LTC4008 Linear Technology, LTC4008 Datasheet - Page 12

• Manufacturer Part Number: LTC4008
• Description: Multi-Chemistry Battery Charger
• Manufacturer: Linear Technology

www.DataSheet4U.com
APPLICATIO S I FOR ATIO
LTC4008
Charger Current Programming
The basic formula for charging current is:
V
and R
since internal currents and voltages are trimmed for this
value. Pick R
CSP and BAT to be close to 100mV during maximum
charger current. Then R
the above equation for R
Table 2. Recommended R
Charging current can be programmed by pulse width
modulating R
quency higher than a few kHz (Figure 5). C
increased to reduce the ripple caused by the R
switching. The compensation capacitor at I
ably need to be increased also to improve stability and
12
REF
I
R
I
CHARGE MAX
MAX
PROG
= 1.19V. This leaves two degrees of freedom: R
1.0
2.0
3.0
4.0
PROG
(A)
=
. The 3.01k input resistors must not be altered
(
Figure 5. PWM Current Programming
R
SENSE
R
5V
0V
PROG
SENSE
SENSE
)
0.100
0.050
0.033
0.025
=
by setting the average voltage between
(Ω) 1%
U
V
with a switch Q1 to R
REF
•
I
R
V
CHARGE MAX
SNS
PROG
PROG
REF
LTC4008
• .
PROG
PROG
U
3 01
and R
10
• .
R
Q1
2N7002
can be determined by solving
3 01
4008 F05
SENSE
.
(
k
0.25
0.25
0.5
0.5
PROG
Ω
R
k
SENSE
(W)
C
/
Ω
)
W
PROG
R
Resistor Values
+
PROG
0 035
.
R
102k
R
Z
PROG
– .
PROG
PROG
V
TH
0 035
26.7
26.7
26.7
26.7
U
(kΩ) 1%
will prob-
at a fre-
must be
V
SENSE
PROG
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
This threshold works well when R
not yield a 10% charging current indication if R
different value. There are situations where a standard
value of R
current when using the preferred R
cases, where the full-scale voltage across R
±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
across R
82.5mV, normally R
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
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 consider-
ation of the ripple current. Input referred maximum com-
parator threshold is 117mV, which is the same ratio of
1.4x the DC target. Input referred I
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.
PROG
= 26.7k, the maximum charging current is still 2.5A
SENSE
SENSE
SENSE
will not allow the desired value of charging
value is 0.033Ω. In this case, the voltage
at maximum charging current is only
PROG
would be 30.1k but the nominal
REV
PROG
PROG
threshold is scaled
is 26.7k, but will
PROG
value. In these
SENSE
= 400mV.
PROG
is within
4008fa
is a