation. The characteristics for the
following parameters: R
FWD
, R
ON
, V
FWD
, and I
FWD
are
specifi ed with the aid of Figure 4.
CONSTANT
I
ON
CONSTANT
R
ON
FORWARD VOLTAGE (V)
CURRENT (A)
CONSTANT
V
ON
SCHOTTKY
DIODE
LTC4055
SLOPE: 1/R
FWD
SLOPE: 1/R
DIO,ON
I
MAX
I
FWD
0
V
FWD
4055 F04
Figure 4. LTC4055/LTC4055-1 vs Schottky Diode Forward Voltage Drop