LTC4077

13

4077fa

Stability Considerations

Theconstant-voltagemodefeedbackloopisstablewithout

any compensation provided a battery is connected to the

charger output. However, a 1µF capacitor with a 1Ω series

resistor is recommended at the BAT pin to keep the ripple

voltage low when the battery is disconnected.

When the charger is in constant-current mode, the charge

current program pin (IDC, IUSB or IUSBL) is in the feed-

back loop, not the battery. The constant-current mode

stability is affected by the impedance at the charge current

program pin. With no additional capacitance on this pin,

the charger is stable with program resistor values as high

as 20k; however, additional capacitance on these nodes

reduces the maximum allowed program resistor.

Power Dissipation

When designing the battery charger circuit, it is not neces-

sary to design for worst-case power dissipation scenarios

because the LTC4077 automatically reduces the charge

current during high power conditions. The conditions

that cause the LTC4077 to reduce charge current through

thermal feedback can be approximated by considering the

power dissipated in the IC. Most of the power dissipation

is generated from the internal MOSFET pass device, thus,

the power dissipation is calculated to be:

temperature at which the thermal feedback begins to

protect the IC is:

Example:AnLTC4077operatingfroma5Vwalladapter(on

theDCINinput)isprogrammedtosupply800mAfull-scale

currenttoadischargedLi-Ionbatterywithavoltageof3.3V.

the ambient temperature at which the LTC4077 will begin

to reduce the charge current is approximately:

The LTC4077 can be used above 50.6°C ambient, but

the charge current will be reduced from 800mA. The ap-

proximate current at a given ambient temperature can be

approximated by:

I

C T

V

V

BAT

A

IN

BAT

JA

=

°

105

±

(

±

•

Using the previous example with an ambient tem-

perature of 60°C, the charge current will be reduced to

approximately:

I

C

C

V

V

C W

C

C A

I

mA

BAT

BAT

=

°

°

°

=

°

°

=

105

60

5

3 3

40

45

68

662

±

(

± . ) •

/

/

It is important to remember that LTC4077 applications do

notneedtobedesignedforworst-casethermalconditions,

since the IC will automatically reduce power dissipation

when the junction temperature reaches approximately

105°C.

APPLICATIO S I FOR ATIO