LTC4008_15 Datasheet, PDF (14/24 Pages) Linear Technology – 4A, High Effi ciency, Multi-Chemistry Battery Charger

LTC4008
APPLICATIONS INFORMATION
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.
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 recommended
values for R8 and R9 for popular float voltages.
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
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. Kemet T495 series
of “Surge Robust” low ESR tantalums are rated for high
surge conditions such as battery to ground.
4008fb
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