LTC4059_15 Datasheet, PDF (9/12 Pages) Linear Technology – 900mA Linear Li-Ion Battery Chargers with Thermal Regulation in 2 2 DFN

LTC4059/LTC4059A
APPLICATIO S I FOR ATIO
Power Dissipation
The conditions that cause the LTC4059/LTC4059A to
reduce charge current through thermal feedback can be
approximated by considering the power dissipated in the
IC. For high charge currents, the LTC4059 power dissipa-
tion is approximately:
P_D = (V_CC – V_BAT) • I_BAT
where P_D is the power dissipated, V_CC is the input supply
voltage, V_BAT is the battery voltage and I_BAT is the charge
current. It is not necessary to perform any worst-case
power dissipation scenarios because the LTC4059/
LTC4059A will automatically reduce the charge current to
maintain the die temperature at approximately 115°C.
However, the approximate ambient temperature at which
the thermal feedback begins to protect the IC is:
T_A = 115°C – P_Dθ_JA
T_A = 115°C – (V_CC – V_BAT) • I_BAT • θ_JA
Example: Consider an LTC4059 operating from a 5V wall
adapter providing 900mA to a 3.7V Li-Ion battery. The
ambient temperature above which the LTC4059/LTC4059A
begin to reduce the 900mA charge current is approximately:
T_A = 115°C – (5V – 3.7V) • (900mA) • 50°C/W
T_A = 115°C – 1.17W • 50°C/W = 115°C – 59°C
T_A = 56°C
The LTC4059 can be used above 56°C, but the charge
current will be reduced from 900mA. The approximate
current at a given ambient temperature can be calculated:
( ) ^{IBAT =}
115°C – T_A
V_CC – V_BAT • θ_JA
Using the previous example with an ambient temperature
of 65°C, the charge current will be reduced to approximately:
I_BAT
=
115°C –
(^{5V – 3.7V})
65°C
• 50°C/W
=
50°C
65°C/A
I_BAT = 770mA
Furthermore, the voltage at the PROG pin will change
proportionally with the charge current as discussed in the
Programming Charge Current section.
It is important to remember that LTC4059/LTC4059A
applications do not need to be designed for worst-case
thermal conditions since the IC will automatically reduce
power dissipation when the junction temperature reaches
approximately 115°C.
Board Layout Considerations
In order to be able to deliver maximum charge current
under all conditions, it is critical that the exposed metal
pad on the backside of the LTC4059/LTC4059A package is
soldered to the PC board ground. Correctly soldered to a
2500mm² double sided 1oz copper board the LTC4059/
LTC4059A have a thermal resistance of approximately
60°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 60°C/W. As an example, a correctly soldered LTC4059/
LTC4059A can deliver over 900mA to a battery from a 5V
supply at room temperature. Without a backside thermal
connection, this number could drop to less than 500mA.
Stability Considerations
The LTC4059 contains two control loops: constant voltage
and constant current. The constant-voltage loop is stable
without any compensation when a battery is connected
with low impedance leads. Excessive lead length, how-
ever, may add enough series inductance to require a
bypass capacitor of at least 1µF from BAT to GND. Further-
more, a 4.7µF capacitor with a 0.2Ω to 1Ω series resistor
from BAT to GND is required to keep ripple voltage low
when the battery is disconnected.
High value capacitors with very low ESR (especially ce-
ramic) reduce the constant-voltage loop phase margin.
Ceramic capacitors up to 22µF may be used in parallel with
a battery, but larger ceramics should be decoupled with
0.2Ω to 1Ω of series resistance.
In constant-current mode, the PROG pin is in the feedback
loop, not the battery. Because of the additional pole
created by PROG pin capacitance, capacitance on this pin
must be kept to a minimum. With no additional capaci-
tance on the PROG pin, the charger is stable with program
resistor values as high as 12k. However, additional ca-
pacitance on this node reduces the maximum allowed
4059fb
9