LP38503-ADJ_15 Datasheet, PDF (12/27 Pages) Texas Instruments – LP3850x-ADJ, LP3850xA-ADJ 3-A FlexCap Low Dropout Linear Regulator for 2.7-V to 5.5-V Inputs

LP38501-ADJ, LP38503-ADJ
SNVS522I – AUGUST 2007 – REVISED AUGUST 2015
www.ti.com
Feature Description (continued)
Figure 16 shows that the output voltage starts “correcting” back upwards after less than a microsecond, and has
fully reversed direction after about 1.2 µs. This very rapid reaction is a result of the maximum loop bandwidth (full
load is being delivered) and the feedforward effect kicking on the drive to the FET before feedback gets fully
around the loop.
In cases where extremely fast load changes occur, and output voltage regulation better than 10% is required, the
output capacitance must be increased. When selecting capacitors, it must be understood that the better
performing ones usually cost the most. For fast changing loads, the internal parasitics of ESR (equivalent series
resistance) and ESL (equivalent series inductance) degrade the capacitor’s ability to source current quickly to the
load. The best capacitor types for transient performance are (in order):
1. Multilayer Ceramic: with the lowest values of ESR and ESL, they can have ESR values in the range of a few
milli Ohms. Disadvantage: capacitance values above about 22 µF significantly increase in cost.
2. Low-ESR Aluminum Electrolytics: these are aluminum types (like OSCON) with a special electrolyte which
provides extremely low ESR values, and are the closest to ceramic performance while still providing large
amounts of capacitance. These are cheaper (by capacitance) than ceramic.
3. Solid tantalum: can provide several hundred µF of capacitance, transient performance is slightly worse than
OSCON type capacitors, cheaper than ceramic in large values.
4. General purpose aluminum electrolytics: cheap and provide a lot of capacitance, but give the worst
performance.
As a first example, larger values of ceramic capacitance show how much reduction can be obtained from the
200-mV output change (Figure 16) which was seen with only a 10-µF ceramic output capacitor. In Figure 18, the
10-µF output capacitor is increased to 22 µF. The 200-mV transient is reduced to about 160 mV, which is from
about 11% of V_OUT down to about 9%.
Figure 18. 22-µF Ceramic Output Capacitor
In Figure 19, the output capacitance is increased to 47 µF ceramic. It can be seen that the output transient is
further reduced down to about 120 mV, which is still about 6.6% of the output voltage. This shows that a 5X
increase in ceramic capacitance from the original 10 µF only reduced the peak voltage transient amplitude by
about 40%.
Figure 19. 47-µF Ceramic Output Capacitor
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