LTC4002

13

4002f

Input and Output Capacitors

Since the input capacitor is assumed to absorb all input

switching ripple current in the converter, it must have an

adequate ripple current rating. Worst-case RMS ripple cur-

rent is approximately one-half of output charge current.

Actual capacitance value is not critical. Solid tantalum

capacitors have a high ripple current rating in a relatively

small surface mount package, but caution must be used

when tantalum capacitors are used for input bypass. High

input surge currents can be created when the adapter is

hot-plugged to the charger and solid tantalum capacitors

have a known failure mechanism when subjected to very

high turn-on surge currents. Selecting the highest pos-

sible voltage rating on the capacitor will minimize prob-

lems. Consult with the manufacturer before use.

mined by the ESR required to minimize ripple voltage and

mately bounded by:

∆≤ ∆

+

⎛

⎝

⎜

⎞

⎠

⎟

V

I ESR

fC

OUT

L

OSC OUT

1

8

highest at maximum input voltage. Typically, once the ESR

requirement is satisfied, the capacitance is adequate for

filtering and has the necessary RMS current rating.

APPLICATIO S I FOR ATIO

Switching ripple current splits between the battery and the

output capacitor depending on the ESR of the output ca-

pacitor and the battery impedance. EMI considerations

usually make it desirable to minimize ripple current in the

battery leads. Ferrite beads or an inductor may be added

to increase battery impedance at the 500kHz switching

frequency. If the ESR of the output capacitor is 0.2Ω and

the battery impedance is raised to 4Ω with a bead or induc-

tor, only 5% of the current ripple will flow in the battery.

Design Example

As a design example, take a charger with the following

First, calculate the SENSE resistor :

Choose the inductor for about 65% ripple current at the

L

V

kHz

A

V

V

H

=

()(

)(

)

⎛

⎝

⎜

⎞

⎠

⎟ =µ

4

500

0 65 1 5

1

4

22

6 713

..

–.

Selecting a standard value of 6.8µH results in a maximum

ripple current of :

∆ =

()

µ

()

⎛

⎝

⎜

⎞

⎠

⎟ =

I

V

kHz

H

V

V

mA

L

4

500

6 8

1

4

22

962 6

.

–.