ACT8828 datasheet(48/49 Pages) ACTIVE-SEMI

48 / 49 page

ActivePath

TM CHARGER

ACT8828

Rev 3, 24-Sep-09

Active-Semi

Innovative Power

- 48 -

www.active-semi.com

ActivePMU

TM and ActivePathTM are trademarks of Active-Semi.

2CTM is a trademark of Philips Electronics.

Figure 12:

CHG_IN Bypass Options for USB or Wall Adaptor Supplies

FUNCTIONAL DESCRIPTION CONT’D

resulting

from

the

current

limit

slew

rate

(100mA/µS) limitations of the USB bus, the

CHG_IN bypass capacitance value must to be

between 4.7µF and 10µF for the ACT8828.

Ceramic

capacitors

are

often

preferred

for

bypassing applications due to their small size and

good surge current ratings, but care must be taken

in applications that can encounter hot plug

conditions as their very low ESR, in combination

with the inductance of the cable, can create a high-

Q filter that induces excessive ringing at the

CHG_IN pin. This ringing can couple to the output

and be mistaken as loop instability, or the ringing

may be large enough to damage the input itself.

Although the CHG_IN pin is designed for maximum

robustness and an absolute maximum voltage

rating of 14V for transients, attention must be given

to bypass techniques to ensure safe operation.

As a result, design of the CHG_IN bypass must

take care to “de-Q” the filter. This can be

accomplished by connecting a 1Ω resistor in series

with a ceramic capacitor (as shown in Figure 12), or

by using a tantalum or electrolytic capacitor to

utilize it’s higher ESR to dampen the ringing. For

additional protection in extreme situations, Zener

diodes with 12V clamp voltages may also be used.

In any case, it is always critical to evaluate voltage

transients at the ACT8828 CHG_IN pin with an

oscilloscope to ensure safe operation.

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