LP38501-ADJ, LP38503-ADJ

www.ti.com

SNVS522I – AUGUST 2007 – REVISED AUGUST 2015

Feature Description (continued)

In general, managing load transients is done by paralleling ceramic capacitance with a larger bulk capacitance.

In this way, the ceramic can source current during the rapidly changing edge and the bulk capacitor can support

the load current after the first initial spike in current.

In the next test, the same 10-µF ceramic capacitor is paralleled with a general-purpose (less expensive)

aluminum electrolytic whose capacitance is 220 µF. As shown in Figure 20, there is a small improvement over

the 200 mV peak seen with the 10-µF ceramic capacitor alone. By adding the 220 µF aluminum capacitor, the

peak is reduced to about 160 mV (the same peak value as seen with a 22-µF ceramic capacitor alone).

Figure 20. 10-µF Ceramic Paralleled by 220-µF Generic Aluminum Electrolytic

A solid Tantalum works better, so the aluminum electrolytic is replaced by a 220-µF Tantalum (Figure 21). The

peak amplitude of the output transient is now reduced to about 130 mV, just slightly less efficient than the value

of the 47-µF ceramic capacitor alone.

Figure 21. 10-µF Ceramic Paralleled by 220-µF Tantalum

The OSCON (ultra low ESR) aluminum electrolytic is the best of the electrolytics. Figure 22 shows the output

voltage transient is reduced down to about 90 mV (about 5% of VOUT) when a 220-µF OSCON is added to the 10

µF ceramic. This indicates that some kind of ultra-low ESR aluminum electrolytic used in parallel with some

ceramic capacitance is probably the best approach for extremely fast transients, but each application must be

dialed in for it’s specific load requirements.

Copyright © 2007–2015, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LP38501-ADJ LP38503-ADJ