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LTC4066EPF View Datasheet(PDF) - Linear Technology

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LTC4066/LTC4066-1
APPLICATIONS INFORMATION
Consider an example where the VTH(Adapter) is to be set
somewhere around 4.5V. Resistance on the WALL pin
should be kept relatively low (~10k) in order to prevent
false tripping of the wall comparator due to leakages as-
sociated with the switching element used to connect the
adapter to OUT. Pick R2 to be 10k and solve for R1:
R1
=
R2
⎝⎜
VTH(Adapter)
VWALL
1⎞⎠⎟
R1 =
10k
⎛⎝⎜
4.5V
1.225V
– 1⎞⎠⎟
=
10k
2.67
=
26.7k
The nearest 1% resistor is 26.7k. Therefore, R1 = 26.7k
and the rising trip point should be 4.50V.
VHYST(Adapter)
30mV
⎛⎝⎜1+
26.7k
10k
⎞⎠⎟
110.1mV
The hysteresis is going to be approximately 110mV for
this example.
Power Dissipation
The conditions that cause the LTC4066/LTC4066-1 to
reduce charge current due to the thermal protection
feedback can be approximated by considering the power
dissipated in the part. For high charge currents and a wall
adapter applied to VOUT, the LTC4066/LTC4066-1 power
dissipation is approximately:
PD = (VOUT – VBAT) • IBAT
where PD is the power dissipated, VOUT is the supply
voltage, VBAT is the battery voltage and IBAT is the battery
charge current. It is not necessary to perform any worst-
case power dissipation scenarios because the LTC4066/
LTC4066-1 will automatically reduce the charge current
to maintain the die temperature at approximately 105°C.
However, the approximate ambient temperature at which
the thermal feedback begins to protect the IC is:
TA = 105°C – PD θJA
TA = 105°C – (VOUT – VBAT) • IBAT θJA
Example: Consider an LTC4066/LTC4066-1 operating from
a wall adapter with 5V at VOUT providing 0.8A to a 3V
Li-Ion battery. The ambient temperature above which the
LTC4066/LTC4066-1 will begin to reduce the 0.8A charge
current, is approximately:
TA = 105°C – (5V – 3V) • 0.8A • 37°C/W
TA = 105°C – 1.6W • 37°C/W = 105°C – 59°C = 46°C
The LTC4066/LTC4066-1 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 – TA
VOUT – VBAT θJA
Consider the above example with an ambient tem-
perature of 55°C. The charge current will be reduced to
approximately:
IBAT
=
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 LTC4066/LTC4066-1 package is
soldered to the board. Correctly soldered to a 2500mm2
double-sided 1oz. copper board, the LTC4066/LTC4066-1
has a thermal resistance of approximately 37°C/W. Failure
4066fc
24
 

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