LP3996 Datasheet, PDF (13/22 Pages) National Semiconductor (TI) – Dual Linear Regulator with 300mA and 150mA Outputs and Power-On-Reset

LP3996
www.ti.com
SNVS360D – NOVEMBER 2006 – REVISED OCTOBER 2013
Capacitor Characteristics
The LP3996 is designed to work with ceramic capacitors on the output to take advantage of the benefits they
offer. For capacitance values in the range of 0.47 µF to 4.7 µF, ceramic capacitors are the smallest, least
expensive and have the lowest ESR values, thus making them best for eliminating high frequency noise. The
ESR of a typical 1.0 µF ceramic capacitor is in the range of 20 mΩ to 40 mΩ, which easily meets the ESR
requirement for stability for the LP3996.
For both input and output capacitors, careful interpretation of the capacitor specification is required to ensure
correct device operation. The capacitor value can change greatly, depending on the operating conditions and
capacitor type.
In particular, the output capacitor selection should take account of all the capacitor parameters, to ensure that the
specification is met within the application. The capacitance can vary with DC bias conditions as well as
temperature and frequency of operation. Capacitor values will also show some decrease over time due to aging.
The capacitor parameters are also dependant on the particular case size, with smaller sizes giving poorer
performance figures in general. As an example, Figure 26 shows a typical graph comparing different capacitor
case sizes in a Capacitance vs. DC Bias plot. As shown in the graph, increasing the DC Bias condition can result
in the capacitance value falling below the minimum value given in the recommended capacitor specifications
table (0.7 µF in this case). Note that the graph shows the capacitance out of spec for the 0402 case size
capacitor at higher bias voltages. It is therefore recommended that the capacitor manufacturers’ specifications for
the nominal value capacitor are consulted for all conditions, as some capacitor sizes (that is, 0402) may not be
suitable in the actual application.
100%
0603, 10V, X5R
80%
60%
40%_
20%
_₀
0402, 6.3V, X5R
2.0
3.0
4.0
5.0
1.0
_
_
_
_
DC BIAS (V)
Figure 26. Graph Showing a Typical Variation in Capacitance vs DC Bias
The ceramic capacitor’s capacitance can vary with temperature. The capacitor type X7R, which operates over a
temperature range of –55°C to +125°C, will only vary the capacitance to within ±15%. The capacitor type X5R
has a similar tolerance over a reduced temperature range of –55°C to +85°C. Many large value ceramic
capacitors, larger than 1µF are manufactured with Z5U or Y5V temperature characteristics. Their capacitance
can drop by more than 50% as the temperature varies from 25°C to 85°C. Therefore X7R is recommended over
Z5U and Y5V in applications where the ambient temperature will change significantly above or below 25°C.
Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more
expensive when comparing equivalent capacitance and voltage ratings in the 0.47 µF to 4.7 µF range.
Another important consideration is that tantalum capacitors have higher ESR values than equivalent size
ceramics. This means that while it may be possible to find a tantalum capacitor with an ESR value within the
stable range, it would have to be larger in capacitance (which means bigger and more costly) than a ceramic
capacitor with the same ESR value. It should also be noted that the ESR of a typical tantalum will increase about
2:1 as the temperature goes from 25°C down to –40°C, so some guard band must be allowed.
Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LP3996
Submit Documentation Feedback
13