LTC4007 datasheet(13/20 Pages) LINER

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LTC4007

4007i

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 IREV 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.

Charger Voltage Programming

Pins CHEM and C3C4 are used to program the charger final

output voltage. The CHEM pin programs Li-Ion battery

chemistry for 4.1V/cell (low) or 4.2V/cell (high). The C3C4

pin selects either 3 series cells (low) or 4 series cells

(high). It is recommended that these pins be shorted to

ground (logic low) or left open (logic high) to effect the

desired logic level. Use open-collector or open-drain out-

puts when interfacing to the CHEM and 3C4C pins from a

logic control circuit.

Table 3. Charger Voltage Programming

VFINAL (V)

3C4C

CHEM

12.3

LOW

12.6

LOW

HIGH

16.4

HIGH

LOW

16.8

HIGH

Setting the Timer Resistor

The charger termination timer is designed for a range of

1hour to 3 hour with a

±15% uncertainty. The timer is

programmed by the resistor RRT using the following

equation:

tTIMER = 227 • RRT • 175pF

It is important to keep the parasitic capacitance on the RT

pin to a minimum. The trace connecting RT to RRT should

be as short as possible.

Soft-Start

The LTC4007 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

APPLICATIO S I FOR ATIO

current starts ramping up when ITH voltage reaches 0.8V

and full current is achieved with ITH at 2V. With a 0.12

µF

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 rela-

tively 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 capaci-

tors 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.

The relatively high ESR of an aluminum electrolytic for C1,

located at the AC adapter input terminal, is helpful in

reducing ringing during the hot-plug event. Refer to AN88

for more information.

Figure 7. tTIMER vs RRT

RRT (kΩ)

100

200

140

200

300

350

4007 F07

160

180

120

150

250

400 450

500

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