Datasheet

LTC4008
13
4008fb
APPLICATIONS INFORMATION
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 internal currents and voltages are trimmed for this
value. Pick R
SENSE
by setting the average voltage between
CSP 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
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 5). C
PROG
must be increased
to reduce the ripple caused by the R
PROG
switching. The
compensation capacitor at I
TH
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 volt-
age 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
the full-scale voltage is reduced then, R4 = R5 = 2.49k
and R
PROG
= 26.7k, the maximum charging current is
still 2.5A but the FLAG trip point is maintained at 10%
of full scale.
There are other effects to consider. The voltage across the
current comparator is scaled to obtain the same values as
the 100mV sense voltage target, but the input referred sense
voltage is reduced, causing some careful consideration of
the ripple current. Input referred maximum comparator
threshold is 117mV, which is the same ratio of 1.4x the
DC target. Input referred I
REV
threshold is scaled back to
–24mV. The current at which the switcher starts will be
reduced as well so there is some risk of boost activity.
These concerns can be addressed by using a slightly larger
inductor to compensate for the reduction of tolerance to
ripple current.
R
Z
102k
C
PROG
4008 F05
LTC4008
PROG
Q1
2N7002
R
PROG
0V
5V
10
Figure 5. PWM Current Programming