LTC4008 Datasheet, PDF (12/24 Pages) Linear Technology – 4A, High Efficiency, Multi-Chemistry Battery Charger

LTC4008
APPLICATIO S I FOR ATIO
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
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_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
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 5). C_PROG must be
increased to reduce the ripple caused by the R_PROG
switching. The compensation capacitor at I_TH will prob-
ably need to be increased also to improve stability and
LTC4008
PROG
10
R_PROG
C_PROG
R_Z
102k
5V
Q1
0V
2N7002
4008 F05
Figure 5. PWM Current Programming
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
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 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_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.
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