LTC4069 datasheet(14/18 Pages) LINER

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LTC4069

4069fc

For more information www.linear.com/LTC4069

4069 F06

RNOM

8.87k

RNTC

10k

VCC

–

TOO COLD

TOO HOT

NTC_ENABLE

604

0.76 • VCC

0.35 • VCC

0.016 • VCC

NTC

Figure 6. NTC Circuits

applicaTions inForMaTion

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

The LTC4069 can be used above 70°C, but the charge

current will be reduced from 750mA. The approximate

current at a given ambient temperature can be calculated:

IBAT =

115

°C– TA

VCC – VBAT

(

)•θJA

Using the previous example with an ambient temperature

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

approximately:

IBAT =

115

°C–73°C

5V –3.6V

(

)•60°C/W

°C

°C/A

= 500mA

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

sincetheICwillautomaticallylimitpowerdissipationwhen

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,cautionmustbeexercisedwhenusingmulti-layer

ceramic capacitors. Because of the self-resonant and high

Qcharacteristicsofsometypesofceramiccapacitors,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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