14

LTC4065/LTC4065A

4065fb

length, however, may add enough series inductance to

require a bypass capacitor of at least 1

µF from BAT to

GND. Furthermore, a 4.7

µF capacitor with a 0.2Ω to 1Ω

series resistor from BAT to GND is required to keep ripple

voltage low when the battery is disconnected.

High value capacitors with very low ESR (especially ce-

ramic) may reduce the constant-voltage loop phase mar-

gin. Ceramic capacitors up to 22

µF may be used in parallel

with a battery, but larger ceramics should be decoupled

with 0.2

Ω to 1Ω of series resistance.

In constant-current mode, the PROG pin is in the feedback

loop, not the battery. Because of the additional pole

created by the PROG pin capacitance, capacitance on this

pin must be kept to a minimum. With no additional

capacitance on the PROG pin, the charger is stable with

program resistor values as high as 25k. However, addi-

tional capacitance on this node reduces the maximum

allowed program resistor. The pole frequency at the PROG

pin should be kept above 100kHz. Therefore, if the PROG

equation should be used to calculate the maximum resis-

R

C

PROG

PROG

≤

π

1

••

Average, rather than instantaneous, battery current may

be of interest to the user. For example, if a switching power

supply operating in low current mode is connected in

parallel with the battery, the average current being pulled

out of the BAT pin is typically of more interest than the

instantaneous current pulses. In such a case, a simple RC

filter can be used on the PROG pin to measure the average

battery current as shown in Figure 4. A 10K resistor has

been added between the PROG pin and the filter capacitor

to ensure stability.

Power Dissipation

The conditions that cause the LTC4065/LTC4065A to

reduce charge current through thermal feedback can be

approximated by considering the power dissipated in the

IC. For high charge currents, the LTC4065/LTC4065A

power dissipation is approximately:

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

power dissipation scenarios because the LTC4065 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:

Example: Consider an LTC4065/LTC4065A operating from

a 5V wall adapter providing 750mA to a 3.6V Li-Ion

battery. The ambient temperature above which the

LTC4065/LTC4065A will begin to reduce the 750mA charge

current is approximately:

The LTC4065/LTC4065A can be used above 70

°C, but the

charge current will be reduced from 750mA. The approxi-

mate current at a given ambient temperature can be

calculated:

I

CT

VV

BAT

A

CC

BAT

JA

=

°

()

115

–

–•

θ

Using the previous example with an ambient temperature

of 73

°C, the charge current will be reduced to approxi-

mately:

I

CC

VV

C W

C

CA

mA

°°

=

°

°

=

115

73

53 6

60

42

84

500

–

–.

•

/

/

4065 F04

CHARGE

CURRENT

MONITOR

CIRCUITRY

LTC4065

PROG

GND

10k

Figure 4. Isolating Capacitive Load on the PROG Pin and Filtering

APPLICATIO S I FOR ATIO