LTC3550-1_15 Datasheet, PDF (18/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550-1
APPLICATIO S I FOR ATIO
Thermal Considerations
The battery charger’s thermal regulation feature and the
buck regulator’s high efficiency make it unlikely that enough
power is dissipated to exceed the LTC3550-1 maximum
junction temperature. Nevertheless, it is a good idea to
do some thermal analysis for worst-case conditions.
The junction temperature, T_J, is given by: T_J = T_A + T_RISE
where T_A is the ambient temperature. The temperature
rise is given by:
T_RISE = P_D • θ_JA
where P_D is the power dissipated and θ_JA is the thermal
resistance from the junction of the die to the ambient
temperature.
In most applications the buck regulator does not dissipate
much heat due to its high efficiency. The majority of the
LTC3550-1 power dissipation occurs when charging a
battery. Fortunately, the LTC3550-1 automatically reduces
the charge current during high power conditions using
a patented thermal regulation circuit. Thus, there is no
need to design for worst-case power dissipation scenarios
because the LTC3550-1 ensures that the battery charger
power dissipation never raises the junction temperature
above a preset value of 105°C. In the unlikely case that
the junction temperature is forced above 105°C (due to
abnormally high ambient temperatures or excessive buck
regulator power dissipation), the battery charge current will
be reduced to zero and thus dissipate no heat. As an added
measure of protection, even if the junction temperature
reaches approximately 150°C, the buck regulator’s power
switches will be turned off and the SW node will become
high impedance.
The conditions that cause the LTC3550-1 to reduce charge
current through thermal feedback can be approximated by
considering the power dissipated in the IC. The approxi-
mate ambient temperature at which the thermal feedback
begins to protect the IC is:
T_A = 105°C – T_RISE
T_A = 105°C – (P_D • θ_JA)
T_A = 105°C – (P_D(CHARGER) + P_D(BUCK)) • θ_JA
(4)
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Most of the charger’s power dissipation is generated from
the internal charger MOSFET. Thus, the power dissipation
is calculated to be:
P_D(CHARGER) = (V_IN – V_BAT) • I_BAT
(5)
V_IN is the charger supply voltage (either DCIN or USBIN),
V_BAT is the battery voltage and I_BAT is the charge cur-
rent.
Example: An LTC3550-1 operating from a 5V wall adapter
(on the DCIN input) is programmed to supply 650mA
full-scale current to a discharged Li-Ion battery with a
voltage of 2.7V.
The charger power dissipation is calculated to be:
P_D(CHARGER) = (5V – 2.7V) • 650mA = 1.495W
For simplicity, assume the buck regulator is disabled and
dissipates no power (P_D(BUCK) = 0). For a properly soldered
DHC16 package, the thermal resistance (θ_JA) is 40°C/W.
Thus, the ambient temperature at which the LTC3550-1
charger will begin to reduce the charge current is:
T_A = 105°C – 1.495W • 40°C/W
T_A = 105°C – 59.8°C
T_A = 45.2°C
The LTC3550-1 can be used above 45.2°C ambient, but
the charge current will be reduced from 650mA. Assum-
ing no power dissipation from the buck converter, the
approximate current at a given ambient temperature can
be approximated by:
I_BAT
=
105°C – T_A
(V_IN – V_BAT) • θ_JA
(6)
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to approxi-
mately:
I_BAT
=
105°C – 60°C
(5V – 2.7V) • 40°C/W
=
45°C
92°C/A
I_BAT = 489mA
Because the regulator typically dissipates significantly less
heat than the charger (even in worst-case situations), the
calculations here should work well as an approximation.
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