LTC3552 Datasheet, PDF (18/24 Pages) Linear Technology – Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter

LTC3552
APPLICATIO S I FOR ATIO
Thermal Considerations
The battery charger’s thermal regulation feature and the
switching regulator’s high efficiency make it unlikely that
the LTC3552 will dissipate enough power to exceed its
maximum junction temperature. However, in applications
where the LTC3552 is running at high ambient temperature
with low supply voltage and high duty cycles, the power
dissipated may result in excessive junction temperatures.
To prevent the LTC3552 from exceeding the maximum
junction temperature, the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated will raise the
junction temperature above the maximum rating. The
temperature rise is given by:
T_RISE = P_D • θ_JA
where P_D is the power dissipated and θ_JA is the ther-
mal resistance from the junction of the die to the
ambient temperature. The junction temperature, T_J, is
given by:
T_J = T_RISE + T_AMBIENT
As an example, consider the case when the battery char-
ger is idle, and both regulators are operating at an input
voltage of 2.7V with a load current of 400mA and 800mA
and an ambient temperature of 70°C. From the Typical
Performance Characteristics graph of Switch Resistance,
the R_DS(ON) resistance of the main switch is 0.425Ω.
Therefore, power dissipated by each regulator is:
P_D = I² • R_DS(ON) = 272mW and 68mW
The DHC16 package junction-to-ambient thermal resis-
tance, θ_JA, is 40°C/W. Therefore, the junction temperature
of the regulator operating in a 70°C ambient temperature
is approximately:
T_J = (0.272 + 0.068) • 40 + 70 = 83.6°C
which is below the absolute maximum junction tempera-
ture of 125°C.
The majority of the LTC3552 power dissipation comes from
the battery charger. Fortunately, the LTC3552 automatically
reduces the charge current during high power conditions
using a patented thermal regulation circuit. Thus, it is
not necessary to design for worst-case power dissipa-
tion scenarios. The conditions that cause the LTC3552 to
18
reduce charge current through thermal feedback can be
approximated by considering the power dissipated in the
IC. The approximate ambient temperature at which the
thermal feedback begins to protect the IC is:
T_A = 120°C – P_Dθ_JA
T_A = 120°C – (P_D(CHARGER) + P_D(REGULATOR)) • θ_JA
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
V_IN is the charger supply voltage, V_BAT is the battery volt-
age and I_BAT is the charge current.
Example: An LTC3552 operating from a 5V supply is
programmed to supply 800mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.3V. For sim-
plicity, assume the regulators are disabled and dissipate
no power.
The charger power dissipation is calculated to be:
P_D(CHARGER) = (5V – 3.3V) • 800mA = 1.36W
Thus, the ambient temperature at which the LTC3552
charger begins to reduce the charge current is approxi-
mately:
T_A = 120°C – 1.36W • 40°C/W
T_A = 120°C – 54.4°C
T_A = 65.6°C
The LTC3552 can be used above 65°C ambient but the
charge current will be reduced from the programmed
800mA. The approximate current at a given ambient
temperature can be approximated by:
I_BAT
=
120°C – T_A
(V_IN – V_BAT) • θ_JA
Using the previous example with an ambient temperature
of 70°C (and no heat dissipation from the regulator), the
charge current will be reduced to approximately:
I_BAT
=
120°C – 70°C
(5V – 3.3V) • 40°C/W
=
50°C
68°C/A
I_BAT = 735mA
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