LTC4008 datasheet(13/24 Pages) LINER

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LTC4008

4008fa

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)

R9 (k

Ω) 0.25%

R8 (k

Ω) 0.25%

8.2

24.9

147

8.4

26.1

158

12.3

140

12.6

16.9

162

16.4

11.5

147

16.8

13.3

174

Soft-Start

The LTC4008 is soft started by the 0.12

µF capacitor on the

ITH pin. On start-up, ITH pin voltage will rise quickly to 0.5V,

then ramp up at a rate set by the internal 40

µA pull-up

current and the external capacitor. Battery charging

current starts ramping up when ITH voltage reaches 0.8V

and full current is achieved with ITH 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 1

µF 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.

53.6k

CPROG

0.0047

µF

4008 F06

LTC4008

2N7002

26.7k

PROG

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):

VFLOAT = VREF (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.

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