LTC4008 Datasheet, PDF (13/24 Pages) Linear Technology – 4A, High Efficiency, Multi-Chemistry Battery Charger

LTC4008
APPLICATIO S I FOR ATIO
Battery Conditioning
Some batteries require a small charging current to condi-
tion them when they are severely depleted. The charging
current is switched to a high rate after the battery voltage
has reached a “safe” voltage to do so. Figure 6 illustrates
how to do this 2-level charging. When Q1 is on, the charger
current is set to maximum. When Q1 is off, the charging
current is set to 10% of the maximum.
Table 3
FLOAT VOLTAGE (V)
8.2
8.4
12.3
12.6
16.4
16.8
R9 (kΩ) 0.25%
24.9
26.1
15
16.9
11.5
13.3
R8 (kΩ) 0.25%
147
158
140
162
147
174
LTC4008
PROG
10
R1
26.7k
C_PROG
0.0047µF
Q1
R2
2N7002 ^53.6k
4008 F06
Figure 6. 2-Level Current Programming
Charger Voltage Programming
A resistor divider, R8 and R9 (see Figure 10), programs
the final float voltage of the charger. The equation for float
voltage is (the input bias current of EA is typically –4nA and
can be ignored):
V_FLOAT = V_REF (1 + R8/R9)
It is recommended that the sum of R8 and R9 not be less
than 100k. Accuracy of the LTC4008 voltage reference is
±0.8% at 25°C, and ±1% over the full temperature range.
This leads to the possibility that very accurate (0.1%)
resistors might be needed for R8 and R9. Actually, the
temperature of the LTC4008 will rarely exceed 50°C near
the float voltage because charging currents have tapered
to a low level, so 0.25% resistors will normally provide the
required level of overall accuracy. Table 3 contains recom-
mended values for R8 and R9 for popular float voltages.
Soft-Start
The LTC4008 is soft started by the 0.12µF capacitor on the
I_TH pin. On start-up, I_TH pin voltage will rise quickly to 0.5V,
then ramp up at a rate set by the internal 40µA pull-up
current and the external capacitor. Battery charging
current starts ramping up when I_TH voltage reaches 0.8V
and full current is achieved with I_TH at 2V. With a 0.12µF
capacitor, time to reach full charge current is about 2ms
and it is assumed that input voltage to the charger will
reach full value in less than 2ms. The capacitor can be
increased up to 1µF if longer input start-up times are
needed.
Input and Output Capacitors
The input capacitor (C2) is assumed to absorb all input
switching ripple current in the converter, so it must have
adequate ripple current rating. Worst-case RMS ripple
current will be equal to one-half of output charging
current. Actual capacitance value is not critical. Solid
tantalum low ESR capacitors have high ripple current
rating in a relatively small surface mount package, but
caution must be used when tantalum capacitors are used
for input or output bypass. High input surge currents can
be created when the adapter is hot-plugged to the charger
or when a battery is connected to the charger. Solid
tantalum capacitors have a known failure mechanism
when subjected to very high turn-on surge currents. Only
Kemet T495 series of “Surge Robust” low ESR tantalums
are rated for high surge conditions such as battery to
ground.
4008fa
13