13

LTC4065L/LTC4065LX

4065lxfa

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

Due to the low charge currents, it is unlikely that the

LTC4065L will reduce charge current through thermal

feedback. Nonetheless, the LTC4065L power dissipation

can be approximated by:

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

power dissipation scenarios because the LTC4065L 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:

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

APPLICATIO S I FOR ATIO

4065L F04

CHARGE

CURRENT

MONITOR

CIRCUITRY

LTC4065L

PROG

GND

10k