LTC4007_1 Datasheet, PDF (13/24 Pages) Linear Technology – 4A, High Efficiency, Standalone Li-Ion Battery Charger

LTC4007
APPLICATIO S I FOR ATIO
Battery Detection
It is generally not good practice to connect a battery while
the charger is running. The timer is in an unknown state
and the charger could provide a large surge current into
the battery for a brief time. The Figure 5 circuit keeps the
charger shut down and the timer reset while a battery is not
connected.
ADAPTER
POWER
SWITCH CLOSED
WHEN BATTERY
CONNECTED
LTC4007
1 DCIN
24 SHDN
4007 F05
Figure 5
Charger Current Programming
The basic formula for charging current is:
I_CHARGE(MAX)
=
V_REF
• 3.01kΩ / R_PROG
R_SENSE
– 0.035V
V_REF = 1.19V
This leaves two degrees of freedom: R_SENSE and R_PROG.
The 3.01k input resistors must not be altered since internal
currents and voltages are trimmed for this value. Pick
R_SENSE by setting the average voltage between C_SP and
BAT to be close to 100mV during maximum charger
current. Then R_PROG can be determined by solving the
above equation for R_PROG.
R_PROG
=
R_SENSE
•
V_REF • 3.01kΩ
I_CHARGE(MAX) +
0.035V
Table 2. Recommended R_SNS and R_PROG Resistor Values
I_MAX (A)
R_SENSE (Ω) 1% R_SENSE (W)
R_PROG (kΩ) 1%
1.0
0.100
0.25
26.7
2.0
0.050
0.25
26.7
3.0
0.033
0.5
26.7
4.0
0.025
0.5
26.7
LTC4007
PROG
11
R_PROG
C_PROG
R_Z
102k
5V
Q1
0V
2N7002
4007 F06
Figure 6. PWM Current Programming
Charging current can be programmed by pulse width
modulating R_PROG with a switch Q1 to R_PROG at a fre-
quency higher than a few kHz (Figure 6). C_PROG must be
increased to reduce the ripple caused by the R_PROG
switching. The compensation capacitor at ITH will prob-
ably need to be increased also to improve stability and
prevent large overshoot currents during start-up condi-
tions. Charging current will be proportional to the duty
cycle of the switch with full current at 100% duty cycle and
zero current when Q1 is off.
Maintaining C/10 Accuracy
The C/10 comparator threshold that drives the FLAG pin
has a fixed threshold of approximately V_PROG = 400mV.
This threshold works well when R_PROG is 26.7k, but will
not yield a 10% charging current indication if R_PROG is a
different value. There are situations where a standard
value of R_SENSE will not allow the desired value of charging
current when using the preferred R_PROG value. In these
cases, where the full-scale voltage across R_SENSE is within
±20mV of the 100mV full-scale target, the input resistors
connected to CSP and BAT can be adjusted to provide the
desired maximum programming current as well as the
correct FLAG trip point.
For example, the desired max charging current is 2.5A but
the best R_SENSE value is 0.033Ω. In this case, the voltage
across R_SENSE at maximum charging current is only
82.5mV, normally R_PROG would be 30.1k but the nominal
FLAG trip point is only 5% of maximum charging current.
If the input resistors are reduced by the same amount as
4007fa
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