ltc4065edc-4.4 Linear Technology Corporation, ltc4065edc-4.4 Datasheet - Page 13

• Manufacturer Part Number: ltc4065edc-4.4
• Description: Standalone 750ma Li-ion Battery Charger In 2 ? 2 Dfn
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Stability Considerations
The LTC4065-4.4 contain two control loops: constant-
voltage and constant-current. The constant-voltage loop
is stable without any compensation when a battery is
connected with low impedance leads. Excessive lead
length, however, may add enough series inductance to
require a bypass capacitor of at least 1µF from BAT to
GND. Furthermore, a 4.7µF 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 22µF may be used in parallel
with a battery, but larger ceramics should be decoupled
with 0.2Ω 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
pin is loaded with a capacitance, C
equation should be used to calculate the maximum resis-
tance value for R
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
R
PROG
≤
2
π
•
PROG
10
U
5
1
•
:
C
PROG
U
W
PROG
, the following
U
Figure 4. Isolating Capacitive Load on the PROG Pin and Filtering
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 LTC4065-4.4 to reduce
charge current through thermal feedback can be approxi-
mated by considering the power dissipated in the IC. For
high charge currents, the LTC4065-4.4 power dissipation
is approximately:
Where P
voltage, V
current. It is not necessary to perform any worst-case
power dissipation scenarios because the LTC4065-4.4
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:
P
T
T
A
A
D
= 115°C – P
= 115°C – (V
= (V
D
BAT
CC
is the power dissipated, V
– V
LTC4065-4.4
is the battery voltage and I
GND
BAT
PROG
D
CC
) • I
• θ
– V
JA
BAT
BAT
R
PROG
10k
) • I
4065 F04
BAT
LTC4065-4.4
C
CC
FILTER
• θ
is the input supply
JA
BAT
CHARGE
CURRENT
MONITOR
CIRCUITRY
is the charge
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