Datasheet

LTC4007
13
4007fc
APPLICATIONS INFORMATION
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.
Charging current can be programmed by pulse width
modulating R
PROG
with a switch Q1 to R
PROG
at a frequency
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 probably need to be
increased also to improve stability and prevent large
overshoot currents during start-up conditions. 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 fi xed 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
1
DCIN
LTC4007
ADAPTER
POWER
SWITCH CLOSED
WHEN BATTERY
CONNECTED
24
SHDN
4007 F05
Figure 5
Charger Current Programming
The basic formula for charging current is:
I
VkR V
R
CHARGE MAX
REF PROG
SENSE
()
•. / –.
=
Ω3 01 0 035
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 in-
ternal 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
Vk
RI V
PROG
REF
SENSE CHARGE MAX
=
Ω
+
•.
•.
()
301
0 035
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
R
Z
102k
C
PROG
4007 F06
PROG
11
LTC4007
Q1
2N7002
R
PROG
0V
5V
Figure 6. PWM Current Programming