LTC4069 datasheet(13/16 Pages) LINER

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LTC4069

4069fb

APPLICATIONS INFORMATION

battery current as shown in Figure 5. A 10k resistor has

been added between the PROG pin and the ﬁlter capacitor

to ensure stability.

Power Dissipation

The conditions that cause the LTC4069 to reduce charge

current through thermal feedback can be approximated

by considering the power dissipated in the IC. For high

charge currents, the LTC4069 power dissipation is

approximately:

PD = (VCC – VBAT) • IBAT

where PD is the power dissipated, VCC is the input supply

voltage, VBAT is the battery voltage and IBAT is the charge

current. It is not necessary to perform any worst-case

power dissipation scenarios because the LTC4069 will

automatically reduce the charge current to maintain the

die temperature at approximately 115°C. However, the

approximate ambient temperature at which the thermal

feedback begins to protect the IC is:

TA = 115°C – PD • θJA

TA = 115°C – (VCC – VBAT) • IBAT • θJA

Example: Consider an LTC4069 operating from a 5V wall

adapter providing 750mA to a 3.6V Li-Ion battery. The

ambient temperature above which the LTC4069 will begin

to reduce the 750mA charge current is approximately:

TA = 115°C – (5V – 3.6V) • (750mA) • 60°C/W

TA = 115°C – (1.05W • 60°C/W) = 115°C – 63°C

TA = 52°C

TheLTC4069canbeusedabove70°C,butthechargecurrent

will be reduced from 750mA. The approximate current at

a given ambient temperature can be calculated:

BAT

()

115

–

–• θ

Using the previous example with an ambient temperature

of 73°C, the charge current will be reduced to

approximately:

C W

BAT =

°°

()

115

53 6

500

–

–.

•

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 LTC4069 applications do

not need to be designed for worst-case thermal conditions

since the IC will automatically limit power dissipation when

the junction temperature reaches approximately 115°C.

Board Layout Considerations

In order to deliver maximum charge current under all

conditions, it is critical that the exposed metal pad on the

backside of the LTC4069 package is soldered to the PC board

copper and extending out to relatively large copper areas

or internal copper layers connected using vias. Correctly

soldered to a 2500mm2 double-sided 1 oz. copper board

the LTC4069 has a thermal resistance of approximately

60°C/W. Failure to make thermal contact between the

Exposed Pad on the backside of the package and the copper

board will result in thermal resistances far greater than

60°C/W. As an example, a correctly soldered LTC4069 can

deliver over 750mA to a battery from a 5V supply at room

temperature. Without a backside thermal connection, this

number could drop to less than 500mA.

VCC Bypass Capacitor

Many types of capacitors can be used for input bypassing;

however, caution must be exercised when using multi-layer

ceramic capacitors. Because of the self-resonant and high

Q characteristics of some types of ceramic capacitors, high

voltage transients can be generated under some start-up

conditions, such as connecting the charger input to a live

power source. For more information, refer to Application

Note 88.

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