0

1.0

2.0

_

3.0

_

4.0

_

5.0

_

DC Bias (V)

100%

80%

60%

40%

20%

_

0402, 6.3V, X5R

0603, 10V, X5R

LP5990

www.ti.com

SNVS438B – APRIL 2007 – REVISED DECEMBER 2007

CAPACITOR CHARACTERISTICS

The LP5990 is designed to work with ceramic capacitors on the input and output to take advantage of the

benefits they offer. For capacitance values in the range of 1.0

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 LP5990

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 conditions of operation 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.Capacitance value 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 particular case size with smaller sizes giving poorer

performance figures in general. As an example Figure 16 shows a typical graph showing a comparison of

capacitor case sizes in a Capacitance versus DC Bias plot. As shown in the graph, as a result of the DC Bias

condition, the capacitance value may drop below the minimum capacitance value given in the recommended

capacitor table (0.3µF 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 recommend that the capacitor manufacturer's specifications

for the nominal value capacitor are consulted for all conditions as some capacitors may not be suited in the

application.

The temperature performance of ceramic capacitors varies by type and manufacturer. Most large value ceramic

capacitors (

≥2.2 µF) are manufactured with Z5U or Y5V temperature characteristics, which results in the

capacitance dropping by more than 50% as the temperature goes from 25°C to 85°C.

A better choice for temperature coefficient in a ceramic capacitor is X7R. This type of capacitor is the most stable

and holds the capacitance within ±15% over the temperature range. 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.

Figure 16.

NO-LOAD STABILITY

The LP5990 will remain stable and in regulation with no external load.

Copyright © 2007, Texas Instruments Incorporated

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