LTC4065L/
LTC4065LX/LTC4065L-4.1
APPLICATIONS INFORMATION
In constant-current mode, the PROG pin is in the feedback
loop,notthebattery.Becauseoftheadditionalpolecreated
bythePROGpincapacitance,capacitanceonthispinmust
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, additional capacitance on
this node reduces the maximum allowed program resis-
tor. The pole frequency at the PROG pin should be kept
above 100kHz. Therefore, if the PROG pin is loaded with
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:
a capacitance, C
, the following equation should be
PROG
P = (V – V ) • I
BAT
D
CC
BAT
usedtocalculatethemaximumresistancevalueforR
:
PROG
Where P is the power dissipated, V is the input supply
D
CC
1
RPROG
≤
voltage, V is the battery voltage and I is the charge
2π •105 •CPROG
BAT
BAT
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:
Average,ratherthaninstantaneous,batterycurrentmaybe
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
T = 115°C – P • θ
JA
A
D
T = 115°C – (V – V ) • I • θ
A
CC
BAT
BAT
JA
LTC4065L
CHARGE
10k
CURRENT
MONITOR
CIRCUITRY
PROG
GND
R
C
FILTER
PROG
4065L F04
Figure 4. Isolating Capacitive Load on the PROG Pin and Filtering
4065lfb
14
Linear Systems [ Linear Systems ]
