AN2370

Power dissipation

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5

Power dissipation

The conditions that cause the STC4054 to reduce charge current through thermal feedback

can be approximated by considering the power dissipated in the IC. For high charge

currents, the STC4054 power dissipation is approximately:

Equation 2

power dissipation scenarios because the STC4054 will automatically reduce the charge

current to maintain the die temperature at approximately 120°C.

However, the approximate ambient temperature at which the thermal feedback begins to

protect the IC is:

Equation 3

Equation 4

Example: Consider an STC4054 operating from a 5 V wall adapter providing 400 mA to a

3.7 V Li-Ion battery. The ambient temperature above which the STC4054 will begin to

reduce the 400 mA charge current is approximately:

Equation 5

The STC4054 can be used above 42°C, but the charge current will be reduced from

400 mA. The approximate current at a given ambient temperature can be calculated:

Equation 6

Using the previous example with an ambient temperature of 65°C, the charge current will be

reduced to approximately:

Equation 7

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

STC4054 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 120 °C.

P

D

V

CC

V

BAT

–

() I

BAT

•

=

T

A

120

° CP

–

=

T

A

120

° CV

CC

V

BAT

–

() I

BAT

•θ

JA

–

=

T

A

120

° C5 V 3.7 V

–

()

400mA

()

•

150

°

C

W

-----

•

42

° C

=

–

=

I

BAT

120

° CT

A

–

V

CC

V

BAT

–

() θ

A

•

-------------------------------------------------

=

I

BAT

120

° C65° C

–

5V 3.7V

–

() 150°

C

W

-----

•

---------------------------------------------------------

282mA

=

=