0402, 6.3V, X5R

0603, 10V, X5R

0

1.0

2.0

3.0

4.0

5.0

DC BIAS (V)

20%

40%

60%

80%

100%

LP3990

www.ti.com

SNVS251J – MAY 2004 – REVISED SEPTEMBER 2014

8.2.2.6 Capacitor Characteristics

The LP3990 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-µF ceramic capacitor is in the range of 20 m

Ω to 40 mΩ, which easily meets the ESR

requirement for stability for the LP3990.

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 14 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 (for example, 0402) may not

be suitable in the actual application.

Figure 14. 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 1 µF 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 and X5R types are 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.

8.2.2.7 Enable Control

The LP3990 features an active high Enable pin, EN, which turns the device on when pulled high. When not

enabled the regulator output is off and the device typically consumes 2 nA.

regulator output permanently on.

Copyright © 2004–2014, Texas Instruments Incorporated

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