LTC4055-1_15 Datasheet, PDF (20/24 Pages) Linear Technology – USB Power Controller and Li-Ion Charger

LTC4055/LTC4055-1
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:
( ) ^{IBAT =}
105°C – T_A
V_OUT – V_BAT • θ_JA
Consider the above example with an ambient temperature
of 55°C. The charge current will be reduced to approxi-
mately:
( ) ^{IBAT =}
105°C – 55°C ₌ 50°C _{= 0.675A}
5V – 3V • 37°C/W 74°C/A
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 2500mm²
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
specified with the aid of Figure 4.
I_MAX
LTC4055
SLOPE: 1/R_DIO,ON
CONSTANT
I_ON
CONSTANT
R_ON
I_FWD
SLOPE: 1/R_FWD
SCHOTTKY
DIODE
CONSTANT
V_ON
0
V_FWD
FORWARD VOLTAGE (V)
4055 F04
Figure 4. LTC4055/LTC4055-1 vs Schottky Diode Forward Voltage Drop
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