LTC4006 datasheet(13/20 Pages) LINER

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LTC4006

4006fa

Soft-Start

The LTC4006 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 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 Appli-

cation Note 88 for more information.

Highest possible voltage rating on the capacitor will mini-

mize problems. Consult with the manufacturer before use.

Alternatives include new high capacity ceramic (at least

µF) from Tokin, United Chemi-Con/Marcon, et al. Other

alternative capacitors include OS-CON capacitors from

Sanyo.

The output capacitor (C3) is also assumed to absorb

output switching current ripple. The general formula for

capacitor current is:

APPLICATIO S I FOR ATIO

RMS

BAT

DCIN

()⎛

⎝⎜

⎞

⎠⎟

()( )

029

.–

For example:

VDCIN = 19V, VBAT = 12.6V, L1 = 10µH, and

f = 300kHz, IRMS = 0.41A.

EMI considerations usually make it desirable to minimize

ripple current in the battery leads, and beads or inductors

may be added to increase battery impedance at the 300kHz

switching frequency. Switching ripple current splits be-

tween the battery and the output capacitor depending on

the ESR of the output capacitor and the battery impedance.

If the ESR of C3 is 0.2

Ω and the battery impedance is

raised to 4

Ω with a bead or inductor, only 5% of the

current ripple will flow in the battery.

Inductor Selection

Higher operating frequencies allow the use of smaller

inductor and capacitor values. A higher frequency gener-

ally results in lower efficiency because of MOSFET gate

charge losses. In addition, the effect of inductor value on

ripple current and low current operation must also be

considered. The inductor ripple current

∆IL decreases

with higher frequency and increases with higher VIN.

∆ = ()( )

⎛

⎝⎜

⎞

⎠⎟

OUT

1–

Accepting larger values of

∆IL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is

∆IL = 0.4(IMAX). In no case should

∆IL exceed 0.6(IMAX) due to limits imposed by IREV and

CA1. Remember the maximum

∆IL occurs at the maxi-

mum input voltage. In practice 10

µH is the lowest value

recommended for use.

Lower charger currents generally call for larger inductor

values. Use Table 4 as a guide for selecting the correct

inductor value for your application.

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