Datasheet

LTC4062
15
4062fb
APPLICATIO S I FOR ATIO
WUUU
User-Selectable Charge Termination
Connecting the TIMER pin to V
CC
selects user-selectable
charge termination, in which all of the internal termination
features are disabled. The charge cycle continues indefi-
nitely until the charger is shut down through the EN pin.
The I
DET
pin programs the behavior of the CHRG output in
the same manner as when using charge time termination.
If the I
DET
pin is not connected, the CHRG output remains
in its pull-down state until the charger is shut down.
With user-selectable charge termination, the SmartStart
feature is disabled; when the charger is powered on or
enabled, the LTC4062 automatically begins charging, re-
gardless of the battery voltage. Figure 5 describes charger
operation when user-selectable charge termination is used.
Programming C/10 Current Detection/Termination
In most cases, an external resistor, R
DET
, is needed to set
the charge current detection threshold, I
DETECT
. However,
when setting I
DETECT
to be 1/10th of I
CHG
, the I
DET
pin can
be connected directly to the PROG pin. This reduces the
component count, as shown in Figure 6.
When PROG and I
DET
are connected in this way, the full-
scale charge current, I
CHG
, is programmed with a different
equation:
R
V
I
I
V
R
PROG
CHG
CHG
PROG
==
500 500
,
Stability Considerations
The battery charger constant voltage mode feedback loop
is stable without any compensation provided a battery is
connected. However, a 1µF capacitor with a 1Ω series
resistor to GND is recommended at the BAT pin to reduce
noise when no battery is present.
When the charger is in constant current mode, the PROG
pin is in the feedback loop, not the battery. The constant
current stability is affected by the impedance at the PROG
pin. With no additional capacitance on the PROG pin, the
charger is stable with program resistor values as high as
10kΩ; however, additional capacitance on this node re-
duces the maximum allowed program resistor value.
Power Dissipation
When designing the battery charger circuit, it is not
necessary to design for worst-case power dissipation
scenarios because the LTC4062 automatically reduces
the charge current during high power conditions. The
conditions that cause the LTC4062 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 charger
MOSFET. Thus, the power dissipation is calculated to be
approximately:
P
D
= (V
CC
– V
BAT
) • I
BAT
P
D
is the power dissipated, V
CC
is the input supply voltage,
V
BAT
is the battery voltage and I
BAT
is the charge current.
The approximate ambient temperature at which the ther-
mal feedback begins to protect the IC is:
T
A
= 105°C – P
D
• θ
JA
T
A
= 105°C – (V
CC
– V
BAT
) • I
BAT
• θ
JA
+
V
CC
PROG
I
DET
V
IN
BAT
500mA
TIMER
R
DET
2k
R
PROG
2k
LTC4062
GND
+
V
CC
PROG
I
DET
V
IN
4062 F06
BAT
500mA
TIMER
R
PROG
1k
LTC4062
GND
C/5
C/5
Figure 6. Two Circuits That Charge at 500mA
Full-Scale Current and Terminate at 50mA