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LTC3552

3552f

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:

TRISE = PD • θJA

where PD is the power dissipated and θJA is the ther-

mal resistance from the junction of the die to the

ambient temperature. The junction temperature, TJ, is

given by:

TJ = TRISE + TAMBIENT

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 RDS(ON) resistance of the main switch is 0.425Ω.

Therefore, power dissipated by each regulator is:

PD = I

2 • R

DS(ON) = 272mW and 68mW

The DHC16 package junction-to-ambient thermal resis-

tance, θJA, is 40°C/W. Therefore, the junction temperature

of the regulator operating in a 70°C ambient temperature

is approximately:

TJ = (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

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:

TA = 120°C – PDθJA

TA = 120°C – (PD(CHARGER) + PD(REGULATOR)) • θJA

Most of the charger’s power dissipation is generated from

the internal charger MOSFET. Thus, the power dissipation

is calculated to be:

PD(CHARGER) = (VIN – VBAT) • IBAT

VIN is the charger supply voltage, VBAT is the battery volt-

age and IBAT 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:

PD(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:

TA = 120°C – 1.36W • 40°C/W

TA = 120°C – 54.4°C

TA = 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:

BAT

120

–

(–

•

)

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:

C W

BAT =

°°

120

53 3

–

(– .

) •

BAT

= 735

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