LTC3552-1 Linear Technology, LTC3552-1 Datasheet - Page 16

• Manufacturer Part Number: LTC3552-1
• Description: Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter
• Manufacturer: Linear Technology

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LTC3552-1
APPLICATIO S I FOR ATIO
Thermal Considerations
The battery charger’s thermal regulation feature and the
switching regulator’s high effi ciency make it unlikely that
the LTC3552-1 will dissipate enough power to exceed
its maximum junction temperature. However, in applica-
tions where the LTC3552-1 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 LTC3552-1 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:
where P
mal resistance from the junction of the die to the
ambient temperature. The junction temperature, T
given by:
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
Therefore, power dissipated by each regulator is:
The DHC16 package junction-to-ambient thermal resis-
tance, θ
of the regulator operating in a 70°C ambient temperature
is approximately:
which is below the absolute maximum junction tempera-
ture of 125°C.
The majority of the LTC3552-1 power dissipation comes
from the battery charger. Fortunately, the LTC3552-1 au-
16
charge storage and very low ESR at the switching fre-
quency. Other losses include diode conduction losses
during dead-time and inductor core losses generally
account for less than 2% total additional loss.
T
T
P
T
RISE
J
J
D
= T
= (0.272 + 0.068) • 40 + 70 = 83.6°C
DS(ON)
= I
JA
= P
D
2
RISE
, is 40°C/W. Therefore, the junction temperature
• R
is the power dissipated and θ
D
resistance of the main switch is 0.425Ω.
+ T
DS(ON)
• θ
AMBIENT
JA
U
= 272mW and 68mW
U
W
JA
is the ther-
U
J
, is
tomatically 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 LTC3552-1
to 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:
Most of the charger’s power dissipation is generated from
the internal charger MOSFET. Thus, the power dissipation
is calculated to be:
V
age and I
Example: An LTC3552-1 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
simplicity, assume the regulators are disabled and dis-
sipate no power.
The charger power dissipation is calculated to be:
Thus, the ambient temperature at which the LTC3552-1
charger begins to reduce the charge current is approxi-
mately:
The LTC3552-1 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:
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:
IN
T
T
P
P
T
T
T
I
BAT
A
A
is the charger supply voltage, V
A
A
A
D(CHARGER)
D(CHARGER)
= 120°C – P
= 120°C – (P
= 120°C – 1.36W • 40°C/W
= 120°C – 54.4°C
= 65.6°C
=
BAT
(
V
IN
is the charge current.
120
–
= (V
= (5V – 3.3V) • 800mA = 1.36W
V
D
°
BAT
D(CHARGER)
θ
C T
IN
JA
–
)
– V
•
A
θ
BAT
JA
) • I
+ P
BAT
D(REGULATOR)
BAT
is the battery volt-
) • θ
JA
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